New drag and drop API (#4571)

This commit implements a new API for drag and drop, with a `DataTransfer` type which abstracts over the various clipboard/drag and drop APIs across different platforms. I built this on top of #2429 although admittedly I ended up removing pretty much all of their work while I was reworking the design.

This is being built in order to help support [drag-and-drop work](https://github.com/slint-ui/slint/issues/1967) in Slint's winit backend. As part of that work, I did extensive research on how drag-and-drop and clipboard APIs are implemented across different platforms, and wrote a (still WIP) research document that can be found [here](https://gist.github.com/eira-fransham/06750cf8d25ade08d362a0ca8dfafe06).

The new API is inspired by the browser's [`DataTransfer`](https://developer.mozilla.org/en-US/docs/Web/API/DataTransfer) API.

Cargo.toml

@@ -36,6 +36,7 @@ rwh_06 = { package = "raw-window-handle", version = "0.6", features = ["std"] }
 serde = { version = "1", features = ["serde_derive"] }
 smol_str = "0.3"
 tracing = { version = "0.1.40", default-features = false }
+url = "2"
 
 # Dev dependencies.
 image = { version = "0.25.0", default-features = false }

typos.toml

@@ -5,6 +5,7 @@ TME_LEAVE = "TME_LEAVE"             # From windows_sys::Win32::UI::Input::Keyboa
 XF86_Calculater = "XF86_Calculater" # From xkbcommon_dl::keysyms::XF86_Calculater
 ptd = "ptd"                         # From windows_sys::Win32::System::Com::FORMATETC { ptd, ..}
 requestor = "requestor"             # From x11_dl::xlib::XSelectionEvent { requestor ..}
+unknwn = "unknwn"                   # Windows SDK header filename `unknwn.h`
 
 [files]
 extend-exclude = ["*.drawio"]

winit-appkit/Cargo.toml

@@ -36,6 +36,8 @@ objc2-app-kit = { workspace = true, features = [
     "NSControl",
     "NSCursor",
     "NSDragging",
+    "NSDraggingItem",
+    "NSDraggingSession",
     "NSEvent",
     "NSGraphics",
     "NSGraphicsContext",
@@ -46,6 +48,7 @@ objc2-app-kit = { workspace = true, features = [
     "NSOpenGLView",
     "NSPanel",
     "NSPasteboard",
+    "NSPasteboardItem",
     "NSResponder",
     "NSRunningApplication",
     "NSScreen",

winit-appkit/src/app_state.rs

@@ -1,4 +1,5 @@
 use std::cell::{Cell, OnceCell, RefCell};
+use std::collections::HashMap;
 use std::mem;
 use std::rc::Rc;
 use std::sync::Arc;
@@ -6,22 +7,30 @@ use std::time::Instant;
 
 use dispatch2::MainThreadBound;
 use objc2::MainThreadMarker;
-use objc2_app_kit::{NSApplication, NSApplicationActivationPolicy, NSRunningApplication};
+use objc2::rc::{Retained, Weak};
+use objc2_app_kit::{
+    NSApplication, NSApplicationActivationPolicy, NSDragOperation, NSRunningApplication,
+};
 use objc2_foundation::NSNotification;
 use winit_common::core_foundation::{EventLoopProxy, MainRunLoop};
 use winit_common::event_handler::EventHandler;
 use winit_core::application::ApplicationHandler;
+use winit_core::data_transfer::DataTransferId;
 use winit_core::event::{StartCause, WindowEvent};
-use winit_core::event_loop::ControlFlow;
+use winit_core::event_loop::{ControlFlow, DndAction};
 use winit_core::window::WindowId;
 
 use super::event_loop::{ActiveEventLoop, notify_windows_of_exit, stop_app_immediately};
 use super::menu;
 use super::observer::EventLoopWaker;
+use crate::dnd::Pasteboards;
+use crate::window_delegate::WindowDelegate;
 
 #[derive(Debug)]
 pub(super) struct AppState {
     mtm: MainThreadMarker,
+    drag_state: RefCell<Option<DragState>>,
+    pasteboards: Pasteboards,
     activation_policy: Option<NSApplicationActivationPolicy>,
     default_menu: bool,
     activate_ignoring_other_apps: bool,
@@ -43,10 +52,17 @@ pub(super) struct AppState {
     start_time: Cell<Option<Instant>>,
     wait_timeout: Cell<Option<Instant>>,
     pending_redraw: RefCell<Vec<WindowId>>,
+    windows: RefCell<HashMap<WindowId, MainThreadBound<Weak<WindowDelegate>>>>,
     // NOTE: This is strongly referenced by our `NSWindowDelegate` and our `NSView` subclass, and
     // as such should be careful to not add fields that, in turn, strongly reference those.
 }
 
+#[derive(Debug)]
+pub(crate) struct DragState {
+    pub id: DataTransferId,
+    pub valid_actions: Vec<DndAction>,
+}
+
 // SAFETY: Creating `MainThreadBound` in a `const` context, where there is no concept of the
 // main thread.
 static GLOBAL: MainThreadBound<OnceCell<Rc<AppState>>> =
@@ -65,6 +81,8 @@ impl AppState {
 
         let this = Rc::new(Self {
             mtm,
+            pasteboards: Default::default(),
+            drag_state: Default::default(),
             activation_policy,
             default_menu,
             activate_ignoring_other_apps,
@@ -82,6 +100,7 @@ impl AppState {
             waker: RefCell::new(EventLoopWaker::new()),
             start_time: Cell::new(None),
             wait_timeout: Cell::new(None),
+            windows: Default::default(),
             pending_redraw: RefCell::new(vec![]),
         });
 
@@ -96,6 +115,20 @@ impl AppState {
             .clone()
     }
 
+    pub fn with_window_delegate_on_main<F, R>(&self, id: WindowId, func: F) -> Option<R>
+    where
+        F: FnOnce(Retained<WindowDelegate>) -> R + Send,
+        R: Send,
+    {
+        self.windows.borrow_mut().get(&id)?.get_on_main(move |delegate| delegate.load().map(func))
+    }
+
+    pub fn register_window(&self, window: &Retained<WindowDelegate>, mtm: MainThreadMarker) {
+        let id = window.id();
+        let window_downgraded = Weak::from_retained(window);
+        self.windows.borrow_mut().insert(id, MainThreadBound::new(window_downgraded, mtm));
+    }
+
     // NOTE: This notification will, globally, only be emitted once,
     // no matter how many `EventLoop`s the user creates.
     pub fn did_finish_launching(self: &Rc<Self>, _notification: &NSNotification) {
@@ -371,6 +404,23 @@ impl AppState {
         };
         self.waker.borrow_mut().start_at(min_timeout(wait_timeout, app_timeout));
     }
+
+    pub fn pasteboards(&self) -> &Pasteboards {
+        &self.pasteboards
+    }
+
+    pub fn drag_state(&self) -> &RefCell<Option<DragState>> {
+        &self.drag_state
+    }
+
+    pub(crate) fn proposed_drag_action(
+        &self,
+        source_operations: NSDragOperation,
+    ) -> Option<DndAction> {
+        self.drag_state().borrow().as_ref().and_then(|drag_state| {
+            crate::dnd::preferred_drag_operation(source_operations, &drag_state.valid_actions)
+        })
+    }
 }
 
 /// Returns the minimum `Option<Instant>`, taking into account that `None`

winit-appkit/src/cursor.rs

@@ -14,6 +14,7 @@ use objc2_foundation::{
 };
 use winit_core::cursor::{CursorIcon, CursorImage, CustomCursorProvider, CustomCursorSource};
 use winit_core::error::{NotSupportedError, RequestError};
+use winit_core::icon::{Icon, RgbaIcon};
 
 #[derive(Clone, Debug, PartialEq, Eq, Hash)]
 pub struct CustomCursor(pub(crate) Retained<NSCursor>);
@@ -42,6 +43,40 @@ impl CustomCursor {
     }
 }
 
+pub(crate) fn image_from_icon(icon: &Icon) -> Result<Retained<NSImage>, RequestError> {
+    let rgba_icon = icon
+        .cast_ref::<RgbaIcon>()
+        .ok_or(NotSupportedError::new("Only RGBA icons can be converted to `NSImage`"))?;
+
+    let width = rgba_icon.width();
+    let height = rgba_icon.height();
+
+    let bitmap = unsafe {
+        NSBitmapImageRep::initWithBitmapDataPlanes_pixelsWide_pixelsHigh_bitsPerSample_samplesPerPixel_hasAlpha_isPlanar_colorSpaceName_bytesPerRow_bitsPerPixel(
+            NSBitmapImageRep::alloc(),
+            std::ptr::null_mut::<*mut c_uchar>(),
+            width as isize,
+            height as isize,
+            8,
+            4,
+            true,
+            false,
+            NSDeviceRGBColorSpace,
+            width as isize * 4,
+            32,
+        )
+    }.ok_or_else(|| os_error!("Initializing the `NSBitmapImageRep` failed"))?;
+
+    let bitmap_data =
+        unsafe { slice::from_raw_parts_mut(bitmap.bitmapData(), rgba_icon.buffer().len()) };
+    bitmap_data.copy_from_slice(rgba_icon.buffer());
+
+    let image = NSImage::initWithSize(NSImage::alloc(), NSSize::new(width.into(), height.into()));
+    image.addRepresentation(&bitmap);
+
+    Ok(image)
+}
+
 pub(crate) fn cursor_from_image(cursor: &CursorImage) -> Result<Retained<NSCursor>, RequestError> {
     let width = cursor.width();
     let height = cursor.height();

winit-appkit/src/dnd.rs

@@ -0,0 +1,499 @@
+use std::cell::RefCell;
+use std::collections::HashMap;
+use std::io;
+use std::ops::{BitOr, ControlFlow};
+use std::sync::{Arc, OnceLock};
+
+use dispatch2::MainThreadBound;
+use objc2::rc::{Retained, Weak};
+use objc2::runtime::AnyObject;
+use objc2::{AnyThread, DefinedClass as _, MainThreadMarker, Message, define_class, msg_send};
+use objc2_app_kit::{
+    NSDragOperation, NSPasteboard, NSPasteboardType, NSPasteboardTypeFileURL, NSPasteboardTypeHTML,
+    NSPasteboardTypePNG, NSPasteboardTypeSound, NSPasteboardTypeString, NSPasteboardTypeTIFF,
+    NSPasteboardWriting, NSPasteboardWritingOptions,
+};
+use objc2_foundation::{NSArray, NSData, NSObject, NSObjectProtocol, NSString};
+use winit_core::data_transfer::{
+    DataTransfer, DataTransferId, DataTransferSend, SendData, TransferType, TypeHint, TypedData,
+};
+use winit_core::event_loop::DndAction;
+use winit_core::window::WindowId;
+
+/// A thin wrapper around [`NSPasteboardType`], implementing [`TransferType`].
+#[derive(PartialEq, Eq, Debug, Clone)]
+pub struct PasteboardType {
+    hint: Option<TypeHint>,
+    // We need to convert `NSString` to `str` since `NSString` isn't `Send`/`Sync`
+    inner: Arc<str>,
+}
+
+impl PasteboardType {
+    fn from_hint(hint: TypeHint) -> Option<Self> {
+        let hint_to_pasteboard_type = unsafe {
+            [
+                (TypeHint::UriList, NSPasteboardTypeFileURL),
+                (TypeHint::Plaintext, NSPasteboardTypeString),
+                (TypeHint::Html, NSPasteboardTypeHTML),
+                (TypeHint::Image { extension_hint: Some("png") }, NSPasteboardTypePNG),
+                (TypeHint::Image { extension_hint: Some("tiff") }, NSPasteboardTypeTIFF),
+                (TypeHint::Audio { extension_hint: None }, NSPasteboardTypeSound),
+            ]
+        };
+
+        hint_to_pasteboard_type.into_iter().find_map(|(haystack, inner)| {
+            (haystack.matches(&hint))
+                .then(|| Self { hint: Some(hint), inner: inner.to_string().into() })
+        })
+    }
+}
+
+impl From<Retained<NSPasteboardType>> for PasteboardType {
+    fn from(value: Retained<NSPasteboardType>) -> Self {
+        let pasteboard_type_to_hint = unsafe {
+            [
+                // Just in case the source application uses the deprecated method, we handle it
+                // here
+                #[expect(deprecated)]
+                (objc2_app_kit::NSFilenamesPboardType, TypeHint::UriList),
+                (NSPasteboardTypeFileURL, TypeHint::UriList),
+                (NSPasteboardTypeString, TypeHint::Plaintext),
+                (NSPasteboardTypeHTML, TypeHint::Html),
+                (NSPasteboardTypePNG, TypeHint::Image { extension_hint: Some("png") }),
+                (NSPasteboardTypeTIFF, TypeHint::Image { extension_hint: Some("tiff") }),
+                (NSPasteboardTypeSound, TypeHint::Audio { extension_hint: None }),
+            ]
+        };
+
+        let hint = pasteboard_type_to_hint
+            .iter()
+            .find_map(|(pb_type, hint)| (**pb_type == *value).then_some(hint));
+
+        Self { hint: hint.copied(), inner: value.to_string().into() }
+    }
+}
+
+impl TransferType for PasteboardType {
+    fn hint(&self) -> Option<winit_core::data_transfer::TypeHint> {
+        self.hint
+    }
+
+    fn matches(&self, other: &dyn TransferType) -> bool {
+        if let Some(other_pb_type) = other.cast_ref::<Self>() {
+            *self == *other_pb_type
+        } else {
+            // If either hint is `None`, return false
+            self.hint().is_some_and(|hint| other.hint() == Some(hint))
+        }
+    }
+}
+
+/// A thin wrapper around [`NSPasteboard`], implementing [`DataTransfer`].
+#[derive(Debug)]
+pub struct Pasteboard {
+    transfer_id: DataTransferId,
+    ns_pasteboard: MainThreadBound<Retained<NSPasteboard>>,
+    types: OnceLock<Arc<[PasteboardType]>>,
+}
+
+impl Clone for Pasteboard {
+    fn clone(&self) -> Self {
+        let inner = self.ns_pasteboard.get_on_main(|inner| {
+            MainThreadBound::new(inner.clone(), MainThreadMarker::new().unwrap())
+        });
+
+        Self { transfer_id: self.transfer_id, ns_pasteboard: inner, types: self.types.clone() }
+    }
+}
+
+impl Pasteboard {
+    fn new(
+        transfer_id: DataTransferId,
+        ns_pasteboard: MainThreadBound<Retained<NSPasteboard>>,
+    ) -> Self {
+        Self { transfer_id, ns_pasteboard, types: Default::default() }
+    }
+
+    /// Get the array of [`PasteboardType`]s advertized by this [`Pasteboard`].
+    pub fn types(&self) -> &[PasteboardType] {
+        self.types.get_or_init(|| {
+            self.ns_pasteboard.get_on_main(|pb| {
+                pb.types()
+                    .map(|types| types.into_iter().map(PasteboardType::from).collect::<Vec<_>>())
+                    .unwrap_or_default()
+                    .into()
+            })
+        })
+    }
+
+    /// Get the `DataTransferId` of this pasteboard.
+    pub fn id(&self) -> DataTransferId {
+        self.transfer_id
+    }
+
+    /// Get a typed reader for this pasteboard. This is only necessary in the cross-platform case,
+    /// as a user downcasting to the platform-specific type can just access the `NSPasteboard`
+    /// directly.
+    pub(crate) fn with_type(&self, type_: PasteboardTypeSpec) -> PasteboardValue {
+        PasteboardValue { type_, pasteboard: self.clone() }
+    }
+}
+
+impl DataTransfer for Pasteboard {
+    fn for_each_available_type<'this>(
+        &'this self,
+        func: &'_ mut dyn FnMut(&'this dyn TransferType) -> std::ops::ControlFlow<()>,
+    ) {
+        let _ = self.types().iter().map(|mime| mime as &dyn TransferType).try_for_each(func);
+    }
+}
+
+#[derive(Debug, Clone)]
+pub(crate) enum PasteboardTypeSpec {
+    PasteboardType(PasteboardType),
+    TypeHint(TypeHint),
+}
+
+impl PasteboardTypeSpec {
+    pub(crate) fn from_dyn(type_: &dyn TransferType) -> Option<Self> {
+        match type_.cast_ref::<PasteboardType>() {
+            Some(pb_type) => Some(Self::PasteboardType(pb_type.clone())),
+            None => type_.hint().map(Into::into),
+        }
+    }
+}
+
+impl From<TypeHint> for PasteboardTypeSpec {
+    fn from(value: TypeHint) -> Self {
+        match PasteboardType::from_hint(value) {
+            Some(pb_type) => Self::PasteboardType(pb_type),
+            None => Self::TypeHint(value),
+        }
+    }
+}
+
+impl PasteboardTypeSpec {
+    fn pasteboard_type(&self) -> Option<&PasteboardType> {
+        match self {
+            PasteboardTypeSpec::PasteboardType(pasteboard_type) => Some(pasteboard_type),
+            PasteboardTypeSpec::TypeHint(_) => None,
+        }
+    }
+}
+
+pub fn dnd_action_to_ns_drag_operation(value: DndAction) -> NSDragOperation {
+    match value {
+        DndAction::Copy => NSDragOperation::Copy,
+        DndAction::Move => NSDragOperation::Move,
+        DndAction::Link => NSDragOperation::Link,
+        DndAction::Private => NSDragOperation::Private,
+        _ => NSDragOperation::empty(),
+    }
+}
+
+pub fn ns_drag_operation_to_dnd_action(value: NSDragOperation) -> Option<DndAction> {
+    [
+        (NSDragOperation::Copy, DndAction::Copy),
+        (NSDragOperation::Move, DndAction::Move),
+        (NSDragOperation::Link, DndAction::Link),
+        (NSDragOperation::Private, DndAction::Private),
+        // Sometimes the OS returns `Generic`, in which case we just fall back to `Copy`.
+        (NSDragOperation::Generic, DndAction::Copy),
+    ]
+    .into_iter()
+    .find_map(|(appkit, winit)| value.contains(appkit).then_some(winit))
+}
+
+pub fn dnd_actions_to_ns_drag_operation(value: &[DndAction]) -> NSDragOperation {
+    value
+        .iter()
+        .copied()
+        .map(dnd_action_to_ns_drag_operation)
+        .fold(NSDragOperation::empty(), BitOr::bitor)
+}
+
+pub fn preferred_drag_operation(
+    value: NSDragOperation,
+    preference: &[DndAction],
+) -> Option<DndAction> {
+    preference
+        .iter()
+        .find(|action| value.intersects(dnd_action_to_ns_drag_operation(**action)))
+        .copied()
+}
+
+/// A thin wrapper around [`NSPasteboard`], implementing [`TypedData`].
+#[derive(Debug)]
+pub struct PasteboardValue {
+    // The concept of "top-level" types for a pasteboard doesn't always make sense on macOS due to
+    // the use of `pasteboardItems`, so we allow using `TypeHint` instead to preserve the user's
+    // intention.
+    type_: PasteboardTypeSpec,
+    pasteboard: Pasteboard,
+}
+
+impl TypedData for PasteboardValue {
+    fn type_(&self) -> &dyn TransferType {
+        match &self.type_ {
+            PasteboardTypeSpec::PasteboardType(pasteboard_type) => {
+                pasteboard_type as &dyn TransferType
+            },
+            PasteboardTypeSpec::TypeHint(type_hint) => type_hint,
+        }
+    }
+
+    fn try_read(&self) -> Option<Box<dyn io::BufRead>> {
+        self.try_as_bytes()
+            .ok()
+            .map(|bytes| Box::new(io::Cursor::new(bytes)) as Box<dyn io::BufRead>)
+    }
+
+    fn try_as_bytes(&self) -> io::Result<Vec<u8>> {
+        let type_ = self.type_.clone();
+        self.pasteboard
+            .ns_pasteboard
+            .get_on_main(|pasteboard| {
+                let bytes =
+                    pasteboard.dataForType(&NSString::from_str(&type_.pasteboard_type()?.inner))?;
+                Some(bytes.to_vec())
+            })
+            .ok_or_else(|| {
+                io::Error::other(format!(
+                    "NSPasteboard doesn't advertise a binary representation for type {:?}",
+                    self.type_
+                ))
+            })
+    }
+
+    fn try_as_uris(&self) -> io::Result<Vec<String>> {
+        // TODO: We should probably use `readObjects`, need to check how that works.
+        if self.type_().hint() != Some(TypeHint::UriList) {
+            return Err(io::ErrorKind::InvalidData.into());
+        }
+
+        self.pasteboard.ns_pasteboard.get_on_main(|pasteboard| {
+            let Some(items) = pasteboard.pasteboardItems() else {
+                // The pasteboard didn't expose any items, so we try with the deprecated method.
+                #[expect(deprecated)]
+                let property_list = match pasteboard
+                    .propertyListForType(unsafe { objc2_app_kit::NSFilenamesPboardType })
+                {
+                    Some(property_list) => property_list,
+                    None => {
+                        return pasteboard
+                            .stringForType(unsafe { NSPasteboardTypeFileURL })
+                            .map(|ns_str| vec![ns_str.to_string()])
+                            .ok_or_else(|| io::ErrorKind::InvalidData.into());
+                    },
+                };
+
+                let paths = property_list
+                    .downcast::<NSArray>()
+                    .unwrap()
+                    .into_iter()
+                    .map(|file| file.downcast::<NSString>().unwrap().to_string())
+                    .collect();
+
+                return Ok(paths);
+            };
+
+            Ok(items
+                .into_iter()
+                .filter_map(|item| item.stringForType(unsafe { NSPasteboardTypeFileURL }))
+                .map(|ns_str| ns_str.to_string())
+                .collect())
+        })
+    }
+
+    fn try_as_string(&self) -> io::Result<String> {
+        let type_ = self.type_.clone();
+
+        self.pasteboard.ns_pasteboard.get_on_main(|pasteboard| {
+            pasteboard
+                .stringForType(&NSString::from_str(
+                    &type_.pasteboard_type().ok_or(io::ErrorKind::InvalidData)?.inner,
+                ))
+                .map(|ns_str| ns_str.to_string())
+                .ok_or_else(|| io::ErrorKind::InvalidData.into())
+        })
+    }
+}
+
+#[derive(Debug)]
+struct ActivePasteboard {
+    window_id: WindowId,
+    pb: MainThreadBound<Weak<NSPasteboard>>,
+}
+
+#[derive(Debug, Default)]
+pub struct Pasteboards {
+    inner: RefCell<HashMap<DataTransferId, ActivePasteboard>>,
+}
+
+impl Pasteboards {
+    pub fn remove_deloaded_pasteboards(&self) {
+        self.inner.borrow_mut().retain(|_, ActivePasteboard { pb, .. }| {
+            pb.get_on_main(|state| state.load().is_some())
+        });
+    }
+
+    /// If the data transfer exists, update the pasteboard it points to.
+    pub fn set_pasteboard(
+        &self,
+        id: DataTransferId,
+        new_pb: &MainThreadBound<Retained<NSPasteboard>>,
+    ) {
+        let mut inner = self.inner.borrow_mut();
+        if let Some(ActivePasteboard { pb, .. }) = inner.get_mut(&id) {
+            *pb = new_pb.get_on_main(|pb| {
+                MainThreadBound::new(Weak::from_retained(pb), MainThreadMarker::new().unwrap())
+            });
+        }
+    }
+
+    pub fn insert(
+        &self,
+        transfer_id: DataTransferId,
+        pb: &MainThreadBound<Retained<NSPasteboard>>,
+        window_id: WindowId,
+    ) {
+        let mut inner = self.inner.borrow_mut();
+        let transfer = inner.entry(transfer_id).or_insert_with(move || {
+            pb.get_on_main(move |pb| ActivePasteboard {
+                window_id,
+                pb: MainThreadBound::new(Weak::from_retained(pb), MainThreadMarker::new().unwrap()),
+            })
+        });
+
+        transfer.window_id = window_id;
+    }
+
+    pub fn get(&self, id: DataTransferId) -> Option<Pasteboard> {
+        self.inner.borrow().get(&id).and_then(|ActivePasteboard { pb, .. }| {
+            pb.get_on_main(|state| {
+                let pb = state.load()?;
+                let pb = MainThreadBound::new(pb, MainThreadMarker::new().unwrap());
+                Some(Pasteboard::new(id, pb))
+            })
+        })
+    }
+
+    /// Get the window ID that most-recently saw the provided data transfer.
+    pub fn window_id(&self, id: DataTransferId) -> Option<WindowId> {
+        self.inner.borrow().get(&id).map(|active_pasteboard| active_pasteboard.window_id)
+    }
+}
+
+pub(crate) struct PasteboardWriterState {
+    data: Box<dyn DataTransferSend>,
+    // The macOS drag-and-drop API has some confusing aspects when handling multi-drag. The best
+    // we can really do is have the first element contain all the cross-platform items, and
+    // any further items are file paths only.
+    uri: Option<Retained<NSString>>,
+    writable_types: Retained<NSArray<NSPasteboardType>>,
+}
+
+impl PasteboardWriter {
+    pub(crate) fn new(
+        value: Box<dyn DataTransferSend>,
+        uri: Option<Retained<NSString>>,
+    ) -> Retained<Self> {
+        let mut writable_types = Vec::<Retained<NSPasteboardType>>::new();
+        value.for_each_available_type(&mut |type_| {
+            let Some(spec) = PasteboardTypeSpec::from_dyn(type_) else {
+                return ControlFlow::Continue(());
+            };
+
+            let Some(pb_type) = spec.pasteboard_type() else {
+                return ControlFlow::Continue(());
+            };
+
+            writable_types.push(NSString::from_str(&pb_type.inner));
+
+            ControlFlow::Continue(())
+        });
+
+        let pb_writer = Self::alloc().set_ivars(PasteboardWriterState {
+            data: value,
+            uri,
+            writable_types: NSArray::from_retained_slice(&writable_types),
+        });
+
+        // Unsure if there's an easier way to do this, but this is how `WindowDelegate` does it.
+        unsafe { msg_send![super(pb_writer), init] }
+    }
+}
+
+impl PasteboardWriterState {
+    fn data_for_pasteboard_type(
+        &self,
+        pasteboard_type: &NSPasteboardType,
+    ) -> Option<Retained<AnyObject>> {
+        if pasteboard_type == unsafe { NSPasteboardTypeFileURL } {
+            if let Some(out) = self.uri.clone().map(Into::into) {
+                return Some(out);
+            }
+        }
+        let pb_type = PasteboardType::from(pasteboard_type.retain());
+
+        let mut out = None;
+
+        self.data.for_each_available_type(&mut |haystack| {
+            if haystack.matches(&pb_type) {
+                out = self.data.data_for_type(haystack);
+                ControlFlow::Break(())
+            } else {
+                ControlFlow::Continue(())
+            }
+        });
+
+        match out? {
+            // This should be handled separately
+            // TODO: Is there a better way to do this?
+            SendData::Uris(_) => None,
+            SendData::String(string) => Some(NSString::from_str(&string).into()),
+            SendData::Bytes(binary) => Some(NSData::from_vec(binary).into()),
+        }
+    }
+}
+
+define_class!(
+    #[unsafe(super(NSObject))]
+    #[thread_kind = AnyThread]
+    #[name = "WinitPasteboardWriter"]
+    #[ivars = PasteboardWriterState]
+    pub(crate) struct PasteboardWriter;
+
+    unsafe impl NSObjectProtocol for PasteboardWriter {}
+
+    unsafe impl NSPasteboardWriting for PasteboardWriter {
+        #[unsafe(method_id(writableTypesForPasteboard:))]
+        fn writable_types_for_pasteboard(
+            &self,
+            _: &NSPasteboard,
+        ) -> Retained<NSArray<NSPasteboardType>> {
+            let vars = self.ivars();
+            vars.writable_types.clone()
+        }
+
+        #[unsafe(method(writingOptionsForType:pasteboard:))]
+        fn writing_options_for_type(
+            &self,
+            type_: &NSPasteboardType,
+            pasteboard: &NSPasteboard,
+        ) -> NSPasteboardWritingOptions {
+            let _ = type_;
+            let _ = pasteboard;
+            NSPasteboardWritingOptions::empty()
+        }
+
+        #[unsafe(method_id(pasteboardPropertyListForType:))]
+        fn pasteboard_property_list_for_type(
+            &self,
+            type_: &NSPasteboardType,
+        ) -> Option<Retained<AnyObject>> {
+            let vars = self.ivars();
+            vars.data_for_pasteboard_type(type_)
+        }
+    }
+);

winit-appkit/src/event_loop.rs

@@ -1,30 +1,38 @@
+use std::fmt;
 use std::rc::Rc;
 use std::sync::Arc;
 use std::time::{Duration, Instant};
 
 use objc2::rc::{Retained, autoreleasepool};
 use objc2::runtime::ProtocolObject;
-use objc2::{MainThreadMarker, available};
+use objc2::{AnyThread, MainThreadMarker, available};
 use objc2_app_kit::{
     NSApplication, NSApplicationActivationPolicy, NSApplicationDidFinishLaunchingNotification,
-    NSApplicationWillTerminateNotification, NSWindow,
+    NSApplicationWillTerminateNotification, NSDraggingItem, NSWindow,
 };
-use objc2_core_foundation::{CFIndex, CFRunLoopActivity, kCFRunLoopCommonModes};
-use objc2_foundation::{NSNotificationCenter, NSObjectProtocol};
+use objc2_core_foundation::{
+    CFIndex, CFRunLoopActivity, CGPoint, CGRect, CGSize, kCFRunLoopCommonModes,
+};
+use objc2_foundation::{NSArray, NSNotificationCenter, NSObjectProtocol, NSString};
 use rwh_06::HasDisplayHandle;
 use tracing::debug_span;
 use winit_common::core_foundation::{MainRunLoop, MainRunLoopObserver, tracing_observers};
 use winit_common::foundation::create_observer;
 use winit_core::application::ApplicationHandler;
 use winit_core::cursor::{CustomCursor as CoreCustomCursor, CustomCursorSource};
+use winit_core::data_transfer::{
+    DataTransfer, DataTransferId, DataTransferSend, SendData, TransferType, TypeHint,
+};
 use winit_core::error::{EventLoopError, RequestError};
+use winit_core::event::WindowEvent;
 use winit_core::event_loop::pump_events::PumpStatus;
 use winit_core::event_loop::{
-    ActiveEventLoop as RootActiveEventLoop, ControlFlow, DeviceEvents,
-    EventLoopProxy as CoreEventLoopProxy, OwnedDisplayHandle as CoreOwnedDisplayHandle,
+    ActiveEventLoop as RootActiveEventLoop, AsyncRequestSerial, ControlFlow, DeviceEvents,
+    DndAction, DragIcon, EventLoopProxy as CoreEventLoopProxy,
+    OwnedDisplayHandle as CoreOwnedDisplayHandle,
 };
 use winit_core::monitor::MonitorHandle as CoreMonitorHandle;
-use winit_core::window::Theme;
+use winit_core::window::{Theme, WindowId};
 
 use super::app::override_send_event;
 use super::app_state::AppState;
@@ -32,6 +40,8 @@ use super::cursor::CustomCursor;
 use super::event::dummy_event;
 use super::monitor;
 use crate::ActivationPolicy;
+use crate::cursor::image_from_icon;
+use crate::dnd::{PasteboardTypeSpec, PasteboardWriter, dnd_actions_to_ns_drag_operation};
 use crate::window::Window;
 
 #[derive(Debug)]
@@ -126,8 +136,183 @@ impl RootActiveEventLoop for ActiveEventLoop {
     fn rwh_06_handle(&self) -> &dyn rwh_06::HasDisplayHandle {
         self
     }
+
+    fn fetch_data_transfer(
+        &self,
+        id: DataTransferId,
+        type_: &dyn TransferType,
+    ) -> Result<AsyncRequestSerial, RequestError> {
+        let Some(pb) = self.app_state.pasteboards().get(id) else {
+            return Err(RequestError::Ignored);
+        };
+        let Some(window_id) = self.app_state.pasteboards().window_id(id) else {
+            return Err(RequestError::Ignored);
+        };
+
+        let serial = AsyncRequestSerial::get();
+
+        let Some(type_) = PasteboardTypeSpec::from_dyn(type_) else {
+            return Err(os_error!(format!("Pasteboard does not contain type {type_:?}")).into());
+        };
+
+        let data = Arc::new(pb.with_type(type_));
+
+        self.app_state.maybe_queue_with_handler(move |app, event_loop| {
+            app.window_event(event_loop, window_id, WindowEvent::DataTransferReceived {
+                id,
+                serial,
+                value: data,
+            });
+        });
+
+        Ok(serial)
     }
 
+    fn data_transfer(&self, id: DataTransferId) -> Result<Box<dyn DataTransfer>, RequestError> {
+        let Some(pb) = self.app_state.pasteboards().get(id) else {
+            return Err(RequestError::Ignored);
+        };
+
+        Ok(Box::new(pb))
+    }
+
+    fn set_valid_dnd_actions(
+        &self,
+        id: DataTransferId,
+        actions: &[DndAction],
+    ) -> Result<(), RequestError> {
+        let mut state = self.app_state.drag_state().borrow_mut();
+        let Some(drag_state) = &mut *state else {
+            return Err(os_error!(UnknownDataTransfer(id)).into());
+        };
+
+        if drag_state.id != id {
+            return Err(os_error!(UnknownDataTransfer(id)).into());
+        }
+
+        drag_state.valid_actions.clear();
+        drag_state.valid_actions.extend_from_slice(actions);
+
+        Ok(())
+    }
+
+    fn start_drag(
+        &self,
+        source: WindowId,
+        send_data: Box<dyn DataTransferSend>,
+        actions: &[DndAction],
+        icon: Option<DragIcon>,
+    ) -> Result<DataTransferId, RequestError> {
+        let drag_operation = dnd_actions_to_ns_drag_operation(actions);
+
+        self.app_state
+            .with_window_delegate_on_main(source, move |delegate| {
+                let (dragging_rect_offset_x, dragging_rect_offset_y) =
+                    icon.as_ref().map(|icon| (icon.offset_x, icon.offset_y)).unwrap_or_default();
+                let drag_image = icon.and_then(|icon| image_from_icon(&icon.icon).ok());
+
+                let Some(event) = delegate.window().currentEvent() else {
+                    return Err(RequestError::Ignored);
+                };
+
+                let dragging_rect_size = drag_image
+                    .as_ref()
+                    .map(|img| img.size())
+                    // Seemingly we need some kind of dragging rectangle even if no icon is
+                    // supplied.
+                    .unwrap_or(CGSize::new(16., 16.));
+
+                let event_location = event.locationInWindow();
+                let dragging_rect_location = CGPoint::new(
+                    event_location.x + dragging_rect_offset_x as f64,
+                    // Convert generic coordinates (y=0 is top of image) to AppKit coordinates (y=0
+                    // is bottom of image)
+                    event_location.y - dragging_rect_size.height - dragging_rect_offset_y as f64,
+                );
+                let dragging_rect = CGRect::new(dragging_rect_location, dragging_rect_size);
+
+                let mut uris = send_data
+                    .data_for_type(&TypeHint::UriList)
+                    .and_then(|file_uris| {
+                        // TODO: Might not be ideal to do this
+                        let ns_url_from_str = |str: String| NSString::from_str(&str);
+                        // Slightly complicated use of iterators in order to ensure that branches
+                        // have the same opaque type
+                        match file_uris {
+                            SendData::Uris(os_strings) => Some(
+                                None.into_iter().chain(os_strings.into_iter().map(ns_url_from_str)),
+                            ),
+                            SendData::String(string) => Some(
+                                Some(NSString::from_str(&string))
+                                    .into_iter()
+                                    .chain(Vec::new().into_iter().map(ns_url_from_str)),
+                            ),
+                            SendData::Bytes(_) => None,
+                        }
+                    })
+                    .into_iter()
+                    .flatten();
+
+                let first_uri = uris.next();
+
+                let mut pasteboard_items = uris
+                    .map(|ns_url| {
+                        let dragging_item = NSDraggingItem::initWithPasteboardWriter(
+                            NSDraggingItem::alloc(),
+                            ProtocolObject::from_ref(&*ns_url),
+                        );
+
+                        // No dragging frame/contents, icon only applies to the first item.
+
+                        dragging_item
+                    })
+                    .collect::<Vec<_>>();
+
+                let first_dragging_item = NSDraggingItem::initWithPasteboardWriter(
+                    NSDraggingItem::alloc(),
+                    ProtocolObject::from_ref(&*PasteboardWriter::new(send_data, first_uri)),
+                );
+
+                unsafe {
+                    first_dragging_item.setDraggingFrame_contents(
+                        dragging_rect,
+                        drag_image.as_ref().map(AsRef::as_ref),
+                    )
+                };
+
+                pasteboard_items.insert(0, first_dragging_item);
+
+                let pasteboard_items = NSArray::from_retained_slice(&pasteboard_items);
+
+                let session = delegate.window().beginDraggingSessionWithItems_event_source(
+                    &pasteboard_items,
+                    &event,
+                    ProtocolObject::from_ref(&*delegate),
+                );
+
+                let id = DataTransferId::from_raw(session.draggingSequenceNumber() as i64);
+
+                delegate.view().set_dragging_session(session, drag_operation);
+
+                Ok(id)
+            })
+            .ok_or(RequestError::Ignored)?
+    }
+}
+
+/// An operation was attempted on a data transfer ID, but that ID was invalid.
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+pub struct UnknownDataTransfer(pub DataTransferId);
+
+impl fmt::Display for UnknownDataTransfer {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let id = self.0.into_raw();
+        write!(f, "Unknown data transfer with ID {id}")
+    }
+}
+
+impl std::error::Error for UnknownDataTransfer {}
+
 impl rwh_06::HasDisplayHandle for ActiveEventLoop {
     fn display_handle(&self) -> Result<rwh_06::DisplayHandle<'_>, rwh_06::HandleError> {
         let raw = rwh_06::RawDisplayHandle::AppKit(rwh_06::AppKitDisplayHandle::new());

winit-appkit/src/lib.rs

@@ -71,6 +71,7 @@ mod util;
 mod app;
 mod app_state;
 mod cursor;
+mod dnd;
 mod event;
 mod event_loop;
 mod ffi;
@@ -91,6 +92,7 @@ use winit_core::event_loop::ActiveEventLoop;
 use winit_core::monitor::MonitorHandle;
 use winit_core::window::{PlatformWindowAttributes, Window};
 
+pub use self::dnd::{Pasteboard, PasteboardType, PasteboardValue};
 pub use self::event::{physicalkey_to_scancode, scancode_to_physicalkey};
 use self::event_loop::ActiveEventLoop as AppKitActiveEventLoop;
 pub use self::event_loop::{EventLoop, PlatformSpecificEventLoopAttributes};

winit-appkit/src/view.rs

@@ -8,8 +8,9 @@ use objc2::rc::Retained;
 use objc2::runtime::{AnyObject, Sel};
 use objc2::{AnyThread, DefinedClass, MainThreadMarker, define_class, msg_send};
 use objc2_app_kit::{
-    NSApplication, NSCursor, NSEvent, NSEventPhase, NSResponder, NSTextInputClient, NSTrackingArea,
-    NSTrackingAreaOptions, NSView, NSViewLayerContentsRedrawPolicy, NSWindow,
+    NSApplication, NSCursor, NSDragOperation, NSDraggingSession, NSEvent, NSEventPhase,
+    NSResponder, NSTextInputClient, NSTrackingArea, NSTrackingAreaOptions, NSView,
+    NSViewLayerContentsRedrawPolicy, NSWindow,
 };
 use objc2_core_foundation::CGRect;
 use objc2_foundation::{
@@ -114,6 +115,12 @@ pub struct ViewState {
     /// Strong reference to the global application state.
     app_state: Rc<AppState>,
 
+    /// This is for a dragging session that we initiated
+    dragging_session: RefCell<Option<Retained<NSDraggingSession>>>,
+
+    /// This is for a dragging session that we initiated
+    drag_operations: Cell<NSDragOperation>,
+
     cursor_state: RefCell<CursorState>,
     ime_position: Cell<NSPoint>,
     ime_size: Cell<NSSize>,
@@ -789,6 +796,8 @@ impl WinitView {
     ) -> Retained<Self> {
         let this = mtm.alloc().set_ivars(ViewState {
             app_state: Rc::clone(app_state),
+            dragging_session: Default::default(),
+            drag_operations: Cell::new(NSDragOperation::empty()),
             cursor_state: Default::default(),
             ime_position: Default::default(),
             ime_size: Default::default(),
@@ -898,7 +907,7 @@ impl WinitView {
         }
     }
 
-    fn scale_factor(&self) -> f64 {
+    pub(crate) fn scale_factor(&self) -> f64 {
         self.window().backingScaleFactor() as f64
     }
 
@@ -1117,6 +1126,29 @@ impl WinitView {
         self.queue_event(WindowEvent::ModifiersChanged(self.ivars().modifiers.get()));
     }
 
+    pub(crate) fn set_dragging_session(
+        &self,
+        drag: Retained<NSDraggingSession>,
+        action_mask: NSDragOperation,
+    ) {
+        let vars = self.ivars();
+        vars.dragging_session.replace(Some(drag));
+        vars.drag_operations.set(action_mask);
+    }
+
+    pub(crate) fn drag_operations(&self) -> NSDragOperation {
+        self.ivars().drag_operations.get()
+    }
+
+    pub(crate) fn clear_dragging_session(&self, drag: &NSDraggingSession) -> bool {
+        let vars = self.ivars();
+        vars.drag_operations.set(NSDragOperation::empty());
+        let mut dragging_session = vars.dragging_session.borrow_mut();
+        dragging_session
+            .take_if(|session| session.draggingSequenceNumber() == drag.draggingSequenceNumber())
+            .is_some()
+    }
+
     fn mouse_click(&self, event: &NSEvent, button_state: ElementState) {
         let position = self.mouse_view_point(event).to_physical(self.scale_factor());
         let button = mouse_button(event);

winit-appkit/src/window.rs

@@ -36,6 +36,7 @@ impl Window {
         let mtm = window_target.mtm;
         let delegate =
             autoreleasepool(|_| WindowDelegate::new(&window_target.app_state, attributes, mtm))?;
+        window_target.app_state.register_window(&delegate, mtm);
         Ok(Window {
             window: MainThreadBound::new(delegate.window().retain(), mtm),
             delegate: MainThreadBound::new(delegate, mtm),

winit-appkit/src/window_delegate.rs

@@ -6,6 +6,7 @@ use std::ptr;
 use std::rc::Rc;
 use std::sync::{Arc, Mutex};
 
+use dispatch2::MainThreadBound;
 use dpi::{
     LogicalInsets, LogicalPosition, LogicalSize, PhysicalInsets, PhysicalPosition, PhysicalSize,
     Position, Size,
@@ -19,14 +20,14 @@ use objc2::{
 use objc2_app_kit::{
     NSAppKitVersionNumber, NSAppKitVersionNumber10_12, NSAppearance, NSAppearanceCustomization,
     NSAppearanceNameAqua, NSApplication, NSApplicationPresentationOptions, NSBackingStoreType,
-    NSColor, NSDraggingDestination, NSDraggingInfo, NSRequestUserAttentionType, NSScreen,
-    NSToolbar, NSView, NSViewFrameDidChangeNotification, NSWindow, NSWindowButton,
-    NSWindowDelegate, NSWindowLevel, NSWindowOcclusionState, NSWindowOrderingMode,
-    NSWindowSharingType, NSWindowStyleMask, NSWindowTabbingMode, NSWindowTitleVisibility,
-    NSWindowToolbarStyle,
+    NSColor, NSDragOperation, NSDraggingContext, NSDraggingDestination, NSDraggingInfo,
+    NSDraggingSession, NSDraggingSource, NSPasteboardTypeFileURL, NSPasteboardTypeHTML,
+    NSPasteboardTypePNG, NSPasteboardTypeSound, NSPasteboardTypeString, NSPasteboardTypeTIFF,
+    NSRequestUserAttentionType, NSScreen, NSToolbar, NSView, NSViewFrameDidChangeNotification,
+    NSWindow, NSWindowButton, NSWindowDelegate, NSWindowLevel, NSWindowOcclusionState,
+    NSWindowOrderingMode, NSWindowSharingType, NSWindowStyleMask, NSWindowTabbingMode,
+    NSWindowTitleVisibility, NSWindowToolbarStyle,
 };
-#[allow(deprecated)]
-use objc2_app_kit::{NSFilenamesPboardType, NSWindowFullScreenButton};
 use objc2_core_foundation::{CGFloat, CGPoint};
 use objc2_core_graphics::{
     CGAcquireDisplayFadeReservation, CGAssociateMouseAndMouseCursorPosition, CGDisplayCapture,
@@ -44,6 +45,7 @@ use objc2_foundation::{
 use tracing::{debug_span, trace, warn};
 use winit_common::core_foundation::MainRunLoop;
 use winit_core::cursor::Cursor;
+use winit_core::data_transfer::DataTransferId;
 use winit_core::error::{NotSupportedError, RequestError};
 use winit_core::event::{SurfaceSizeWriter, WindowEvent};
 use winit_core::icon::Icon;
@@ -60,6 +62,10 @@ use super::monitor::{self, MonitorHandle, flip_window_screen_coordinates, get_di
 use super::util::cgerr;
 use super::view::WinitView;
 use super::window::{WinitPanel, WinitWindow, window_id};
+use crate::app_state::DragState;
+use crate::dnd::{
+    dnd_action_to_ns_drag_operation, ns_drag_operation_to_dnd_action, preferred_drag_operation,
+};
 use crate::{OptionAsAlt, WindowAttributesMacOS, WindowExtMacOS};
 
 #[derive(Debug)]
@@ -358,37 +364,78 @@ define_class!(
         }
     }
 
+    unsafe impl NSDraggingSource for WindowDelegate {
+        #[unsafe(method(draggingSession:sourceOperationMaskForDraggingContext:))]
+        fn dragging_session_source_operation_mask(
+            &self,
+            _: &NSDraggingSession,
+            _: NSDraggingContext,
+        ) -> NSDragOperation {
+            self.view().drag_operations()
+        }
+
+        #[unsafe(method(draggingSession:endedAtPoint:operation:))]
+        fn dragging_session_ended_at_point(
+            &self,
+            session: &NSDraggingSession,
+            _: NSPoint,
+            operation: NSDragOperation,
+        ) {
+            let id = DataTransferId::from_raw(session.draggingSequenceNumber() as i64);
+            if operation == NSDragOperation::None {
+                self.queue_event(WindowEvent::OutgoingDragCanceled { id });
+            } else {
+                self.queue_event(WindowEvent::OutgoingDragDropped {
+                    id,
+                    action: ns_drag_operation_to_dnd_action(operation),
+                });
+            }
+
+            self.view().clear_dragging_session(session);
+        }
+    }
+
     unsafe impl NSDraggingDestination for WindowDelegate {
         /// Invoked when the dragged image enters destination bounds or frame
         #[unsafe(method(draggingEntered:))]
-        fn dragging_entered(&self, sender: &ProtocolObject<dyn NSDraggingInfo>) -> bool {
+        fn dragging_entered(&self, sender: &ProtocolObject<dyn NSDraggingInfo>) -> NSDragOperation {
             let _entered = debug_span!("draggingEntered:").entered();
 
-            use std::path::PathBuf;
-
-            let pb = sender.draggingPasteboard();
-
-            #[allow(deprecated)]
-            let property_list = match pb.propertyListForType(unsafe { NSFilenamesPboardType }) {
-                Some(property_list) => property_list,
-                None => return false.into(),
-            };
-
-            let paths = property_list
-                .downcast::<NSArray>()
-                .unwrap()
-                .into_iter()
-                .map(|file| PathBuf::from(file.downcast::<NSString>().unwrap().to_string()))
-                .collect();
+            let pb =
+                MainThreadBound::new(sender.draggingPasteboard(), MainThreadMarker::new().unwrap());
 
             let dl = sender.draggingLocation();
             let dl = self.view().convertPoint_fromView(dl, None);
             let position =
                 LogicalPosition::<f64>::from((dl.x, dl.y)).to_physical(self.scale_factor());
 
-            self.queue_event(WindowEvent::DragEntered { paths, position });
+            let window_id = self.id();
 
-            true
+            let vars = self.ivars();
+
+            let source_operations = sender.draggingSourceOperationMask();
+
+            let transfer_id = DataTransferId::from_raw(sender.draggingSequenceNumber() as i64);
+            vars.app_state.pasteboards().insert(transfer_id, &pb, window_id);
+
+            vars.app_state
+                .drag_state()
+                .replace(Some(DragState { id: transfer_id, valid_actions: vec![] }));
+
+            self.queue_event(WindowEvent::DragEntered {
+                id: transfer_id,
+                position: Some(position),
+            });
+
+            let drag_state = vars.app_state.drag_state().borrow();
+
+            drag_state
+                .as_ref()
+                .and_then(|drag_state| {
+                    preferred_drag_operation(source_operations, &drag_state.valid_actions)
+                })
+                .map(dnd_action_to_ns_drag_operation)
+                .unwrap_or(NSDragOperation::empty())
         }
 
         #[unsafe(method(wantsPeriodicDraggingUpdates))]
@@ -400,17 +447,46 @@ define_class!(
         /// Invoked periodically as the image is held within the destination area, allowing
         /// modification of the dragging operation or mouse-pointer position.
         #[unsafe(method(draggingUpdated:))]
-        fn dragging_updated(&self, sender: &ProtocolObject<dyn NSDraggingInfo>) -> bool {
+        fn dragging_updated(&self, sender: &ProtocolObject<dyn NSDraggingInfo>) -> NSDragOperation {
             let _entered = debug_span!("draggingUpdated:").entered();
 
+            let vars = self.ivars();
+
+            let Some(transfer_id) =
+                vars.app_state.drag_state().borrow().as_ref().map(|state| state.id)
+            else {
+                return NSDragOperation::empty();
+            };
+
+            let pb =
+                MainThreadBound::new(sender.draggingPasteboard(), MainThreadMarker::new().unwrap());
+
+            let source_operations = sender.draggingSourceOperationMask();
+
+            vars.app_state.pasteboards().set_pasteboard(transfer_id, &pb);
+
             let dl = sender.draggingLocation();
             let dl = self.view().convertPoint_fromView(dl, None);
             let position =
                 LogicalPosition::<f64>::from((dl.x, dl.y)).to_physical(self.scale_factor());
 
-            self.queue_event(WindowEvent::DragMoved { position });
+            let proposed_action = vars.app_state.proposed_drag_action(source_operations);
 
-            true
+            self.queue_event(WindowEvent::DragPosition {
+                id: transfer_id,
+                position,
+                proposed_action,
+            });
+
+            let drag_state = vars.app_state.drag_state().borrow();
+
+            drag_state
+                .as_ref()
+                .and_then(|drag_state| {
+                    preferred_drag_operation(source_operations, &drag_state.valid_actions)
+                })
+                .map(dnd_action_to_ns_drag_operation)
+                .unwrap_or(NSDragOperation::empty())
         }
 
         /// Invoked when the image is released
@@ -425,30 +501,43 @@ define_class!(
         fn perform_drag_operation(&self, sender: &ProtocolObject<dyn NSDraggingInfo>) -> bool {
             let _entered = debug_span!("performDragOperation:").entered();
 
-            use std::path::PathBuf;
+            let vars = self.ivars();
 
-            let pb = sender.draggingPasteboard();
-
-            #[allow(deprecated)]
-            let property_list = match pb.propertyListForType(unsafe { NSFilenamesPboardType }) {
-                Some(property_list) => property_list,
-                None => return false.into(),
+            let Some(transfer_id) =
+                vars.app_state.drag_state().borrow().as_ref().map(|state| state.id)
+            else {
+                return false.into();
             };
 
-            let paths = property_list
-                .downcast::<NSArray>()
-                .unwrap()
-                .into_iter()
-                .map(|file| PathBuf::from(file.downcast::<NSString>().unwrap().to_string()))
-                .collect();
+            let pb =
+                MainThreadBound::new(sender.draggingPasteboard(), MainThreadMarker::new().unwrap());
+
+            let source_operations = sender.draggingSourceOperationMask();
+            // let operations = ns_drag_operation_to_dnd_actions(source_operations);
+
+            vars.app_state.pasteboards().set_pasteboard(transfer_id, &pb);
 
             let dl = sender.draggingLocation();
             let dl = self.view().convertPoint_fromView(dl, None);
             let position =
                 LogicalPosition::<f64>::from((dl.x, dl.y)).to_physical(self.scale_factor());
 
-            self.queue_event(WindowEvent::DragDropped { paths, position });
+            let proposed_action = vars.app_state.proposed_drag_action(source_operations);
 
+            self.queue_event(WindowEvent::DragPosition {
+                id: transfer_id,
+                position,
+                proposed_action,
+            });
+
+            // Check again, in case the application updated
+            let proposed_action = vars.app_state.proposed_drag_action(source_operations);
+
+            self.queue_event(WindowEvent::DragDropped { id: transfer_id, proposed_action });
+
+            // We assume that if the OS has sent `perform_drag_operation`, that the drag succeeded.
+            // We may want to extend this API in the future to allow signalling that the final drop
+            // failed.
             true
         }
 
@@ -456,6 +545,10 @@ define_class!(
         #[unsafe(method(concludeDragOperation:))]
         fn conclude_drag_operation(&self, _sender: Option<&NSObject>) {
             let _entered = debug_span!("concludeDragOperation:").entered();
+            let vars = self.ivars();
+
+            vars.app_state.pasteboards().remove_deloaded_pasteboards();
+            vars.app_state.drag_state().take();
         }
 
         /// Invoked when the dragging operation is cancelled
@@ -463,13 +556,39 @@ define_class!(
         fn dragging_exited(&self, sender: Option<&ProtocolObject<dyn NSDraggingInfo>>) {
             let _entered = debug_span!("draggingExited:").entered();
 
-            let position = sender.map(|sender| {
+            let vars = self.ivars();
+
+            let Some(transfer_id) =
+                vars.app_state.drag_state().borrow().as_ref().map(|state| state.id)
+            else {
+                return;
+            };
+
+            if let Some(sender) = sender {
+                let pb = MainThreadBound::new(
+                    sender.draggingPasteboard(),
+                    MainThreadMarker::new().unwrap(),
+                );
+                vars.app_state.pasteboards().set_pasteboard(transfer_id, &pb);
+
                 let dl = sender.draggingLocation();
                 let dl = self.view().convertPoint_fromView(dl, None);
-                LogicalPosition::<f64>::from((dl.x, dl.y)).to_physical(self.scale_factor())
-            });
+                let position =
+                    LogicalPosition::<f64>::from((dl.x, dl.y)).to_physical(self.scale_factor());
 
-            self.queue_event(WindowEvent::DragLeft { position });
+                let source_operations = sender.draggingSourceOperationMask();
+                let proposed_action = vars.app_state.proposed_drag_action(source_operations);
+
+                self.queue_event(WindowEvent::DragPosition {
+                    id: transfer_id,
+                    position,
+                    proposed_action,
+                });
+            }
+
+            self.queue_event(WindowEvent::DragLeft { id: transfer_id });
+
+            vars.app_state.drag_state().take();
         }
     }
 
@@ -674,7 +793,7 @@ fn new_window(
         if macos_attrs.titlebar_buttons_hidden {
             for titlebar_button in &[
                 #[allow(deprecated)]
-                NSWindowFullScreenButton,
+                objc2_app_kit::NSWindowFullScreenButton,
                 NSWindowButton::MiniaturizeButton,
                 NSWindowButton::CloseButton,
                 NSWindowButton::ZoomButton,
@@ -756,10 +875,6 @@ fn new_window(
             window.setBackgroundColor(Some(&NSColor::clearColor()));
         }
 
-        // register for drag and drop operations.
-        #[allow(deprecated)]
-        window.registerForDraggedTypes(&NSArray::from_slice(&[unsafe { NSFilenamesPboardType }]));
-
         Some(window)
     })
 }
@@ -837,6 +952,22 @@ impl WindowDelegate {
 
         window.setDelegate(Some(ProtocolObject::from_ref(&*delegate)));
 
+        let drag_types = unsafe {
+            // Advertize support for the set of types which correspond to variants of `TypeHint`.
+            // If the user wants to support other pasteboard types which don't have a cross-platform
+            // equivalent, they can downcast the window and manually call `registerForDraggedTypes`
+            // themselves.
+            NSArray::from_slice(&[
+                NSPasteboardTypeFileURL,
+                NSPasteboardTypeHTML,
+                NSPasteboardTypePNG,
+                NSPasteboardTypeSound,
+                NSPasteboardTypeString,
+                NSPasteboardTypeTIFF,
+            ])
+        };
+        window.registerForDraggedTypes(&drag_types);
+
         // Listen for theme change event.
         //
         // SAFETY: The observer is un-registered in the `Drop` of the delegate.

winit-core/Cargo.toml

@@ -30,6 +30,7 @@ keyboard-types.workspace = true
 rwh_06.workspace = true
 serde = { workspace = true, optional = true }
 smol_str.workspace = true
+url.workspace = true
 
 # `wasm32-unknown-unknown` and `wasm32-none`, but not `wasm32-wasi`.
 [target.'cfg(all(target_family = "wasm", any(target_os = "unknown", target_os = "none")))'.dependencies]

winit-core/src/data_transfer.rs

@@ -0,0 +1,593 @@
+//! Cross-platform abstractions related to data transfer (i.e. clipboard and drag-and-drop).
+//!
+//! > **NOTE**: Interacting with the clipboard is currently not implemented in Winit, and
+//! > this API is only used for drag-and-drop.
+//!
+//! # Quickstart
+//!
+//! The API in this module is used for both sending and receiving data. The flow is detailed below,
+//! but to quickly get started, the relevant APIs are the following:
+//!
+//! ### Receiving a drag-and-drop operation
+//!
+//! - [`DragEntered`](crate::event::WindowEvent::DragEntered) - informs a window that a new drag
+//!   operation has started.
+//! - [`data_transfer`](crate::event_loop::ActiveEventLoop::data_transfer) - get metadata about the
+//!   incoming transfer.
+//! - [`DataTransfer`] - metadata about the incoming transfer, in particular the available types
+//! - [`set_valid_dnd_actions`](crate::event_loop::ActiveEventLoop::set_valid_dnd_actions) - the
+//!   application must set at least some actions as valid in order for the drag to be considered
+//!   accepted.
+//! - [`fetch_data_transfer`](crate::event_loop::ActiveEventLoop::fetch_data_transfer) - request the
+//!   actual data, with a specific type, from the data transfer.
+//! - [`DataTransferReceived`](crate::event::WindowEvent::DataTransferReceived) - the actual data,
+//!   with a specific type, has been received.
+//! - [`TypedData`] - provides methods to read the actual data
+//!
+//! ### Sending a drag-and-drop operation
+//!
+//! - [`DataTransferSend`] - the core trait which defines data to be sent
+//! - [`DataTransferSendBuilder`] - helper to create a new outgoing data transfer from a set of
+//!   types and callbacks that supply data of that type
+//! - [`ActiveEventLoop::start_drag`](crate::event_loop::ActiveEventLoop::start_drag) - the
+//!   application calls this to start a new drag operation
+//! - [`OutgoingDragDropped`](crate::event::WindowEvent::OutgoingDragDropped)/
+//!   [`OutgoingDragCanceled`](crate::event::WindowEvent::OutgoingDragCanceled) - the application
+//!   receives this when the user has ended the drag operation, by dropping the data or by canceling
+//!   the operation respectively
+//!
+//! # Detailed flow
+//!
+//! ## Receiving a drag-and-drop operation
+//!
+//! On all platforms, the process looks something like this:
+//!
+//! - A data transfer advertises a set of types which the data can be interpreted as. While the
+//!   precise implementation depends on platform, there's a set of types which can be safely
+//!   transferred between applications on all platforms (see [`TypeHint`]).
+//!   - For example, if you copy or drag text from a web page, the browser may advertise the text
+//!     formatted using HTML, the text formatted as RTF, and the text with all formatting removed
+//!     simultaneously.
+//! - An application receiving a data transfer chooses one or more types that it understands and
+//!   requests the data in those formats (in practice, it will usually only request a single
+//!   format).
+//! - The source application converts the data stored in its memory to the requested format and
+//!   asynchronously sends it to the target application
+//!
+//! On some platforms, the data is sometimes available synchronously, but all platforms have at
+//! least some method of sending the data asynchronously and some types of data that may _only_ be
+//! sent using the asynchronous interface. Because of this, the API in winit must be asynchronous.
+//!
+//! The flow for a user application that implements drag-and-drop would look something like this:
+//!
+//! - The application receives a [`DragEntered`](crate::event::WindowEvent::DragEntered) event. This
+//!   event supplies a [`DataTransferId`] which can be used to request information or operations on
+//!   the dragged data by using methods on [`Window`](crate::window::Window).
+//! - To make sure that the operating system displays the correct cursor, and that modifier keys
+//!   will change the selected drag action correctly, the application should call
+//!   [`set_valid_dnd_actions`](crate::event_loop::ActiveEventLoop::set_valid_dnd_actions). See
+//!   documentation on that method for details.
+//! - As the drag operation continues, the window will receive
+//!   [`DragPosition`](crate::event::WindowEvent::DragPosition) events.
+//! - At any point during this operation, the receiving application may request either the available
+//!   types or even the data being transferred. This may be useful in cases where the application
+//!   wants to preload the data. For example, an image editor may want to display the image on the
+//!   canvas during the drag operation.
+//! - When the user tries to drop the data onto the window, that window will receive either a
+//!   [`DragDropped`](crate::event::WindowEvent::DragDropped) or
+//!   [`DragLeft`](crate::event::WindowEvent::DragLeft) event if the drag operation was accepted or
+//!   rejected, respectively. See documentation for
+//!   [`set_valid_dnd_actions`](crate::event_loop::ActiveEventLoop::set_valid_dnd_actions) for
+//!   details on accepting/rejecting a drag.
+//!
+//! ## Sending a drag-and-drop operation
+//!
+//! As the source application cannot interact with the ongoing drag while it is in-flight, this flow
+//! is a lot simpler.
+//!
+//! - The application creates a [`DataTransferSend`] with a set of types and associated data. For
+//!   most cases, this can be done with [`DataTransferSendBuilder`].
+//! - The application passes this [`DataTransferSend`] to
+//!   [`ActiveEventLoop::start_drag`](crate::event_loop::ActiveEventLoop::start_drag)`. This is also
+//!   where metadata is set, such as the icon that will be shown during the drag operation.
+//! - When the drag operation completes, the application receives
+//!   [`OutgoingDragDropped`](crate::event::WindowEvent::OutgoingDragDropped) with the resultant
+//!   action, or [`OutgoingDragCanceled`](crate::event::WindowEvent::OutgoingDragCanceled), and
+//!   handles it appropriately. For example, if the drag was successful and the operation is
+//!   [`DndAction::Move`](crate::event_loop::DndAction::Move), then the application would delete the
+//!   source object, since the data has now been transferred somewhere else.
+
+#![warn(missing_docs)]
+
+use std::ops::ControlFlow;
+use std::path::{Path, PathBuf};
+use std::{fmt, io};
+
+use crate::as_any::AsAny;
+
+/// Unique identifier for a data transfer.
+#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct DataTransferId(i64);
+
+impl DataTransferId {
+    /// Convert the [`DataTransferId`] into the underlying integer.
+    ///
+    /// This is useful if you need to pass the ID across an FFI boundary, or store it in an atomic.
+    pub const fn into_raw(self) -> i64 {
+        self.0
+    }
+
+    /// Construct a [`DataTransferId`] from the underlying integer.
+    ///
+    /// This should only be called with integers returned from [`DataTransferId::into_raw`].
+    pub const fn from_raw(id: i64) -> Self {
+        Self(id)
+    }
+}
+
+/// The set of types supported cross-platform.
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
+pub enum TypeHint {
+    /// Plain UTF-8 text (see [`TypedData::try_as_string`]).
+    ///
+    /// **Note for platform implementations**: this hint is _only_ for UTF-8 text. If the platform
+    /// returns plaintext in some format other than UTF-8 by default, a [`TypedData`]
+    /// implementation marked with this type hint should convert to UTF-8.
+    Plaintext,
+    /// A list of URIs in the format defined by the `text/uri-list` MIME type, encoded as UTF-8 (see
+    /// [`TypedData::try_as_uris`]).
+    ///
+    /// **Note for platform implementations**: this hint is _only_ for URIs encoded precisely in the
+    /// format specified above. If the platform uses a different format, a [`TypedData`]
+    /// implementation marked with this type hint should convert to that format.
+    UriList,
+    /// A HTML-formatted string
+    Html,
+    /// An RTF-formatted string
+    Rtf,
+    /// Audio
+    Audio {
+        /// An optional hint for the encoding of the supplied bytes, specified using the standard
+        /// file extension for that audio format, lowercase and without the leading `.`.
+        extension_hint: Option<&'static str>,
+    },
+    /// Image data
+    Image {
+        /// An optional hint for the encoding of the supplied bytes, specified using the standard
+        /// file extension for that image format, lowercase and without the leading `.`.
+        extension_hint: Option<&'static str>,
+    },
+}
+
+impl TypeHint {
+    /// Check whether the two type hints "match".
+    ///
+    /// This is subtly different to direct equality. If one of the types is an image or audio with a
+    /// `None` extension hint, then the other type just needs to match variant (i.e. image/audio),
+    /// the extension does not also have to be `None`.
+    pub fn matches(&self, other: &Self) -> bool {
+        match (self, other) {
+            (Self::Plaintext, Self::Plaintext)
+            | (Self::UriList, Self::UriList)
+            | (Self::Html, Self::Html)
+            | (Self::Rtf, Self::Rtf) => true,
+
+            (
+                Self::Audio { extension_hint: this_ext },
+                Self::Audio { extension_hint: other_ext },
+            )
+            | (
+                Self::Image { extension_hint: this_ext },
+                Self::Image { extension_hint: other_ext },
+            ) => match (this_ext, other_ext) {
+                (Some(this_ext), Some(other_ext)) => this_ext == other_ext,
+                (None, _) | (_, None) => true,
+            },
+
+            _ => false,
+        }
+    }
+}
+
+/// The type of a data transfer.
+///
+/// [`hint`](TransferType::hint) can be called to get the type in
+/// a cross-platform format (see [`TypeHint`])
+pub trait TransferType: AsAny + fmt::Debug {
+    /// Get the cross-platform representation of this type.
+    ///
+    /// If this returns `None`, then this is a platform-dependent type that has no cross-platform
+    /// equivalent.
+    fn hint(&self) -> Option<TypeHint>;
+
+    /// Check whether two dynamically-typed transfer types are equivalent.
+    // Can't use a `PartialEq` bound because it causes a dependency cycle.
+    fn matches(&self, other: &dyn TransferType) -> bool;
+}
+
+impl TransferType for TypeHint {
+    fn hint(&self) -> Option<TypeHint> {
+        Some(*self)
+    }
+
+    fn matches(&self, other: &dyn TransferType) -> bool {
+        other.hint().is_some_and(|hint| self.matches(&hint))
+    }
+}
+
+impl_dyn_casting!(TransferType);
+
+// Replicates the cfg for `url::Url::parse`
+#[cfg(any(unix, windows, target_os = "redox", target_os = "wasi", target_os = "hermit"))]
+fn default_try_as_file_paths<T: TypedData + ?Sized>(data: &T) -> io::Result<Vec<PathBuf>> {
+    data.try_as_uris().and_then(|uris| {
+        uris.into_iter()
+            .map(|uri_string| {
+                Ok(url::Url::parse(&uri_string)
+                    .map_err(|e| io::Error::new(io::ErrorKind::InvalidData, e))?
+                    .to_file_path()
+                    .map_err(|()| io::ErrorKind::InvalidData)?)
+            })
+            .collect()
+    })
+}
+
+// Replicates the cfg for `url::Url::parse`
+//
+// It doesn't matter that this is unimplemented on the web, as we don't currently support
+// drag-and-drop for web targets and the web platform can't directly access paths anyway.
+#[cfg(not(any(unix, windows, target_os = "redox", target_os = "wasi", target_os = "hermit")))]
+fn default_try_as_file_paths<T: TypedData + ?Sized>(_: &T) -> io::Result<Vec<PathBuf>> {
+    Err(io::ErrorKind::Unsupported.into())
+}
+
+/// Data that has been fetched from a data transfer
+///
+/// ### Blocking
+///
+/// Note that this type provides a blocking interface. In cases where reading this type directly on
+/// the event loop would cause a deadlock, the backend will make a best-effort attempt to return an
+/// error with [`io::ErrorKind::Deadlock`]. For now, the only way to access the data is via blocking
+/// on the event loop, so simply retrying the next time an event is received that references the
+/// data transfer should be enough to ensure that the data is accessible.
+pub trait TypedData: AsAny + fmt::Debug + Send + Sync {
+    /// The type of this `TypedData`.
+    fn type_(&self) -> &dyn TransferType;
+
+    /// If this value is readable as bytes, return a reader than can be used to read those bytes.
+    ///
+    /// On some platforms, the reader must be driven incrementally upon each
+    /// [`WindowEvent::DataTransferReceived`](crate::event::WindowEvent::DataTransferReceived)`. If
+    /// you don't need to stream the data and just want the bytes in a single buffer, use
+    /// [`TypedData::try_as_bytes`].
+    fn try_read(&self) -> Option<Box<dyn io::BufRead>>;
+
+    /// If this value is readable as bytes, return those bytes.
+    ///
+    /// If this returns [`WouldBlock`](std::io::ErrorKind::WouldBlock), then it should be called
+    /// again upon next receiving
+    /// [`WindowEvent::DataTransferReceived`](crate::event::WindowEvent::DataTransferReceived)
+    fn try_as_bytes(&self) -> io::Result<Vec<u8>> {
+        let mut reader = self
+            .try_read()
+            .ok_or_else(|| io::Error::other("This `TypedData` is not readable as bytes"))?;
+
+        let mut out = Vec::new();
+
+        reader.read_to_end(&mut out)?;
+
+        Ok(out)
+    }
+
+    /// Read this value as a list of URIs.
+    ///
+    /// If this value is not readable as URIs, return an error.
+    ///
+    /// The returned `String`s should be interpreted as URIs conforming to [RFC 3986](https://www.rfc-editor.org/info/rfc3986/).
+    ///
+    /// If this returns [`WouldBlock`](std::io::ErrorKind::WouldBlock), then it should be called
+    /// again upon next receiving
+    /// [`WindowEvent::DataTransferReceived`](crate::event::WindowEvent::DataTransferReceived)
+    fn try_as_uris(&self) -> io::Result<Vec<String>>;
+
+    /// Read this value as a list of paths.
+    ///
+    /// This is provided as a convenience method to avoid the need for the user to manually parse
+    /// the result of [`try_as_uris`](TypedData::try_as_uris). `try_as_uris` should be preferred
+    /// when the extra complexity is acceptable, as it is more generic.
+    ///
+    /// If this value is not readable as URIs, return an error.
+    ///
+    /// If this returns [`WouldBlock`](std::io::ErrorKind::WouldBlock), then it should be called
+    /// again upon next receiving
+    /// [`WindowEvent::DataTransferReceived`](crate::event::WindowEvent::DataTransferReceived)
+    fn try_as_file_paths(&self) -> io::Result<Vec<PathBuf>> {
+        default_try_as_file_paths(self)
+    }
+
+    /// Read this value as a plain text string.
+    ///
+    /// If this value is not readable as a string, return an error.
+    ///
+    /// If this returns [`WouldBlock`](std::io::ErrorKind::WouldBlock), then it should be called
+    /// again upon next receiving
+    /// [`WindowEvent::DataTransferReceived`](crate::event::WindowEvent::DataTransferReceived)
+    fn try_as_string(&self) -> io::Result<String>;
+}
+
+// Required for `WindowEvent` to implement `PartialEq` - we just implement this on a best-effort
+// basis.
+impl PartialEq for dyn TypedData {
+    fn eq(&self, other: &Self) -> bool {
+        std::ptr::addr_eq(self, other)
+    }
+}
+
+impl_dyn_casting!(TypedData);
+
+/// Metadata about a data transfer. This does not allow actually receiving data, as that is an
+/// asynchronous operation. To fetch the data from the source application, see
+/// [`ActiveEventLoop::fetch_data_transfer`](crate::event_loop::ActiveEventLoop::fetch_data_transfer).
+pub trait DataTransfer: AsAny + fmt::Debug {
+    /// Iterate over each type advertized by this `DataTransfer`. This is just a minor optimization,
+    /// in most cases you should probably use [`has_type`](DataTransfer::has_type) or
+    /// [`available_types`](DataTransfer::available_types).
+    fn for_each_available_type<'this>(
+        &'this self,
+        func: &'_ mut dyn FnMut(&'this dyn TransferType) -> ControlFlow<()>,
+    );
+
+    /// Display the list of all available types.
+    ///
+    /// This is useful if more-complex type matching is required, but for most cases
+    /// [`has_type`](DataTransfer::has_type) should be used.
+    fn available_types(&self) -> Vec<&'_ dyn TransferType> {
+        let mut out = Vec::new();
+
+        self.for_each_available_type(&mut |ty| {
+            out.push(ty);
+            ControlFlow::Continue(())
+        });
+
+        out
+    }
+
+    /// Check if the supplied type is provided by this [`DataTransfer`].
+    ///
+    /// Supplying a [`TypeHint`] as the type is supported on all platforms, but if some
+    /// platform-specific type is required then that platform's implementation of `TransferType` can
+    /// be used.
+    fn has_type(&self, type_: &dyn TransferType) -> bool {
+        let mut found = false;
+        self.for_each_available_type(&mut |haystack| {
+            if haystack.matches(type_) {
+                found = true;
+                ControlFlow::Break(())
+            } else {
+                ControlFlow::Continue(())
+            }
+        });
+
+        found
+    }
+}
+
+impl_dyn_casting!(DataTransfer);
+
+/// Kinds of data that can be sent via a `DataTransfer`.
+///
+/// Some kinds of data cannot be represented by just a binary blob in a cross-platform way.
+/// File URIs on Windows and macOS are represented as arrays of strings, and strings have
+/// different encoding on different platforms. To allow this to be represented, we allow
+/// supplying strings and URIs separately from binary blobs.
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub enum SendData {
+    /// List of URIs.
+    ///
+    /// These should conform to [RFC 3986](https://www.rfc-editor.org/info/rfc3986/).
+    /// If you just want to send file paths, see [`SendData::from_file_paths`].
+    ///
+    /// Note that `SendData` implements `From<String>` and `From<Vec<u8>>`, but _not_
+    /// `From<Vec<String>>`, as it is not necessarily obvious to a reader that `Vec<String>`
+    /// will be interpreted as a URI list. However, it _does_ implement [`From<Url>`](url::Url),
+    /// if you are using the [`url`](https://docs.rs/url/2) crate.
+    Uris(Vec<String>),
+    /// String
+    ///
+    /// This can also be constructed with the [`From<String>`](std::string::String) implementation.
+    String(String),
+    /// Binary blob
+    ///
+    /// This can also be constructed with the [`From<Vec<u8>>`](std::vec::Vec) implementation.
+    Bytes(Vec<u8>),
+}
+
+impl SendData {
+    /// Create [`SendData::Uris`] from an iterator of [`Path`]s.
+    ///
+    /// All paths must be absolute, and on Windows must include either a drive prefix (e.g. `C:\`)
+    /// or a UNC prefix (`\\`). See documentation for [`url::Url::from_file_path`].
+    pub fn from_file_paths<I>(paths: I) -> Option<Self>
+    where
+        I: IntoIterator,
+        I::Item: AsRef<Path>,
+    {
+        // Replicates the cfg for `url::Url::from_file_path`
+        #[cfg(any(unix, windows, target_os = "redox", target_os = "wasi", target_os = "hermit"))]
+        fn from_file_paths_impl<I>(paths: I) -> Option<SendData>
+        where
+            I: IntoIterator,
+            I::Item: AsRef<Path>,
+        {
+            paths
+                .into_iter()
+                .map(url::Url::from_file_path)
+                .map(|result| result.map(String::from))
+                .collect::<Result<Vec<_>, ()>>()
+                .map(SendData::Uris)
+                .ok()
+        }
+
+        // Replicates the cfg for `url::Url::from_file_path`
+        //
+        // It doesn't matter that this is unimplemented on the web, as we don't currently support
+        // drag-and-drop for web targets and the web platform can't directly access paths
+        // anyway.
+        #[cfg(not(any(
+            unix,
+            windows,
+            target_os = "redox",
+            target_os = "wasi",
+            target_os = "hermit"
+        )))]
+        fn from_file_paths_impl<I>(_: I) -> Option<SendData> {
+            None
+        }
+
+        from_file_paths_impl(paths)
+    }
+}
+
+// We monomorphize these `From` implementations instead of making them generic, in order to
+// prevent accidentally casting to the wrong type.
+impl From<String> for SendData {
+    fn from(value: String) -> Self {
+        Self::String(value)
+    }
+}
+
+impl From<Vec<u8>> for SendData {
+    fn from(value: Vec<u8>) -> Self {
+        Self::Bytes(value)
+    }
+}
+
+impl From<Vec<url::Url>> for SendData {
+    fn from(value: Vec<url::Url>) -> Self {
+        Self::Uris(value.into_iter().map(Into::into).collect())
+    }
+}
+
+/// Trait for sending data via a data transfer.
+///
+/// See [`ActiveEventLoop::start_drag`](crate::event_loop::ActiveEventLoop::start_drag) for where
+/// this is used. To build an implementation of this trait dynamically in a cross-platform way, use
+/// [`DataTransferSendBuilder`].
+pub trait DataTransferSend: DataTransfer + Send {
+    /// Get the data for the specified type, or `None` if this value does not supply the given data
+    /// type.
+    fn data_for_type(&self, type_: &dyn TransferType) -> Option<SendData>;
+}
+
+impl_dyn_casting!(DataTransferSend);
+
+type SendDataCallback<T> = Box<dyn Fn(&T, &dyn TransferType) -> Option<SendData> + Send>;
+
+/// Dynamic builder for an implementation of [`DataTransferSend`].
+///
+/// On all platforms, inter-application data transfer (i.e. clipboard and drag-and-drop) works like
+/// so:
+///
+/// - The source advertises a set of types that it can transfer.
+/// - The destination picks one or more of those types to receive.
+/// - The source sends the data for that type.
+///
+/// This type abstracts that in a way that allows data to be sent cross-platform. `T` is an optional
+/// state value, which allows the user to have a single source of truth for their data, converting
+/// it lazily to the requested type.
+pub struct DataTransferSendBuilder<T> {
+    state: T,
+    types: Vec<(Box<dyn TransferType + Send>, SendDataCallback<T>)>,
+}
+
+impl<T> fmt::Debug for DataTransferSendBuilder<T>
+where
+    T: fmt::Debug,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("NewDataTransferBuilder").field("state", &self.state).finish_non_exhaustive()
+    }
+}
+
+impl<T> DataTransfer for DataTransferSendBuilder<T>
+where
+    T: fmt::Debug + Send + 'static,
+{
+    fn for_each_available_type<'this>(
+        &'this self,
+        func: &'_ mut dyn FnMut(&'this dyn TransferType) -> ControlFlow<()>,
+    ) {
+        let _ = self.types.iter().try_for_each(|(ty, _)| func(&**ty));
+    }
+}
+
+impl<T> DataTransferSend for DataTransferSendBuilder<T>
+where
+    T: fmt::Debug + Send + 'static,
+{
+    fn data_for_type(&self, type_: &dyn TransferType) -> Option<SendData> {
+        self.data_for_type(type_)
+    }
+}
+
+impl<T> DataTransferSendBuilder<T> {
+    /// Create a new [`DataTransferSendBuilder`], with a state value which acts as
+    /// the single source of truth for the underlying data.
+    pub fn new(state: T) -> Self {
+        Self { state, types: vec![] }
+    }
+}
+
+impl<T> DataTransferSendBuilder<T> {
+    fn data_for_type(&self, type_: &dyn TransferType) -> Option<SendData> {
+        let (_, func) = self.types.iter().find(|(ty, _)| ty.matches(type_))?;
+
+        func(&self.state, type_)
+    }
+
+    /// Add a callback which converts the builder's state to the given type. In
+    /// most cases, `type_` will be [`TypeHint`].
+    pub fn add_type<Ty, F, O>(&mut self, type_: Ty, func: F) -> &mut Self
+    where
+        Ty: TransferType + Send,
+        F: Fn(&T, &dyn TransferType) -> Option<O> + Send + 'static,
+        O: Into<SendData>,
+    {
+        self.types
+            .push((Box::new(type_), Box::new(move |state, ty| func(state, ty).map(Into::into))));
+        self
+    }
+
+    /// Return a new builder, adding a callback which converts the builder's state
+    /// to the given type.
+    ///
+    /// For cross-platform use, `type_` will be [`TypeHint`]. The closure additionally receives
+    /// a [`TransferType`], which is not necessarily the same as `type_` for the following reasons:
+    ///
+    /// - The OS may have multiple types which are equivalent to the supplied type
+    /// - `TypeHint::Audio` and `TypeHint::Image` with `extension_hint: None` will advertise all
+    ///   supported audio and image formats, in which case the closure may receive a type with an
+    ///   extension chosen by the receiving application.
+    pub fn with_type<Ty, F, O>(mut self, type_: Ty, func: F) -> Self
+    where
+        Ty: TransferType + Send,
+        F: Fn(&T, &dyn TransferType) -> Option<O> + Send + 'static,
+        O: Into<SendData>,
+    {
+        self.add_type(type_, func);
+        self
+    }
+}
+
+impl<T> DataTransferSendBuilder<T>
+where
+    T: fmt::Debug + Send + 'static,
+{
+    /// Consume the builder, returning an implementation of [`DataTransferSend`].
+    ///
+    /// Note that this is only provided for explicitness and ergonomics. [`DataTransferSendBuilder`]
+    /// implements [`DataTransferSend`] and this method is equivalent to [`Box::new`].
+    pub fn build(self) -> Box<dyn DataTransferSend> {
+        Box::new(self)
+    }
+}

winit-core/src/event.rs

@@ -2,8 +2,7 @@
 use std::cell::LazyCell;
 use std::cmp::Ordering;
 use std::f64;
-use std::path::PathBuf;
-use std::sync::{Mutex, Weak};
+use std::sync::{Arc, Mutex, Weak};
 
 use dpi::{PhysicalPosition, PhysicalSize};
 #[cfg(feature = "serde")]
@@ -11,8 +10,9 @@ use serde::{Deserialize, Serialize};
 use smol_str::SmolStr;
 
 use crate::Instant;
+use crate::data_transfer::{DataTransferId, TypedData};
 use crate::error::RequestError;
-use crate::event_loop::AsyncRequestSerial;
+use crate::event_loop::{AsyncRequestSerial, DndAction};
 use crate::keyboard::{self, ModifiersKeyState, ModifiersKeys, ModifiersState};
 #[cfg(doc)]
 use crate::window::Window;
@@ -75,42 +75,104 @@ pub enum WindowEvent {
     /// The window has been destroyed.
     Destroyed,
 
-    /// A file drag operation has entered the window.
+    /// A drag operation has entered the window.
+    ///
+    /// The user can use the `id` to read information about the incoming dragged data, and report
+    /// whether the operation is accepted or rejected back to the operating system (see
+    /// [`crate::event_loop::ActiveEventLoop::set_valid_dnd_actions`](`crate::event_loop::ActiveEventLoop::set_valid_dnd_actions`)).
+    ///
+    /// To read the data being dragged, see
+    /// [`crate::event_loop::ActiveEventLoop::fetch_data_transfer`](`crate::event_loop::ActiveEventLoop::fetch_data_transfer`).
     DragEntered {
-        /// List of paths that are being dragged onto the window.
-        paths: Vec<PathBuf>,
-        /// (x,y) coordinates in pixels relative to the top-left corner of the window. May be
-        /// negative on some platforms if something is dragged over a window's decorations (title
-        /// bar, frame, etc).
-        position: PhysicalPosition<f64>,
-    },
-    /// A file drag operation has moved over the window.
-    DragMoved {
-        /// (x,y) coordinates in pixels relative to the top-left corner of the window. May be
-        /// negative on some platforms if something is dragged over a window's decorations (title
-        /// bar, frame, etc).
-        position: PhysicalPosition<f64>,
-    },
-    /// The file drag operation has dropped file(s) on the window.
-    DragDropped {
-        /// List of paths that are being dragged onto the window.
-        paths: Vec<PathBuf>,
-        /// (x,y) coordinates in pixels relative to the top-left corner of the window. May be
-        /// negative on some platforms if something is dragged over a window's decorations (title
-        /// bar, frame, etc).
-        position: PhysicalPosition<f64>,
-    },
-    /// The file drag operation has been cancelled or left the window.
-    DragLeft {
-        /// (x,y) coordinates in pixels relative to the top-left corner of the window. May be
-        /// negative on some platforms if something is dragged over a window's decorations (title
-        /// bar, frame, etc).
+        /// ID of the data transfer object, see
+        /// [`crate::event_loop::ActiveEventLoop::data_transfer`](`crate::event_loop::ActiveEventLoop::data_transfer`).
+        id: DataTransferId,
+        /// (x,y) coordinates in pixels relative to the top-left corner of the window.
         ///
-        /// ## Platform-specific
+        /// May be negative on some platforms if something is dragged over a window's decorations
+        /// (title bar, frame, etc).
         ///
-        /// - **Windows:** Always emits [`None`].
+        /// Some platforms will provide this on enter, others do not. If
+        /// [`crate::event_loop::ActiveEventLoop::set_valid_dnd_actions`](`crate::event_loop::ActiveEventLoop::set_valid_dnd_actions`)
+        /// is never called, the default state is for the drag operation to be rejected. The
+        /// position is provided here when available to allow the application to accept a drag
+        /// operation as soon as possible, preventing the cursor from flickering from rejected to
+        /// accepted.
         position: Option<PhysicalPosition<f64>>,
     },
+    /// The position of an ongoing drag operation has changed.
+    DragPosition {
+        /// ID of the data transfer object, see
+        /// [`crate::event_loop::ActiveEventLoop::data_transfer`](`crate::event_loop::ActiveEventLoop::data_transfer`).
+        id: DataTransferId,
+        /// (x,y) coordinates in pixels relative to the top-left corner of the window.
+        ///
+        /// May be negative on some platforms if something is dragged over a window's decorations
+        /// (title bar, frame, etc).
+        position: PhysicalPosition<f64>,
+        /// The drag action proposed by the OS, based on the actions supplied in
+        /// [`crate::event_loop::ActiveEventLoop::set_valid_dnd_actions`], the actions available on
+        /// the source, and the held modifier keys.
+        ///
+        /// This may be `None` if the backend has not supplied a valid action. On some platforms
+        /// (in particular, X11), the application is only informed of the proposed action once
+        /// the operation completes.
+        proposed_action: Option<DndAction>,
+    },
+    /// A drag operation has dropped file(s) on the window.
+    DragDropped {
+        /// ID of the data transfer object, see
+        /// [`crate::event_loop::ActiveEventLoop::data_transfer`].
+        id: DataTransferId,
+        /// The drag action proposed by the OS, based on the actions supplied in
+        /// [`crate::event_loop::ActiveEventLoop::set_valid_dnd_actions`], the actions available on
+        /// the source, and the held modifier keys.
+        ///
+        /// This may be `None` if the backend has not supplied a valid action. This is different
+        /// from the drag being canceled: the drag completed successfully, we just don't know
+        /// what action was selected.
+        proposed_action: Option<DndAction>,
+    },
+    /// A drag operation has been canceled or left the window.
+    DragLeft {
+        /// ID of the data transfer object, see
+        /// [`crate::event_loop::ActiveEventLoop::data_transfer`].
+        id: DataTransferId,
+    },
+    /// Data is available for a specific fetch request, see
+    /// [`fetch_data_transfer`](crate::event_loop::ActiveEventLoop::data_transfer).
+    ///
+    /// While winit makes a best effort to only send this event precisely once, on some platforms it
+    /// may not be possible to uniquely determine the window that should receive it. In these
+    /// cases, winit may dispatch the event to all  windows that have access to the data
+    /// transfer. If your application should only process this event once per data transfer, the
+    /// `serial` field can be used to deduplicate it.
+    DataTransferReceived {
+        /// ID of the data transfer object, see
+        /// [`crate::event_loop::ActiveEventLoop::data_transfer`].
+        id: DataTransferId,
+        /// Serial returned from `fetch_data_transfer`.
+        serial: AsyncRequestSerial,
+        /// The data for the transfer, with a specific type.
+        value: Arc<dyn TypedData>,
+    },
+
+    /// A drag operation started with `start_drag` has been dropped.
+    OutgoingDragDropped {
+        /// The ID returned from `start_drag`
+        id: DataTransferId,
+        /// The operation selected by the drop destination.
+        ///
+        /// This may be `None` if the backend has not supplied a valid action. This is different
+        /// from the drag being canceled: the drag completed successfully, we just don't know
+        /// what action was selected.
+        action: Option<DndAction>,
+    },
+    /// A drag operation started with `start_drag` has been canceled.
+    OutgoingDragCanceled {
+        /// The ID returned from `start_drag`
+        id: DataTransferId,
+    },
 
     /// The window gained or lost focus.
     ///
@@ -1582,16 +1644,20 @@ mod tests {
             use crate::event::Ime::Enabled;
             use crate::event::WindowEvent::*;
             use crate::event::{PointerKind, PointerSource};
+            use crate::event_loop::DndAction;
+            use crate::data_transfer::DataTransferId;
+
+            let dnd_data = DataTransferId::from_raw(123);
 
             with_window_event(CloseRequested);
             with_window_event(Destroyed);
             with_window_event(Focused(true));
             with_window_event(Moved((0, 0).into()));
             with_window_event(SurfaceResized((0, 0).into()));
-            with_window_event(DragEntered { paths: vec!["x.txt".into()], position: (0, 0).into() });
-            with_window_event(DragMoved { position: (0, 0).into() });
-            with_window_event(DragDropped { paths: vec!["x.txt".into()], position: (0, 0).into() });
-            with_window_event(DragLeft { position: Some((0, 0).into()) });
+            with_window_event(DragEntered { id: dnd_data, position: None });
+            with_window_event(DragPosition { id: dnd_data, position: (0, 0).into(), proposed_action: Some(DndAction::Copy) });
+            with_window_event(DragDropped { id: dnd_data, proposed_action: Some(DndAction::Copy) });
+            with_window_event(DragLeft { id: dnd_data });
             with_window_event(Ime(Enabled));
             with_window_event(PointerMoved {
                 device_id: None,

winit-core/src/event_loop/mod.rs

@@ -13,9 +13,11 @@ use rwh_06::{DisplayHandle, HandleError, HasDisplayHandle};
 use crate::Instant;
 use crate::as_any::AsAny;
 use crate::cursor::{CustomCursor, CustomCursorSource};
-use crate::error::RequestError;
+use crate::data_transfer::{DataTransfer, DataTransferId, DataTransferSend, TransferType};
+use crate::error::{NotSupportedError, RequestError};
+use crate::icon::Icon;
 use crate::monitor::MonitorHandle;
-use crate::window::{Theme, Window, WindowAttributes};
+use crate::window::{Theme, Window, WindowAttributes, WindowId};
 
 pub trait ActiveEventLoop: AsAny + fmt::Debug {
     /// Creates an [`EventLoopProxy`] that can be used to dispatch user events
@@ -114,8 +116,136 @@ pub trait ActiveEventLoop: AsAny + fmt::Debug {
 
     /// Get the raw-window-handle handle.
     fn rwh_06_handle(&self) -> &dyn HasDisplayHandle;
+
+    /// Request to fetch a type from a [data transfer](crate::data_transfer::DataTransfer).
+    ///
+    /// This may be called multiple times on the same [`DataTransferId`] with different types,
+    /// and may be called at any point during the drag operation, including during handling the
+    /// [`DragDropped`](crate::event::WindowEvent::DragDropped) event. After that event has been
+    /// received, though, the data transfer is not guaranteed to be available. The data is
+    /// _not_ guaranteed to be available during (or after) handling of
+    /// [`DragLeft](crate::event::WindowEvent::DragLeft).
+    ///
+    /// Once available, the data will be supplied to the application with the
+    /// [`DataTransferReceived`](crate::event::WindowEvent::DataTransferReceived) event.
+    fn fetch_data_transfer(
+        &self,
+        id: DataTransferId,
+        type_: &dyn TransferType,
+    ) -> Result<AsyncRequestSerial, RequestError> {
+        let _ = id;
+        let _ = type_;
+        Err(RequestError::NotSupported(NotSupportedError::new(
+            DATA_TRANSFER_UNSUPPORTED_ERROR_MESSAGE,
+        )))
     }
 
+    /// Get a [data transfer](DataTransfer) by its ID.
+    ///
+    /// If the ID is invalid (e.g. if the lifetime of the data transfer has expired), this will
+    /// return an error.
+    fn data_transfer(&self, id: DataTransferId) -> Result<Box<dyn DataTransfer>, RequestError> {
+        let _ = id;
+        Err(RequestError::NotSupported(NotSupportedError::new(
+            DATA_TRANSFER_UNSUPPORTED_ERROR_MESSAGE,
+        )))
+    }
+
+    /// Set a given set of `DndAction`s as the valid actions for the given [`DataTransferId`],
+    /// if the transfer ID is from an incoming drag-and-drop operation.
+    ///
+    /// This allows the OS/compositor to display the correct UI, indicating that the dragged data
+    /// can be dropped. If the data transfer does not exist or is not from a drag-and-drop
+    /// operation, will return an error.
+    ///
+    /// The operating system will consider the drag either accepted or rejected based on the
+    /// set of valid actions supplied using this method, combined with the set of valid actions
+    /// on the drag source. If the drag is rejected at the point that the user finalizes the drop,
+    /// the application will receive [`DragLeft`](crate::event::WindowEvent::DragLeft) instead
+    /// of [`DragDropped`](crate::event::WindowEvent::DragDropped).
+    ///
+    /// Note that _rejecting_ the drag is not the same as _canceling_ the drag. A rejected drag can
+    /// be accepted later and the user can continue dragging it over other potential targets. On
+    /// most platforms, there is no way for an application to explicitly cancel a drag
+    /// operation.
+    ///
+    /// The set of actions is expected to be ordered by preference.
+    fn set_valid_dnd_actions(
+        &self,
+        id: DataTransferId,
+        actions: &[DndAction],
+    ) -> Result<(), RequestError> {
+        let _ = id;
+        let _ = actions;
+        Err(RequestError::NotSupported(NotSupportedError::new(
+            DATA_TRANSFER_UNSUPPORTED_ERROR_MESSAGE,
+        )))
+    }
+
+    /// Initiate a new drag-and-drop operation.
+    ///
+    /// See [`DataTransferSendBuilder`](crate::data_transfer::DataTransferSendBuilder) for how to
+    /// create a new cross-platform data transfer, or [`DataTransferSend`] for a generic trait
+    /// which can be implemented manually.
+    ///
+    /// The [`DataTransferId`] returned from this method, identifying the outgoing drag, is
+    /// currently only used for identifying the drag in the
+    /// [`OutgoingDragDropped`](crate::event::WindowEvent::OutgoingDragDropped) event. In most
+    /// cases, a drag will be started while the mouse is over the window which started it. This
+    /// means that, directly after this method is called, the window will then receive a
+    /// [`DragEntered`](crate::event::WindowEvent::DragEntered) event. However, the ID identifying
+    /// the incoming drag is not guaranteed to be the same as the ID returned from this method.
+    ///
+    /// For most cases, applications can treat all `DragEntered` events the same, whether they were
+    /// initiated by the same application or a different application. However, if the user wants to
+    /// have some kind of special handling for internal drag-and-drop, they will currently need
+    /// to implement it via workaround. On all systems where drag-and-drop is implemented in
+    /// Winit, the application can make the assumption that only a single drag operation can
+    /// occur at one time. Therefore, if `DragEntered` is received between calling this method
+    /// and receiving `OutgoingDragDropped`, then you can assume that it's the same drag.
+    /// In theory, Wayland allows multiple simultaneous drag operations at a time, but Winit does
+    /// not currently guarantee that this is supported correctly for either internal or external
+    /// drag.
+    ///
+    /// ### Arguments
+    ///
+    /// - `source` - The ID of the window that initiated the drag operation.
+    /// - `send_data` - The data provided by this drag operation. See
+    ///   [`DataTransferSendBuilder`](crate::data_transfer::DataTransferSendBuilder).
+    /// - `actions` - The set of valid actions for this drag operation. See [`DndAction`]. On
+    ///   Wayland, this is expected to be ordered by preference.
+    /// - `icon` - The icon to show while dragging.
+    ///
+    /// Some platforms have a more-expressive way of setting the visual component of a drag
+    /// operation. For those platforms, consider using the platform-specific implementation of
+    /// [`DataTransferSend`] for `send_data` and set this field to `None`.
+    ///
+    /// ### Returns
+    ///
+    /// A unique identifier for this drag operation, which will be later supplied by
+    /// [`OutgoingDragDropped`](crate::event::WindowEvent::OutgoingDragDropped).
+    fn start_drag(
+        &self,
+        source: WindowId,
+        send_data: Box<dyn DataTransferSend>,
+        actions: &[DndAction],
+        icon: Option<DragIcon>,
+    ) -> Result<DataTransferId, RequestError> {
+        let _ = source;
+        let _ = send_data;
+        let _ = actions;
+        let _ = icon;
+        Err(RequestError::NotSupported(NotSupportedError::new(
+            DATA_TRANSFER_UNSUPPORTED_ERROR_MESSAGE,
+        )))
+    }
+}
+
+const DATA_TRANSFER_UNSUPPORTED_ERROR_MESSAGE: &str = {
+    "Cross-application data transfer (e.g. drag-and-drop, clipboard) is unsupported on this \
+     platform"
+};
+
 impl HasDisplayHandle for dyn ActiveEventLoop + '_ {
     fn display_handle(&self) -> Result<DisplayHandle<'_>, HandleError> {
         self.rwh_06_handle().display_handle()
@@ -124,6 +254,73 @@ impl HasDisplayHandle for dyn ActiveEventLoop + '_ {
 
 impl_dyn_casting!(ActiveEventLoop);
 
+/// Information needed to initiate a new drag operation.
+pub struct DragIcon {
+    /// The icon to apply to the cursor.
+    pub icon: Icon,
+    /// An x offset applied to the dragged icon.
+    ///
+    /// This is specified in image pixels. 0 means that the left side of the icon will be at
+    /// the cursor.
+    pub offset_x: i32,
+    /// A y offset applied to the dragged icon.
+    ///
+    /// This is specified in image pixels. 0 means that the top of the icon will be at the
+    /// cursor.
+    pub offset_y: i32,
+}
+
+impl From<Icon> for DragIcon {
+    fn from(value: Icon) -> Self {
+        Self { icon: value, offset_x: 0, offset_y: 0 }
+    }
+}
+
+/// The set of available actions for a drag operation.
+///
+/// This is _not_ a bitset, as on some platforms (e.g. Wayland, macOS) the source and/or destination
+/// are expected to provide some kind of order of preference.
+#[repr(u8)]
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub enum DndAction {
+    /// Move the dragged item from the source to the destination.
+    ///
+    /// # Platforms
+    ///
+    /// - Wayland
+    /// - macOS
+    /// - Windows
+    Move,
+    /// Copy the dragged item from the source to the destination.
+    ///
+    /// # Platforms
+    ///
+    /// - X11
+    /// - Wayland
+    /// - macOS
+    /// - Windows
+    Copy,
+    /// A link is established between the source and the destination.
+    ///
+    /// # Platforms
+    ///
+    /// - macOS
+    /// - Windows
+    Link,
+    /// The user will be prompted for what should be done
+    ///
+    /// # Platforms
+    ///
+    /// - Wayland
+    Ask,
+    /// The source and destination will negotiate the drag operation privately
+    ///
+    /// # Platforms
+    ///
+    /// - macOS
+    Private,
+}
+
 /// Control the [`ActiveEventLoop`], possibly from a different thread, without referencing it
 /// directly.
 #[derive(Clone, Debug)]

winit-core/src/lib.rs

@@ -13,6 +13,7 @@ pub mod cursor;
 #[macro_use]
 pub mod error;
 pub mod application;
+pub mod data_transfer;
 pub mod event;
 pub mod event_loop;
 pub mod icon;

winit-wayland/Cargo.toml

@@ -22,6 +22,7 @@ serde = ["dep:serde", "bitflags/serde", "smol_str/serde", "dpi/serde"]
 bitflags.workspace = true
 cursor-icon.workspace = true
 dpi.workspace = true
+percent-encoding.workspace = true
 rwh_06.workspace = true
 serde = { workspace = true, optional = true }
 smol_str.workspace = true
@@ -40,7 +41,7 @@ sctk = { package = "smithay-client-toolkit", version = "0.20.0", default-feature
 sctk-adwaita = { version = "0.11.0", default-features = false, optional = true }
 wayland-backend = { version = "0.3.10", default-features = false, features = ["client_system"] }
 wayland-client = "0.31.10"
-wayland-protocols = { version = "0.32.11", features = ["staging", "unstable"] }
+wayland-protocols = { version = "0.32.12", features = ["staging", "unstable"] }
 wayland-protocols-plasma = { version = "0.3.8", features = ["client"] }
 winit-common = { workspace = true, features = ["xkb", "wayland"] }
 

winit-wayland/src/dnd.rs

@@ -0,0 +1,805 @@
+//! Types related to drag-and-drop and data transfer on Wayland.
+
+use std::ffi::OsStr;
+use std::fmt;
+use std::io::{self, BufRead, Cursor, ErrorKind, Write};
+use std::ops::{BitOr, Deref};
+use std::sync::Arc;
+
+use calloop::PostAction;
+use dpi::{LogicalPosition, PhysicalPosition};
+use sctk::data_device_manager::WritePipe;
+use sctk::data_device_manager::data_device::{DataDeviceData, DataDeviceHandler};
+use sctk::data_device_manager::data_offer::{DataOfferHandler, DragOffer};
+use sctk::data_device_manager::data_source::{DataSourceHandler, DragSource as SctkDragSource};
+use sctk::reexports::client::backend::ObjectId;
+use wayland_client::protocol::wl_data_device::WlDataDevice;
+use wayland_client::protocol::wl_data_device_manager::DndAction as WlDndAction;
+use wayland_client::protocol::wl_data_offer::WlDataOffer;
+use wayland_client::protocol::wl_data_source::WlDataSource;
+use wayland_client::protocol::wl_surface::WlSurface;
+use wayland_client::{Connection, Proxy, QueueHandle};
+use winit_core::data_transfer::{
+    DataTransfer, DataTransferId, DataTransferSend, SendData, TransferType, TypeHint, TypedData,
+};
+use winit_core::event::WindowEvent;
+use winit_core::event_loop::DndAction;
+use winit_core::window::WindowId;
+
+use crate::make_data_transfer_id;
+use crate::state::WinitState;
+
+fn encode_uri_list<I>(uri_list: I) -> Vec<u8>
+where
+    I: IntoIterator,
+    I::Item: AsRef<OsStr>,
+{
+    let mut out = Vec::new();
+
+    for uri in uri_list {
+        out.extend_from_slice(OsStr::new(&uri).as_encoded_bytes());
+        out.extend_from_slice(b"\r\n");
+    }
+
+    out
+}
+
+impl DataSourceHandler for WinitState {
+    fn accept_mime(
+        &mut self,
+        _: &Connection,
+        _: &QueueHandle<Self>,
+        _: &WlDataSource,
+        _: Option<String>,
+    ) {
+        // This method isn't a necessary part of the protocol, it's a holdover from the first
+        // version of DnD in Wayland and now just serves as a hint.
+    }
+
+    fn send_request(
+        &mut self,
+        _: &Connection,
+        _: &QueueHandle<Self>,
+        _: &WlDataSource,
+        mime: String,
+        fd: WritePipe,
+    ) {
+        let Some(data) = self.dnd_state.send_drag_data_mut() else {
+            // TODO: Is there a way to explicitly express that the data was not sent?
+            return;
+        };
+
+        let mime = MimeType::parse(mime);
+
+        let Some(send_data) = data.data_for_type(&mime) else {
+            return;
+        };
+
+        let mut encoder = match send_data {
+            SendData::Uris(strings) => Cursor::new(encode_uri_list(strings)),
+            SendData::String(str) => match mime.parse_charset() {
+                Ok(Charset::Utf8) => Cursor::new(str.into_bytes()),
+                Err(e) => {
+                    tracing::error!("{e}");
+                    return;
+                },
+            },
+            SendData::Bytes(binary) => Cursor::new(binary),
+        };
+
+        let _ = self.loop_handle.insert_source(fd, move |_, file, _| {
+            // Safety: We only mutate `file` in-place and do not replace and drop it.
+            let file = unsafe { file.get_mut() };
+            loop {
+                let Ok(encoded_bytes) = encoder.fill_buf() else {
+                    return PostAction::Remove;
+                };
+
+                match file.write(encoded_bytes) {
+                    Ok(0) => {
+                        break PostAction::Remove;
+                    },
+                    Ok(consumed) => {
+                        encoder.consume(consumed);
+                    },
+                    Err(e) if e.kind() == ErrorKind::WouldBlock => {
+                        break PostAction::Continue;
+                    },
+                    Err(_) => {
+                        break PostAction::Remove;
+                    },
+                }
+            }
+        });
+    }
+
+    fn cancelled(&mut self, _: &Connection, _: &QueueHandle<Self>, _: &WlDataSource) {
+        let Some(current_drag) = self.dnd_state.send_drag() else {
+            return;
+        };
+
+        let window_id = current_drag.window_id;
+        let id = current_drag.data_transfer_id;
+
+        self.events_sink.push_window_event(WindowEvent::OutgoingDragCanceled { id }, window_id);
+    }
+
+    fn dnd_dropped(&mut self, _: &Connection, _: &QueueHandle<Self>, _: &WlDataSource) {
+        let Some(current_drag) = self.dnd_state.send_drag() else {
+            return;
+        };
+
+        let window_id = current_drag.window_id;
+        let id = current_drag.data_transfer_id;
+        let selected_action = current_drag.selected_action;
+
+        self.events_sink.push_window_event(
+            WindowEvent::OutgoingDragDropped {
+                id,
+                action: dnd_action_wl_to_winit(selected_action),
+            },
+            window_id,
+        );
+    }
+
+    fn dnd_finished(
+        &mut self,
+        _: &Connection,
+        _: &QueueHandle<Self>,
+        _: &wayland_client::protocol::wl_data_source::WlDataSource,
+    ) {
+        self.dnd_state.clear_send_drag();
+    }
+
+    fn action(
+        &mut self,
+        _: &Connection,
+        _: &QueueHandle<Self>,
+        _: &WlDataSource,
+        action: WlDndAction,
+    ) {
+        self.dnd_state.set_target_drag_action(action);
+    }
+}
+
+#[derive(Default, Debug, PartialEq, Eq, Clone, Hash)]
+enum Charset {
+    #[default]
+    Utf8,
+}
+
+/// MIME type as string, with an optional hint detected from the MIME type.
+#[derive(Debug, PartialEq, Eq, Clone, Hash)]
+pub struct MimeType {
+    mime: Arc<str>,
+    hint: Option<TypeHint>,
+}
+
+// MIME types
+// Files
+const TEXT_URI_LIST: &str = "text/uri-list";
+// Plaintext
+const TEXT_PLAIN: &str = "text/plain";
+const TEXT_PLAIN_CHARSET_UTF8: &str = "text/plain; charset=utf-8";
+// HTML
+const TEXT_HTML: &str = "text/html";
+const TEXT_HTML_CHARSET_UTF8: &str = "text/html; charset=utf-8";
+// RTF
+const APPLICATION_RTF: &str = "application/rtf";
+// Audio
+const AUDIO_AAC: &str = "audio/aac";
+const AUDIO_AIFF: &str = "audio/aiff";
+const AUDIO_FLAC: &str = "audio/flac";
+const AUDIO_WAV: &str = "audio/wav";
+const AUDIO_WAVE: &str = "audio/wave";
+const AUDIO_X_WAV: &str = "audio/x-wav";
+const AUDIO_VND_WAV: &str = "audio/vnd.wav";
+const AUDIO_VND_WAVE: &str = "audio/vnd.wave";
+const AUDIO_MPEG: &str = "audio/mpeg";
+const AUDIO_OGG: &str = "audio/ogg";
+// Image
+const IMAGE_BMP: &str = "image/bmp";
+const IMAGE_GIF: &str = "image/gif";
+const IMAGE_JPEG: &str = "image/jpeg";
+const IMAGE_PJPEG: &str = "image/pjpeg";
+const IMAGE_PNG: &str = "image/png";
+const IMAGE_SVG: &str = "image/svg+xml";
+const IMAGE_TIFF: &str = "image/tiff";
+const IMAGE_WEBP: &str = "image/webp";
+const IMAGE_X_ICON: &str = "image/x-icon";
+const IMAGE_RAW: &str = "image/x-panasonic-raw";
+
+#[derive(Debug)]
+struct UnexpectedCharsetError<'a>(&'a str);
+
+impl std::error::Error for UnexpectedCharsetError<'_> {}
+
+impl fmt::Display for UnexpectedCharsetError<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "Unsupported charset: {}", self.0)
+    }
+}
+
+impl MimeType {
+    const MIME_HINT_MAP: &[(&str, TypeHint)] = &[
+        // Files
+        (TEXT_URI_LIST, TypeHint::UriList),
+        // Plaintext
+        (TEXT_PLAIN, TypeHint::Plaintext),
+        (TEXT_PLAIN_CHARSET_UTF8, TypeHint::Plaintext),
+        // HTML
+        (TEXT_HTML, TypeHint::Html),
+        (TEXT_HTML_CHARSET_UTF8, TypeHint::Html),
+        // RTF
+        (APPLICATION_RTF, TypeHint::Rtf),
+        // Audio
+        (AUDIO_AAC, TypeHint::Audio { extension_hint: Some("aac") }),
+        (AUDIO_AIFF, TypeHint::Audio { extension_hint: Some("aif") }),
+        (AUDIO_FLAC, TypeHint::Audio { extension_hint: Some("flac") }),
+        (AUDIO_VND_WAV, TypeHint::Audio { extension_hint: Some("wav") }),
+        (AUDIO_VND_WAVE, TypeHint::Audio { extension_hint: Some("wav") }),
+        (AUDIO_WAV, TypeHint::Audio { extension_hint: Some("wav") }),
+        (AUDIO_WAVE, TypeHint::Audio { extension_hint: Some("wav") }),
+        (AUDIO_X_WAV, TypeHint::Audio { extension_hint: Some("wav") }),
+        (AUDIO_OGG, TypeHint::Audio { extension_hint: Some("ogg") }),
+        (AUDIO_MPEG, TypeHint::Audio { extension_hint: Some("mp3") }),
+        // Image
+        (IMAGE_BMP, TypeHint::Image { extension_hint: Some("bmp") }),
+        (IMAGE_GIF, TypeHint::Image { extension_hint: Some("gif") }),
+        (IMAGE_JPEG, TypeHint::Image { extension_hint: Some("jpg") }),
+        (IMAGE_PJPEG, TypeHint::Image { extension_hint: Some("jpg") }),
+        (IMAGE_PNG, TypeHint::Image { extension_hint: Some("png") }),
+        (IMAGE_RAW, TypeHint::Image { extension_hint: Some("raw") }),
+        (IMAGE_SVG, TypeHint::Image { extension_hint: Some("svg") }),
+        (IMAGE_TIFF, TypeHint::Image { extension_hint: Some("tiff") }),
+        (IMAGE_WEBP, TypeHint::Image { extension_hint: Some("webp") }),
+        (IMAGE_X_ICON, TypeHint::Image { extension_hint: Some("ico") }),
+    ];
+
+    // Returns an iterator so that things like the multiple charsets for plaintext/HTML
+    // and the multiple ways of expressing .wav work correctly.
+    pub(crate) fn from_dyn(type_: &dyn TransferType) -> impl Iterator<Item = Self> {
+        let downcast = type_.cast_ref::<Self>().cloned();
+        let downcast_failed = downcast.is_none();
+        // This filter is a bit hacky, but it's the only way to ensure that we always
+        // return the same type.
+        let from_hint = downcast_failed
+            .then_some(
+                Self::MIME_HINT_MAP
+                    .iter()
+                    .filter(move |(_, haystack)| TransferType::matches(haystack, type_))
+                    .map(move |(mime, _)| Self {
+                        mime: mime.to_string().into(),
+                        hint: type_.hint(),
+                    }),
+            )
+            .into_iter()
+            .flatten();
+
+        downcast.into_iter().chain(from_hint)
+    }
+
+    // TODO: We should properly parse MIME types using `mime` or a similar crate.
+    fn parse_charset(&self) -> Result<Charset, UnexpectedCharsetError<'_>> {
+        let Some((_, charset)) = self
+            .mime
+            .split_once(';')
+            .and_then(|(_essence, options)| options.split_once("charset="))
+        else {
+            return Ok(Default::default());
+        };
+
+        let charset = charset.split_once(',').map(|(first, _)| first).unwrap_or(charset).trim();
+
+        if charset == "utf-8" { Ok(Charset::Utf8) } else { Err(UnexpectedCharsetError(charset)) }
+    }
+
+    fn parse(mime: String) -> Self {
+        let hint = Self::MIME_HINT_MAP
+            .iter()
+            .find_map(|(haystack, hint)| (*haystack == &*mime).then_some(*hint))
+            .or_else(|| {
+                if mime.starts_with("image/") {
+                    Some(TypeHint::Image { extension_hint: None })
+                } else if mime.starts_with("audio/") {
+                    Some(TypeHint::Audio { extension_hint: None })
+                } else {
+                    None
+                }
+            });
+
+        Self { mime: mime.into(), hint }
+    }
+}
+
+impl fmt::Display for MimeType {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.mime.fmt(f)
+    }
+}
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+pub struct UnknownTypeHint(pub TypeHint);
+
+impl fmt::Display for UnknownTypeHint {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "Unknown type hint: {:?}", self.0)
+    }
+}
+
+impl TryFrom<TypeHint> for MimeType {
+    type Error = UnknownTypeHint;
+
+    fn try_from(hint: TypeHint) -> Result<Self, Self::Error> {
+        let mime = Self::MIME_HINT_MAP
+            .iter()
+            .find_map(|(mime, haystack)| (*haystack == hint).then_some(*mime))
+            .ok_or(UnknownTypeHint(hint))?;
+
+        Ok(Self { mime: mime.to_owned().into(), hint: Some(hint) })
+    }
+}
+
+impl TransferType for MimeType {
+    fn hint(&self) -> Option<TypeHint> {
+        self.hint
+    }
+
+    fn matches(&self, other: &dyn TransferType) -> bool {
+        if let Some(other_mime) = other.cast_ref::<Self>() {
+            *self == *other_mime
+        } else {
+            // If either hint is `None`, return false
+            self.hint().is_some_and(|hint| other.hint() == Some(hint))
+        }
+    }
+}
+
+type BytesResult = Result<Vec<u8>, Arc<io::Error>>;
+
+/// Typed data transfer from another application.
+#[derive(Debug)]
+pub struct MimeData {
+    mime_type: MimeType,
+    result: BytesResult,
+}
+
+impl MimeData {
+    pub(crate) fn new(mime_type: MimeType, result: BytesResult) -> Self {
+        Self { mime_type, result }
+    }
+
+    fn data(&self) -> io::Result<&[u8]> {
+        fn arc_to_io_error(arc: Arc<io::Error>) -> io::Error {
+            io::Error::new(arc.kind(), arc)
+        }
+
+        self.result.as_deref().map_err(|e| arc_to_io_error(e.clone()))
+    }
+}
+
+impl TypedData for MimeData {
+    fn type_(&self) -> &dyn TransferType {
+        &self.mime_type
+    }
+
+    fn try_read(&self) -> Option<Box<dyn io::BufRead>> {
+        let data = self.data().ok()?.to_owned();
+
+        Some(Box::new(io::Cursor::new(data)))
+    }
+
+    fn try_as_bytes(&self) -> io::Result<Vec<u8>> {
+        self.data().map(ToOwned::to_owned)
+    }
+
+    fn try_as_uris(&self) -> io::Result<Vec<String>> {
+        let data = self.data()?;
+
+        Cursor::new(&data)
+            .lines()
+            .filter(|result| match result {
+                Ok(s) => !s.starts_with('#'),
+                // We want to maintain errors, so the final `collect` returns an error too
+                Err(_) => true,
+            })
+            .collect()
+    }
+
+    fn try_as_string(&self) -> io::Result<String> {
+        let charset = self.mime_type.parse_charset();
+
+        let data = self.data()?;
+
+        match charset {
+            Ok(Charset::Utf8) => String::from_utf8(data.to_vec())
+                .map_err(|err| io::Error::new(io::ErrorKind::InvalidData, err)),
+            Err(e) => Err(io::Error::other(e.to_string())),
+        }
+    }
+}
+
+/// A wrapper around `WlDataOffer`, implementing `DataTransfer`.
+#[derive(Debug, Clone)]
+pub struct DataOffer {
+    mime_types: Arc<[MimeType]>,
+    data: WlDataOffer,
+    available_actions: WlDndAction,
+    data_device_id: ObjectId,
+    serial: u32,
+    window_id: WindowId,
+}
+
+pub(crate) fn dnd_action_winit_to_wl(winit: DndAction) -> WlDndAction {
+    match winit {
+        DndAction::Move => WlDndAction::Move,
+        DndAction::Copy => WlDndAction::Copy,
+        DndAction::Ask => WlDndAction::Ask,
+        _ => WlDndAction::empty(),
+    }
+}
+
+pub(crate) fn dnd_action_wl_to_winit(wl: WlDndAction) -> Option<DndAction> {
+    match wl {
+        WlDndAction::Move => Some(DndAction::Move),
+        WlDndAction::Copy => Some(DndAction::Copy),
+        WlDndAction::Ask => Some(DndAction::Ask),
+        _ => None,
+    }
+}
+
+impl DataOffer {
+    pub(crate) fn transfer_id(&self) -> DataTransferId {
+        make_data_transfer_id(self.data_device_id.clone(), self.serial)
+    }
+
+    pub(crate) fn first_mime_type(&self) -> Option<&MimeType> {
+        self.mime_types.first()
+    }
+
+    pub(crate) fn serial(&self) -> u32 {
+        self.serial
+    }
+
+    pub(crate) fn window_id(&self) -> WindowId {
+        self.window_id
+    }
+
+    pub(crate) fn set_actions(&self, action_set: &[DndAction]) -> bool {
+        let preferred_action = action_set.iter().find_map(|winit| {
+            let wl = dnd_action_winit_to_wl(*winit);
+            self.available_actions.intersects(wl).then_some(wl)
+        });
+
+        let any = preferred_action.is_some();
+
+        let all_actions = action_set
+            .iter()
+            .copied()
+            .map(dnd_action_winit_to_wl)
+            .fold(WlDndAction::empty(), BitOr::bitor);
+
+        self.data.set_actions(all_actions, preferred_action.unwrap_or(WlDndAction::empty()));
+
+        any
+    }
+
+    pub(crate) fn find_type_dyn<'a>(&'a self, type_: &'a dyn TransferType) -> Option<&'a MimeType> {
+        match type_.cast_ref::<MimeType>() {
+            Some(mime_type) => Some(mime_type),
+            None => {
+                let hint = type_.hint()?;
+                self.mime_types.iter().find(|mime_type| {
+                    mime_type.hint().is_some_and(|haystack| haystack.matches(&hint))
+                })
+            },
+        }
+    }
+}
+
+impl Deref for DataOffer {
+    type Target = WlDataOffer;
+
+    fn deref(&self) -> &Self::Target {
+        &self.data
+    }
+}
+
+impl DataTransfer for DataOffer {
+    fn for_each_available_type<'this>(
+        &'this self,
+        func: &'_ mut dyn FnMut(&'this dyn TransferType) -> std::ops::ControlFlow<()>,
+    ) {
+        let _ = self.mime_types.iter().map(|mime| mime as &dyn TransferType).try_for_each(func);
+    }
+}
+
+/// Wrapper for [`WlDataSource`], which exposes the types that are advertised by a data
+/// transfer operation, along with the data that the source represents
+#[derive(Debug)]
+pub struct DragSource {
+    pub(crate) data_transfer_id: DataTransferId,
+    /// The `WlDataSource` generated from `data`.
+    ///
+    /// This is stored internally, as if this source is dropped then the
+    /// drag operation will be cancelled.
+    _data_source: SctkDragSource,
+    /// The supplied [`DataTransferSend`].
+    pub(crate) data: Box<dyn DataTransferSend>,
+    pub(crate) selected_action: WlDndAction,
+    pub(crate) window_id: WindowId,
+    /// (Optionally) an icon for the drag-and-drop operation.
+    _icon: Option<WlSurface>,
+}
+
+impl DragSource {
+    pub(crate) fn new(
+        data_transfer_id: DataTransferId,
+        data_source: SctkDragSource,
+        data: Box<dyn DataTransferSend>,
+        icon: Option<WlSurface>,
+        window_id: WindowId,
+    ) -> Self {
+        Self {
+            data_transfer_id,
+            _data_source: data_source,
+            data,
+            selected_action: WlDndAction::None,
+            window_id,
+            _icon: icon,
+        }
+    }
+
+    /// Per-type data to be sent. See [`DataTransferSend`].
+    pub fn data(&mut self) -> &mut dyn DataTransferSend {
+        &mut *self.data
+    }
+}
+
+/// The current state of an in-progress drag-and-drop operation.
+#[derive(Debug, Default)]
+pub struct DndState {
+    receive_drag: Option<DataOffer>,
+    send_drag: Option<DragSource>,
+}
+
+impl DndState {
+    pub(crate) fn receive_drag(&self) -> Option<&DataOffer> {
+        self.receive_drag.as_ref()
+    }
+
+    pub(crate) fn set_send_drag(&mut self, source: DragSource) {
+        self.send_drag = Some(source);
+    }
+
+    pub(crate) fn send_drag(&self) -> Option<&DragSource> {
+        self.send_drag.as_ref()
+    }
+
+    pub(crate) fn set_target_drag_action(&mut self, action: WlDndAction) {
+        if let Some(source) = &mut self.send_drag {
+            source.selected_action = action;
+        }
+    }
+
+    /// Returns `true` if a drag operation was in progress, `false` if no drag operation was in
+    /// progress.
+    pub(crate) fn clear_send_drag(&mut self) -> bool {
+        self.send_drag.take().is_some()
+    }
+
+    pub(crate) fn send_drag_data_mut(&mut self) -> Option<&mut dyn DataTransferSend> {
+        self.send_drag.as_mut().map(|send| send.data())
+    }
+}
+
+impl DataOfferHandler for WinitState {
+    fn source_actions(
+        &mut self,
+        conn: &Connection,
+        qh: &QueueHandle<Self>,
+        offer: &mut DragOffer,
+        actions: WlDndAction,
+    ) {
+        let _ = actions;
+        let _ = offer;
+        let _ = qh;
+        let _ = conn;
+        // Not implemented, but required for `DataDeviceHandler`.
+    }
+
+    fn selected_action(
+        &mut self,
+        conn: &Connection,
+        qh: &QueueHandle<Self>,
+        offer: &mut DragOffer,
+        actions: WlDndAction,
+    ) {
+        let _ = actions;
+        let _ = offer;
+        let _ = qh;
+        let _ = conn;
+    }
+}
+
+impl DataDeviceHandler for WinitState {
+    fn enter(
+        &mut self,
+        _: &Connection,
+        _: &QueueHandle<Self>,
+        data_device: &WlDataDevice,
+        x: f64,
+        y: f64,
+        wl_surface: &WlSurface,
+    ) {
+        let Some(data) = data_device.data::<DataDeviceData>() else {
+            return;
+        };
+
+        let Some(drag) = data.drag_offer() else {
+            // Selections are not yet implemented
+            return;
+        };
+
+        let window_id = crate::make_wid(wl_surface);
+
+        let current_drag = drag.with_mime_types(|types| DataOffer {
+            mime_types: types
+                .iter()
+                .map(|str| MimeType::parse(str.clone()))
+                .collect::<Vec<_>>()
+                .into(),
+            available_actions: drag.source_actions,
+            serial: drag.serial,
+            data_device_id: data_device.id(),
+            data: drag.inner().clone(),
+            window_id,
+        });
+
+        current_drag.set_actions(&[]);
+
+        let id = current_drag.transfer_id();
+
+        self.dnd_state.receive_drag = Some(current_drag);
+
+        let scale_factor = self
+            .windows
+            .borrow()
+            .get(&window_id)
+            .map(|window| window.lock().unwrap().scale_factor())
+            .unwrap_or(1.);
+        let position: PhysicalPosition<f64> = LogicalPosition::new(x, y).to_physical(scale_factor);
+
+        self.events_sink.push_window_event(
+            WindowEvent::DragEntered { id, position: Some(position) },
+            window_id,
+        );
+    }
+
+    fn leave(&mut self, _: &Connection, _: &QueueHandle<Self>, data_device: &WlDataDevice) {
+        let Some(data) = data_device.data::<DataDeviceData>() else {
+            return;
+        };
+
+        if let Some(current_drag) = self.dnd_state.receive_drag() {
+            self.events_sink.push_window_event(
+                WindowEvent::DragLeft { id: current_drag.transfer_id() },
+                current_drag.window_id(),
+            );
+
+            if let Some(receive_drag) = self.dnd_state.receive_drag.take() {
+                receive_drag.finish();
+            }
+        }
+
+        if let Some(drag) = data.drag_offer() {
+            drag.destroy();
+        }
+        if let Some(selection) = data.selection_offer() {
+            selection.destroy();
+        }
+    }
+
+    fn motion(
+        &mut self,
+        _: &Connection,
+        _: &QueueHandle<Self>,
+        data_device: &WlDataDevice,
+        x: f64,
+        y: f64,
+    ) {
+        let Some(data) = data_device.data::<DataDeviceData>() else {
+            return;
+        };
+        let Some(drag) = data.drag_offer() else {
+            // Selections (copy/paste) are not yet implemented
+            return;
+        };
+
+        // `selected_action` should only contain a single flag, but we check with `contains`
+        // just in case we or the compositor misunderstood the spec.
+        let proposed_action = if drag.selected_action.contains(WlDndAction::Move) {
+            Some(DndAction::Move)
+        } else if drag.selected_action.contains(WlDndAction::Copy) {
+            Some(DndAction::Copy)
+        } else if drag.selected_action.contains(WlDndAction::Ask) {
+            Some(DndAction::Ask)
+        } else {
+            None
+        };
+
+        let Some(current_drag) = self.dnd_state.receive_drag() else {
+            return;
+        };
+
+        let window_id = crate::make_wid(&drag.surface);
+
+        let scale_factor = self
+            .windows
+            .borrow()
+            .get(&window_id)
+            .map(|window| window.lock().unwrap().scale_factor())
+            .unwrap_or(1.);
+        let position: PhysicalPosition<f64> = LogicalPosition::new(x, y).to_physical(scale_factor);
+
+        self.events_sink.push_window_event(
+            WindowEvent::DragPosition { id: current_drag.transfer_id(), position, proposed_action },
+            window_id,
+        );
+    }
+
+    fn selection(&mut self, _: &Connection, _: &QueueHandle<Self>, _: &WlDataDevice) {
+        // We don't handle selections right now.
+    }
+
+    fn drop_performed(
+        &mut self,
+        _: &Connection,
+        _: &QueueHandle<Self>,
+        data_device: &WlDataDevice,
+    ) {
+        let Some(data) = data_device.data::<DataDeviceData>() else {
+            return;
+        };
+        let Some(drag) = data.drag_offer() else {
+            // Selections (copy/paste) are not yet implemented
+            return;
+        };
+
+        let Some(current_drag) = self.dnd_state.receive_drag() else {
+            return;
+        };
+
+        let window_id = crate::make_wid(&drag.surface);
+
+        // `selected_action` should only contain a single flag, but we check with `contains`
+        // just in case we or the compositor misunderstood the spec.
+        let proposed_action = if drag.selected_action.contains(WlDndAction::Move) {
+            Some(DndAction::Move)
+        } else if drag.selected_action.contains(WlDndAction::Copy) {
+            Some(DndAction::Copy)
+        } else if drag.selected_action.contains(WlDndAction::Ask) {
+            Some(DndAction::Ask)
+        } else {
+            None
+        };
+
+        self.events_sink.push_window_event(
+            WindowEvent::DragDropped { id: current_drag.transfer_id(), proposed_action },
+            window_id,
+        );
+
+        if let Some(receive_drag) = self.dnd_state.receive_drag.take() {
+            receive_drag.finish();
+        }
+
+        if let Some(drag) = data.drag_offer() {
+            drag.destroy();
+        }
+        if let Some(selection) = data.selection_offer() {
+            selection.destroy();
+        }
+    }
+}
+
+sctk::delegate_data_device!(WinitState);

winit-wayland/src/event_loop/mod.rs

@@ -1,35 +1,46 @@
 //! The event-loop routines.
 
 use std::cell::{Cell, RefCell};
-use std::io::Result as IOResult;
-use std::mem;
+use std::io::{self, Read, Result as IOResult};
+use std::ops::BitOr;
 use std::os::fd::OwnedFd;
 use std::os::unix::io::{AsFd, AsRawFd, BorrowedFd, RawFd};
 use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::{Arc, Condvar, Mutex};
 use std::thread::JoinHandle;
 use std::time::{Duration, Instant};
+use std::{fmt, mem};
 
+use calloop::PostAction;
 use calloop::ping::Ping;
 use dpi::LogicalSize;
 use rustix::event::{PollFd, PollFlags};
 use rustix::pipe::{self, PipeFlags};
+use sctk::data_device_manager::{ReadPipe, data_offer};
 use sctk::reexports::calloop_wayland_source::WaylandSource;
 use sctk::reexports::client::{Connection, QueueHandle, globals};
+use sctk::shell::WaylandSurface;
 use tracing::warn;
+use wayland_client::Proxy;
+use wayland_client::protocol::wl_data_device_manager::DndAction as WlDndAction;
+use wayland_client::protocol::wl_shm::Format;
 use winit_core::application::ApplicationHandler;
 use winit_core::cursor::{CustomCursor as CoreCustomCursor, CustomCursorSource};
+use winit_core::data_transfer::{DataTransfer, DataTransferId, DataTransferSend, TransferType};
 use winit_core::error::{EventLoopError, NotSupportedError, OsError, RequestError};
 use winit_core::event::{DeviceEvent, StartCause, SurfaceSizeWriter, WindowEvent};
 use winit_core::event_loop::pump_events::PumpStatus;
 use winit_core::event_loop::{
-    ActiveEventLoop as RootActiveEventLoop, ControlFlow, DeviceEvents,
-    OwnedDisplayHandle as CoreOwnedDisplayHandle,
+    ActiveEventLoop as RootActiveEventLoop, AsyncRequestSerial, ControlFlow, DeviceEvents,
+    DndAction, DragIcon, OwnedDisplayHandle as CoreOwnedDisplayHandle,
 };
+use winit_core::icon::RgbaIcon;
 use winit_core::monitor::MonitorHandle as CoreMonitorHandle;
 use winit_core::window::Theme;
 
+use crate::dnd::{MimeData, dnd_action_winit_to_wl};
 use crate::types::cursor::WaylandCustomCursor;
+use crate::{DragSource, MimeType, image_to_buffer, make_data_transfer_id};
 
 mod proxy;
 pub mod sink;
@@ -678,8 +689,233 @@ impl RootActiveEventLoop for ActiveEventLoop {
     fn rwh_06_handle(&self) -> &dyn rwh_06::HasDisplayHandle {
         self
     }
+
+    fn fetch_data_transfer(
+        &self,
+        id: DataTransferId,
+        type_: &dyn TransferType,
+    ) -> Result<AsyncRequestSerial, RequestError> {
+        let state = self.state.borrow_mut();
+        let Some(current_drag) = state.dnd_state.receive_drag() else {
+            return Err(RequestError::Ignored);
+        };
+
+        if current_drag.transfer_id() != id {
+            return Err(RequestError::Ignored);
         }
 
+        let Some(mime_type) = current_drag.find_type_dyn(type_) else {
+            return Err(RequestError::Ignored);
+        };
+
+        let mime_type_str = mime_type.to_string();
+
+        // create a pipe
+        let (readfd, writefd) =
+            pipe::pipe_with(PipeFlags::CLOEXEC | PipeFlags::NONBLOCK).map_err(|e| os_error!(e))?;
+
+        let async_request_serial = AsyncRequestSerial::get();
+
+        let mut buffer = Vec::new();
+        let window_id = current_drag.window_id();
+        let mut mime_type = Some(mime_type.clone());
+
+        let _ = state.loop_handle.insert_source(ReadPipe::from(readfd), move |_, file, state| {
+            // SAFETY: We do not overwrite the referent of `file`
+            let file = unsafe { file.get_mut() };
+
+            let result = match file.read_to_end(&mut buffer) {
+                Err(e) if e.kind() == io::ErrorKind::WouldBlock => {
+                    return PostAction::Continue;
+                },
+                Ok(0) => Ok(mem::take(&mut buffer)),
+                Ok(_) => {
+                    return PostAction::Continue;
+                },
+                Err(e) => Err(Arc::new(e)),
+            };
+
+            state.events_sink.push_window_event(
+                WindowEvent::DataTransferReceived {
+                    id,
+                    serial: async_request_serial,
+                    // `unwrap` is safe here, as we always return `PostAction::Remove` in this
+                    // branch.
+                    value: Arc::new(MimeData::new(mime_type.take().unwrap(), result)),
+                },
+                window_id,
+            );
+
+            PostAction::Remove
+        });
+
+        current_drag.accept(current_drag.serial(), Some(mime_type_str.clone()));
+        data_offer::receive_to_fd(current_drag, mime_type_str, writefd);
+
+        Ok(async_request_serial)
+    }
+
+    fn data_transfer(&self, id: DataTransferId) -> Result<Box<dyn DataTransfer>, RequestError> {
+        let state = self.state.borrow();
+        let Some(state) = state.dnd_state.receive_drag() else {
+            return Err(RequestError::Ignored);
+        };
+
+        if state.transfer_id() != id {
+            return Err(RequestError::Ignored);
+        }
+
+        Ok(Box::new(state.clone()))
+    }
+
+    fn set_valid_dnd_actions(
+        &self,
+        id: DataTransferId,
+        actions: &[DndAction],
+    ) -> Result<(), RequestError> {
+        let state = self.state.borrow();
+        let Some(state) = state.dnd_state.receive_drag() else {
+            return Err(os_error!(UnknownDataTransfer(id)).into());
+        };
+
+        if state.transfer_id() != id {
+            return Err(os_error!(UnknownDataTransfer(id)).into());
+        }
+
+        let any_actions = state.set_actions(actions);
+        let accepted_type =
+            if any_actions { state.first_mime_type().map(|mime| mime.to_string()) } else { None };
+        // Some compositors won't even send the "dropped" event if no type
+        // has been accepted, so we need to accept _something_ here. The
+        // application can accept further types by fetching the data, but
+        // this will at least mean that waiting until the drop to start
+        // fetching data won't prevent the drop from working at all.
+        state.accept(state.serial(), accepted_type);
+
+        Ok(())
+    }
+
+    fn start_drag(
+        &self,
+        source: WindowId,
+        send_data: Box<dyn DataTransferSend>,
+        action_mask: &[DndAction],
+        icon: Option<DragIcon>,
+    ) -> Result<DataTransferId, RequestError> {
+        const NO_POINTER_CAP_ERROR_MSG: &str =
+            "Tried to initiate drag, but source window does not have the pointer capability";
+
+        let mut state = self.state.borrow_mut();
+        let dnd_actions = action_mask
+            .iter()
+            .copied()
+            .map(dnd_action_winit_to_wl)
+            .fold(WlDndAction::empty(), BitOr::bitor);
+
+        let data_device_manager = state
+            .data_device_manager_state
+            .as_ref()
+            .ok_or(NotSupportedError::new("Tried to initiate drag, but data device not enabled"))?;
+
+        let mut mime_types = Vec::new();
+        send_data.for_each_available_type(&mut |ty_| {
+            for mime in MimeType::from_dyn(ty_) {
+                mime_types.push(mime);
+            }
+
+            std::ops::ControlFlow::Continue(())
+        });
+
+        let data_source = data_device_manager.create_drag_and_drop_source(
+            &self.queue_handle,
+            mime_types,
+            dnd_actions,
+        );
+
+        let icon_surface = {
+            let mut pool = state.image_pool.lock().unwrap();
+            icon.and_then(|icon| {
+                let rgba = icon.icon.cast_ref::<RgbaIcon>()?;
+
+                let width = rgba.width().try_into().ok()?;
+                let height = rgba.height().try_into().ok()?;
+
+                let buffer =
+                    image_to_buffer(width, height, rgba.buffer(), Format::Argb8888, &mut pool)
+                        .ok()?;
+
+                let surface = state.compositor_state.create_surface(&self.queue_handle);
+                if surface.version() >= 5 {
+                    buffer.attach_to(&surface).ok()?;
+                    surface.offset(icon.offset_x, icon.offset_y);
+                } else {
+                    surface.attach(Some(buffer.wl_buffer()), icon.offset_x, icon.offset_y);
+                }
+
+                Some(surface)
+            })
+        };
+
+        // New scope to ensure we drop the locks as soon as possible.
+        let transfer_id = {
+            let windows = state.windows.borrow();
+            let source_window_mutex = windows
+                .get(&source)
+                .ok_or(os_error!("Tried to initiate drag, but source window ID was invalid"))?;
+            let source_window_state = source_window_mutex.lock().unwrap();
+            let source_surface = source_window_state.window.wl_surface();
+
+            let seat = source_window_state
+                .focused_seats()
+                .find_map(|seat_id| {
+                    // HACK: How do we get the correct seat for pointers here?
+                    state.seats.get(seat_id).filter(|seat| seat.data_device().is_some())
+                })
+                .ok_or(NotSupportedError::new(NO_POINTER_CAP_ERROR_MSG))?;
+            let data_device =
+                seat.data_device().ok_or(NotSupportedError::new(NO_POINTER_CAP_ERROR_MSG))?;
+
+            let serial = seat
+                .pointer_data()
+                .ok_or(NotSupportedError::new(NO_POINTER_CAP_ERROR_MSG))?
+                .latest_button_serial();
+
+            data_source.start_drag(data_device, source_surface, icon_surface.as_ref(), serial);
+
+            make_data_transfer_id(data_device.inner().id(), serial)
+        };
+
+        // For some reason, if we commit before starting the drag then the offset isn't applied.
+        // This doesn't seem to be documented anywhere, and it's possible that it's a bug in KDE.
+        if let Some(surface) = &icon_surface {
+            surface.commit();
+        }
+
+        state.dnd_state.set_send_drag(DragSource::new(
+            transfer_id,
+            data_source,
+            send_data,
+            icon_surface,
+            source,
+        ));
+
+        Ok(transfer_id)
+    }
+}
+
+/// An operation was attempted on a data transfer ID, but that ID was invalid.
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+pub struct UnknownDataTransfer(pub DataTransferId);
+
+impl fmt::Display for UnknownDataTransfer {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let id = self.0.into_raw();
+        write!(f, "Unknown data transfer with ID {id}")
+    }
+}
+
+impl std::error::Error for UnknownDataTransfer {}
+
 impl ActiveEventLoop {
     fn clear_exit(&self) {
         self.exit.set(None)

winit-wayland/src/lib.rs

@@ -18,13 +18,16 @@
 #![allow(clippy::mutable_key_type)]
 
 use std::ffi::c_void;
+use std::hash::BuildHasher;
 use std::ptr::NonNull;
 
 use dpi::{LogicalSize, PhysicalSize};
 use sctk::reexports::client::Proxy;
+use sctk::reexports::client::backend::ObjectId;
 use sctk::reexports::client::protocol::wl_surface::WlSurface;
 use sctk::shm::slot::{Buffer, CreateBufferError, SlotPool};
 use wayland_client::protocol::wl_shm::Format;
+use winit_core::data_transfer::DataTransferId;
 use winit_core::event_loop::ActiveEventLoop as CoreActiveEventLoop;
 use winit_core::window::{
     ActivationToken, PlatformWindowAttributes, Window as CoreWindow, WindowId,
@@ -34,6 +37,7 @@ macro_rules! os_error {
     ($error:expr) => {{ winit_core::error::OsError::new(line!(), file!(), $error) }};
 }
 
+mod dnd;
 mod event_loop;
 mod output;
 mod seat;
@@ -41,6 +45,7 @@ mod state;
 mod types;
 mod window;
 
+pub use self::dnd::{DataOffer, DragSource, MimeData, MimeType};
 pub use self::event_loop::{ActiveEventLoop, EventLoop};
 pub use self::window::Window;
 
@@ -149,6 +154,17 @@ fn make_wid(surface: &WlSurface) -> WindowId {
     WindowId::from_raw(surface.id().as_ptr() as usize)
 }
 
+/// Create a `DataTransferId` for the given data device and serial.
+///
+/// It's currently unclear if this will result in the same ID when transferring to the same
+/// application.
+#[inline]
+fn make_data_transfer_id(data_device_id: ObjectId, serial: u32) -> DataTransferId {
+    const BUILD_HASHER: foldhash::fast::FixedState = foldhash::fast::FixedState::with_seed(0);
+
+    DataTransferId::from_raw(BUILD_HASHER.hash_one((data_device_id, serial)) as i64)
+}
+
 /// The default routine does floor, but we need round on Wayland.
 fn logical_to_physical_rounded(size: LogicalSize<u32>, scale_factor: f64) -> PhysicalSize<u32> {
     let width = size.width as f64 * scale_factor;

winit-wayland/src/seat/mod.rs

@@ -3,6 +3,7 @@
 use std::sync::Arc;
 
 use foldhash::HashMap;
+use sctk::data_device_manager::data_device::DataDevice;
 use sctk::reexports::client::backend::ObjectId;
 use sctk::reexports::client::protocol::wl_seat::WlSeat;
 use sctk::reexports::client::protocol::wl_touch::WlTouch;
@@ -64,6 +65,9 @@ pub struct WinitSeatState {
     /// The hold pointer gesture bound on the seat.
     pointer_gesture_hold: Option<ZwpPointerGestureHoldV1>,
 
+    /// The drag-and-drop state
+    data_device: Option<DataDevice>,
+
     /// The keyboard bound on the seat.
     keyboard_state: Option<KeyboardState>,
 
@@ -78,6 +82,14 @@ impl WinitSeatState {
     pub fn new() -> Self {
         Default::default()
     }
+
+    pub(crate) fn data_device(&self) -> Option<&DataDevice> {
+        self.data_device.as_ref()
+    }
+
+    pub(crate) fn pointer_data(&self) -> Option<&WinitPointerData> {
+        self.pointer.as_ref().and_then(|pointer| pointer.pointer().data())
+    }
 }
 
 impl SeatHandler for WinitState {
@@ -129,6 +141,11 @@ impl SeatHandler for WinitState {
                     )
                     .expect("failed to create pointer with present capability.");
 
+                seat_state.data_device = self
+                    .data_device_manager_state
+                    .as_ref()
+                    .map(|device| device.get_data_device(queue_handle, &seat));
+
                 seat_state.relative_pointer = self.relative_pointer.as_ref().map(|manager| {
                     manager.get_relative_pointer(
                         themed_pointer.pointer(),
@@ -225,6 +242,8 @@ impl SeatHandler for WinitState {
                     pointer_gesture_hold.destroy();
                 }
 
+                seat_state.data_device = None;
+
                 if let Some(pointer) = seat_state.pointer.take() {
                     let pointer_data = pointer.pointer().winit_data();
 

winit-wayland/src/seat/pointer/mod.rs

@@ -22,19 +22,19 @@ use sctk::reexports::protocols::wp::viewporter::client::wp_viewport::WpViewport;
 
 use sctk::compositor::SurfaceData;
 use sctk::globals::GlobalData;
+use sctk::seat::SeatState;
 use sctk::seat::pointer::{
     PointerData, PointerDataExt, PointerEvent, PointerEventKind, PointerHandler,
 };
-use sctk::seat::SeatState;
 
 use dpi::{LogicalPosition, PhysicalPosition};
 use winit_core::event::{
-    ElementState, MouseButton, MouseScrollDelta, PointerKind, PointerSource, TouchPhase,
-    WindowEvent, ButtonSource,
+    ButtonSource, ElementState, MouseButton, MouseScrollDelta, PointerKind, PointerSource,
+    TouchPhase, WindowEvent,
 };
 
-use crate::state::WinitState;
 use crate::WindowId;
+use crate::state::WinitState;
 
 pub mod pointer_gesture;
 pub mod relative_pointer;

winit-wayland/src/seat/pointer/relative_pointer.rs

@@ -3,8 +3,8 @@
 use std::ops::Deref;
 
 use sctk::reexports::client::globals::{BindError, GlobalList};
-use sctk::reexports::client::{delegate_dispatch, Dispatch};
 use sctk::reexports::client::{Connection, QueueHandle};
+use sctk::reexports::client::{Dispatch, delegate_dispatch};
 use sctk::reexports::protocols::wp::relative_pointer::zv1::{
     client::zwp_relative_pointer_manager_v1::ZwpRelativePointerManagerV1,
     client::zwp_relative_pointer_v1::{self, ZwpRelativePointerV1},
@@ -12,8 +12,8 @@ use sctk::reexports::protocols::wp::relative_pointer::zv1::{
 
 use sctk::globals::GlobalData;
 
-use winit_core::event::DeviceEvent;
 use crate::state::WinitState;
+use winit_core::event::DeviceEvent;
 
 /// Wrapper around the relative pointer.
 #[derive(Debug)]

winit-wayland/src/state.rs

@@ -4,6 +4,7 @@ use std::sync::{Arc, Mutex};
 
 use foldhash::HashMap;
 use sctk::compositor::{CompositorHandler, CompositorState};
+use sctk::data_device_manager::DataDeviceManagerState;
 use sctk::output::{OutputHandler, OutputState};
 use sctk::reexports::calloop::LoopHandle;
 use sctk::reexports::client::backend::ObjectId;
@@ -23,6 +24,7 @@ use sctk::subcompositor::SubcompositorState;
 use winit_core::error::OsError;
 
 use crate::WindowId;
+use crate::dnd::DndState;
 use crate::event_loop::sink::EventSink;
 use crate::output::MonitorHandle;
 use crate::seat::{
@@ -113,12 +115,18 @@ pub struct WinitState {
     /// Viewporter state on the given window.
     pub viewporter_state: Option<ViewporterState>,
 
+    /// Data device manager state on the given window.
+    pub data_device_manager_state: Option<DataDeviceManagerState>,
+
     /// Fractional scaling manager.
     pub fractional_scaling_manager: Option<FractionalScalingManager>,
 
     /// Blur manager.
     pub blur_manager: Option<BgrEffectManager>,
 
+    /// Drag-and-drop state.
+    pub dnd_state: DndState,
+
     /// Loop handle to re-register event sources, such as keyboard repeat.
     pub loop_handle: LoopHandle<'static, Self>,
 
@@ -168,6 +176,17 @@ impl WinitState {
                 (None, None)
             };
 
+        let data_device_manager_state = match DataDeviceManagerState::bind(globals, queue_handle) {
+            Ok(state) => Some(state),
+            Err(e) => {
+                tracing::warn!(
+                    "Data device manager not available, clipboard and drag-and-drop disabled: \
+                     {e:?}"
+                );
+                None
+            },
+        };
+
         let shm = Shm::bind(globals, queue_handle).map_err(|err| os_error!(err))?;
         let image_pool = Arc::new(Mutex::new(SlotPool::new(2, &shm).unwrap()));
 
@@ -191,9 +210,12 @@ impl WinitState {
             window_compositor_updates: Vec::new(),
             window_events_sink: Default::default(),
             viewporter_state,
+            data_device_manager_state,
             fractional_scaling_manager,
             blur_manager: BgrEffectManager::new(globals, queue_handle).ok(),
 
+            dnd_state: Default::default(),
+
             seats,
             text_input_state: TextInputState::new(globals, queue_handle).ok(),
 

winit-wayland/src/window/state.rs

@@ -243,6 +243,12 @@ impl WindowState {
         }
     }
 
+    // HACK: Currently to get the data device to initiate a drag-and-drop, we iterate through all
+    // focused seats to find one with a pointer capability. This is definitely wrong.
+    pub(crate) fn focused_seats(&self) -> impl Iterator<Item = &ObjectId> {
+        self.seat_focus.iter()
+    }
+
     /// Apply closure on the given pointer.
     fn apply_on_pointer<F: FnMut(&ThemedPointer<WinitPointerData>, &WinitPointerData)>(
         &self,

winit-win32/Cargo.toml

@@ -19,6 +19,7 @@ rwh_06.workspace = true
 serde = { workspace = true, optional = true }
 smol_str.workspace = true
 tracing.workspace = true
+url.workspace = true
 winit-core.workspace = true
 
 # Platform-specific
@@ -32,7 +33,9 @@ windows-sys = { workspace = true, features = [
     "Win32_Media",
     "Win32_System_Com_StructuredStorage",
     "Win32_System_Com",
+    "Win32_System_DataExchange",
     "Win32_System_LibraryLoader",
+    "Win32_System_Memory",
     "Win32_System_Ole",
     "Win32_Security",
     "Win32_System_SystemInformation",

winit-win32/src/definitions.rs

@@ -3,8 +3,9 @@
 
 use std::ffi::c_void;
 
-use windows_sys::Win32::Foundation::{HWND, POINTL};
+use windows_sys::Win32::Foundation::{HWND, POINT, POINTL};
 use windows_sys::Win32::System::Com::{FORMATETC, STGMEDIUM};
+use windows_sys::Win32::UI::Shell::SHDRAGIMAGE;
 use windows_sys::core::{BOOL, GUID, HRESULT};
 
 pub type IUnknown = *mut c_void;
@@ -37,7 +38,7 @@ pub struct IDataObjectVtbl {
         pformatetc: *const FORMATETC,
         pmedium: *mut STGMEDIUM,
     ) -> HRESULT,
-    QueryGetData:
+    pub QueryGetData:
         unsafe extern "system" fn(This: *mut IDataObject, pformatetc: *const FORMATETC) -> HRESULT,
     pub GetCanonicalFormatEtc: unsafe extern "system" fn(
         This: *mut IDataObject,
@@ -47,7 +48,7 @@ pub struct IDataObjectVtbl {
     pub SetData: unsafe extern "system" fn(
         This: *mut IDataObject,
         pformatetc: *const FORMATETC,
-        pformatetcOut: *const FORMATETC,
+        pmedium: *const STGMEDIUM,
         fRelease: BOOL,
     ) -> HRESULT,
     pub EnumFormatEtc: unsafe extern "system" fn(
@@ -69,6 +70,81 @@ pub struct IDataObjectVtbl {
     ) -> HRESULT,
 }
 
+#[repr(C)]
+pub struct IEnumFORMATETCVtbl {
+    pub parent: IUnknownVtbl,
+    pub Next: unsafe extern "system" fn(
+        This: *mut IEnumFORMATETC,
+        celt: u32,
+        rgelt: *mut FORMATETC,
+        pceltFetched: *mut u32,
+    ) -> HRESULT,
+    pub Skip: unsafe extern "system" fn(This: *mut IEnumFORMATETC, celt: u32) -> HRESULT,
+    pub Reset: unsafe extern "system" fn(This: *mut IEnumFORMATETC) -> HRESULT,
+    pub Clone: unsafe extern "system" fn(
+        This: *mut IEnumFORMATETC,
+        ppenum: *mut *mut IEnumFORMATETC,
+    ) -> HRESULT,
+}
+
+pub type IDragSourceHelper = *mut c_void;
+
+#[repr(C)]
+pub struct IDragSourceHelperVtbl {
+    pub parent: IUnknownVtbl,
+    pub InitializeFromBitmap: unsafe extern "system" fn(
+        This: *mut IDragSourceHelper,
+        pshdi: *const SHDRAGIMAGE,
+        pDataObject: *mut IDataObject,
+    ) -> HRESULT,
+    pub InitializeFromWindow: unsafe extern "system" fn(
+        This: *mut IDragSourceHelper,
+        hwnd: HWND,
+        ppt: *const POINT,
+        pDataObject: *mut IDataObject,
+    ) -> HRESULT,
+}
+
+pub type IDropTargetHelper = *mut c_void;
+
+#[repr(C)]
+pub struct IDropTargetHelperVtbl {
+    pub parent: IUnknownVtbl,
+    pub DragEnter: unsafe extern "system" fn(
+        This: *mut IDropTargetHelper,
+        hwndTarget: HWND,
+        pDataObject: *mut IDataObject,
+        ppt: *const POINT,
+        dwEffect: u32,
+    ) -> HRESULT,
+    pub DragLeave: unsafe extern "system" fn(This: *mut IDropTargetHelper) -> HRESULT,
+    pub DragOver: unsafe extern "system" fn(
+        This: *mut IDropTargetHelper,
+        ppt: *const POINT,
+        dwEffect: u32,
+    ) -> HRESULT,
+    pub Drop: unsafe extern "system" fn(
+        This: *mut IDropTargetHelper,
+        pDataObject: *mut IDataObject,
+        ppt: *const POINT,
+        dwEffect: u32,
+    ) -> HRESULT,
+    pub Show: unsafe extern "system" fn(This: *mut IDropTargetHelper, fShow: BOOL) -> HRESULT,
+}
+
+pub type IDropSource = *mut c_void;
+
+#[repr(C)]
+pub struct IDropSourceVtbl {
+    pub parent: IUnknownVtbl,
+    pub QueryContinueDrag: unsafe extern "system" fn(
+        This: *mut IDropSource,
+        fEscapePressed: BOOL,
+        grfKeyState: u32,
+    ) -> HRESULT,
+    pub GiveFeedback: unsafe extern "system" fn(This: *mut IDropSource, dwEffect: u32) -> HRESULT,
+}
+
 #[repr(C)]
 pub struct IDropTargetVtbl {
     pub parent: IUnknownVtbl,
@@ -130,6 +206,22 @@ pub struct ITaskbarList2 {
     pub lpVtbl: *const ITaskbarList2Vtbl,
 }
 
+/// Defined in `objidl.h`.
+pub const IID_IDataObject: GUID = GUID::from_u128(0x0000010e_0000_0000_c000_000000000046);
+
+/// Defined in `oleidl.h`.
+pub const IID_IDropSource: GUID = GUID::from_u128(0x00000121_0000_0000_c000_000000000046);
+
+/// Defined in `objidl.h`.
+pub const IID_IEnumFORMATETC: GUID = GUID::from_u128(0x00000103_0000_0000_c000_000000000046);
+
+/// Defined in `shobjidl_core.h`.
+pub const IID_IDragSourceHelper: GUID = GUID::from_u128(0xde5bf786_477a_11d2_839d_00c04fd918d0);
+
+/// Defined in `shobjidl_core.h`.
+pub const IID_IDropTargetHelper: GUID = GUID::from_u128(0x4657278b_411b_11d2_839a_00c04fd918d0);
+
+/// Defined in `shobjidl_core.h`.
 pub const CLSID_TaskbarList: GUID = GUID {
     data1: 0x56fdf344,
     data2: 0xfd6d,
@@ -137,6 +229,7 @@ pub const CLSID_TaskbarList: GUID = GUID {
     data4: [0x95, 0x8a, 0x00, 0x60, 0x97, 0xc9, 0xa0, 0x90],
 };
 
+/// Defined in `shobjidl_core.h`.
 pub const IID_ITaskbarList: GUID = GUID {
     data1: 0x56fdf342,
     data2: 0xfd6d,
@@ -144,6 +237,7 @@ pub const IID_ITaskbarList: GUID = GUID {
     data4: [0x95, 0x8a, 0x00, 0x60, 0x97, 0xc9, 0xa0, 0x90],
 };
 
+/// Defined in `shobjidl_core.h`.
 pub const IID_ITaskbarList2: GUID = GUID {
     data1: 0x602d4995,
     data2: 0xb13a,

winit-win32/src/drop_handler.rs

@@ -1,240 +0,0 @@
-use std::ffi::{OsString, c_void};
-use std::os::windows::ffi::OsStringExt;
-use std::path::PathBuf;
-use std::ptr;
-use std::sync::atomic::{AtomicUsize, Ordering};
-
-use dpi::PhysicalPosition;
-use tracing::debug;
-use windows_sys::Win32::Foundation::{DV_E_FORMATETC, HWND, POINT, POINTL, S_OK};
-use windows_sys::Win32::Graphics::Gdi::ScreenToClient;
-use windows_sys::Win32::System::Com::{DVASPECT_CONTENT, FORMATETC, TYMED_HGLOBAL};
-use windows_sys::Win32::System::Ole::{CF_HDROP, DROPEFFECT_COPY, DROPEFFECT_NONE};
-use windows_sys::Win32::UI::Shell::{DragFinish, DragQueryFileW, HDROP};
-use windows_sys::core::{GUID, HRESULT};
-use winit_core::event::WindowEvent;
-
-use crate::definitions::{
-    IDataObject, IDataObjectVtbl, IDropTarget, IDropTargetVtbl, IUnknown, IUnknownVtbl,
-};
-
-#[repr(C)]
-pub struct FileDropHandlerData {
-    pub interface: IDropTarget,
-    refcount: AtomicUsize,
-    window: HWND,
-    send_event: Box<dyn Fn(WindowEvent)>,
-    cursor_effect: u32,
-    valid: bool, /* If the currently hovered item is not valid there must not be any
-                  * `DragLeft` emitted */
-}
-
-pub struct FileDropHandler {
-    pub data: *mut FileDropHandlerData,
-}
-
-#[allow(non_snake_case)]
-impl FileDropHandler {
-    pub(crate) fn new(window: HWND, send_event: Box<dyn Fn(WindowEvent)>) -> FileDropHandler {
-        let data = Box::new(FileDropHandlerData {
-            interface: IDropTarget { lpVtbl: &DROP_TARGET_VTBL as *const IDropTargetVtbl },
-            refcount: AtomicUsize::new(1),
-            window,
-            send_event,
-            cursor_effect: DROPEFFECT_NONE,
-            valid: false,
-        });
-        FileDropHandler { data: Box::into_raw(data) }
-    }
-
-    // Implement IUnknown
-    pub unsafe extern "system" fn QueryInterface(
-        _this: *mut IUnknown,
-        _riid: *const GUID,
-        _ppvObject: *mut *mut c_void,
-    ) -> HRESULT {
-        // This function doesn't appear to be required for an `IDropTarget`.
-        // An implementation would be nice however.
-        unimplemented!();
-    }
-
-    pub unsafe extern "system" fn AddRef(this: *mut IUnknown) -> u32 {
-        let drop_handler_data = unsafe { Self::from_interface(this) };
-        let count = drop_handler_data.refcount.fetch_add(1, Ordering::Release) + 1;
-        count as u32
-    }
-
-    pub unsafe extern "system" fn Release(this: *mut IUnknown) -> u32 {
-        let drop_handler = unsafe { Self::from_interface(this) };
-        let count = drop_handler.refcount.fetch_sub(1, Ordering::Release) - 1;
-        if count == 0 {
-            // Destroy the underlying data
-            drop(unsafe { Box::from_raw(drop_handler as *mut FileDropHandlerData) });
-        }
-        count as u32
-    }
-
-    pub unsafe extern "system" fn DragEnter(
-        this: *mut IDropTarget,
-        pDataObj: *const IDataObject,
-        _grfKeyState: u32,
-        pt: POINTL,
-        pdwEffect: *mut u32,
-    ) -> HRESULT {
-        let drop_handler = unsafe { Self::from_interface(this) };
-        let mut pt = POINT { x: pt.x, y: pt.y };
-        unsafe {
-            ScreenToClient(drop_handler.window, &mut pt);
-        }
-        let position = PhysicalPosition::new(pt.x as f64, pt.y as f64);
-        let mut paths = Vec::new();
-        let hdrop = unsafe { Self::iterate_filenames(pDataObj, |path| paths.push(path)) };
-        drop_handler.valid = hdrop.is_some();
-        if drop_handler.valid {
-            (drop_handler.send_event)(WindowEvent::DragEntered { paths, position });
-        }
-        drop_handler.cursor_effect =
-            if drop_handler.valid { DROPEFFECT_COPY } else { DROPEFFECT_NONE };
-        unsafe {
-            *pdwEffect = drop_handler.cursor_effect;
-        }
-
-        S_OK
-    }
-
-    pub unsafe extern "system" fn DragOver(
-        this: *mut IDropTarget,
-        _grfKeyState: u32,
-        pt: POINTL,
-        pdwEffect: *mut u32,
-    ) -> HRESULT {
-        let drop_handler = unsafe { Self::from_interface(this) };
-        if drop_handler.valid {
-            let mut pt = POINT { x: pt.x, y: pt.y };
-            unsafe {
-                ScreenToClient(drop_handler.window, &mut pt);
-            }
-            let position = PhysicalPosition::new(pt.x as f64, pt.y as f64);
-            (drop_handler.send_event)(WindowEvent::DragMoved { position });
-        }
-        unsafe {
-            *pdwEffect = drop_handler.cursor_effect;
-        }
-
-        S_OK
-    }
-
-    pub unsafe extern "system" fn DragLeave(this: *mut IDropTarget) -> HRESULT {
-        let drop_handler = unsafe { Self::from_interface(this) };
-        if drop_handler.valid {
-            (drop_handler.send_event)(WindowEvent::DragLeft { position: None });
-        }
-
-        S_OK
-    }
-
-    pub unsafe extern "system" fn Drop(
-        this: *mut IDropTarget,
-        pDataObj: *const IDataObject,
-        _grfKeyState: u32,
-        pt: POINTL,
-        pdwEffect: *mut u32,
-    ) -> HRESULT {
-        let drop_handler = unsafe { Self::from_interface(this) };
-        if drop_handler.valid {
-            let mut pt = POINT { x: pt.x, y: pt.y };
-            unsafe {
-                ScreenToClient(drop_handler.window, &mut pt);
-            }
-            let position = PhysicalPosition::new(pt.x as f64, pt.y as f64);
-            let mut paths = Vec::new();
-            let hdrop = unsafe { Self::iterate_filenames(pDataObj, |path| paths.push(path)) };
-            (drop_handler.send_event)(WindowEvent::DragDropped { paths, position });
-            if let Some(hdrop) = hdrop {
-                unsafe {
-                    DragFinish(hdrop);
-                }
-            }
-        }
-        unsafe {
-            *pdwEffect = drop_handler.cursor_effect;
-        }
-
-        S_OK
-    }
-
-    unsafe fn from_interface<'a, InterfaceT>(this: *mut InterfaceT) -> &'a mut FileDropHandlerData {
-        unsafe { &mut *(this as *mut _) }
-    }
-
-    unsafe fn iterate_filenames<F>(data_obj: *const IDataObject, mut callback: F) -> Option<HDROP>
-    where
-        F: FnMut(PathBuf),
-    {
-        let drop_format = FORMATETC {
-            cfFormat: CF_HDROP,
-            ptd: ptr::null_mut(),
-            dwAspect: DVASPECT_CONTENT,
-            lindex: -1,
-            tymed: TYMED_HGLOBAL as u32,
-        };
-
-        let mut medium = unsafe { std::mem::zeroed() };
-        let get_data_fn = unsafe { (*(*data_obj).cast::<IDataObjectVtbl>()).GetData };
-        let get_data_result = unsafe { get_data_fn(data_obj as *mut _, &drop_format, &mut medium) };
-        if get_data_result >= 0 {
-            let hdrop = unsafe { medium.u.hGlobal as HDROP };
-
-            // The second parameter (0xFFFFFFFF) instructs the function to return the item count
-            let item_count = unsafe { DragQueryFileW(hdrop, 0xffffffff, ptr::null_mut(), 0) };
-
-            for i in 0..item_count {
-                // Get the length of the path string NOT including the terminating null character.
-                // Previously, this was using a fixed size array of MAX_PATH length, but the
-                // Windows API allows longer paths under certain circumstances.
-                let character_count =
-                    unsafe { DragQueryFileW(hdrop, i, ptr::null_mut(), 0) as usize };
-                let str_len = character_count + 1;
-
-                // Fill path_buf with the null-terminated file name
-                let mut path_buf = Vec::with_capacity(str_len);
-                unsafe {
-                    DragQueryFileW(hdrop, i, path_buf.as_mut_ptr(), str_len as u32);
-                    path_buf.set_len(str_len);
-                }
-
-                callback(OsString::from_wide(&path_buf[0..character_count]).into());
-            }
-
-            Some(hdrop)
-        } else if get_data_result == DV_E_FORMATETC {
-            // If the dropped item is not a file this error will occur.
-            // In this case it is OK to return without taking further action.
-            debug!("Error occurred while processing dropped/hovered item: item is not a file.");
-            None
-        } else {
-            debug!("Unexpected error occurred while processing dropped/hovered item.");
-            None
-        }
-    }
-}
-
-impl Drop for FileDropHandler {
-    fn drop(&mut self) {
-        unsafe {
-            FileDropHandler::Release(self.data as *mut IUnknown);
-        }
-    }
-}
-
-static DROP_TARGET_VTBL: IDropTargetVtbl = IDropTargetVtbl {
-    parent: IUnknownVtbl {
-        QueryInterface: FileDropHandler::QueryInterface,
-        AddRef: FileDropHandler::AddRef,
-        Release: FileDropHandler::Release,
-    },
-    DragEnter: FileDropHandler::DragEnter,
-    DragOver: FileDropHandler::DragOver,
-    DragLeave: FileDropHandler::DragLeave,
-    Drop: FileDropHandler::Drop,
-};

winit-win32/src/event_loop.rs

@@ -63,6 +63,9 @@ use windows_sys::Win32::UI::WindowsAndMessaging::{
 };
 use winit_core::application::ApplicationHandler;
 use winit_core::cursor::{CustomCursor, CustomCursorSource};
+use winit_core::data_transfer::{
+    DataTransfer, DataTransferId, DataTransferSend, TransferType, TypedData,
+};
 use winit_core::error::{EventLoopError, NotSupportedError, RequestError};
 use winit_core::event::{
     DeviceEvent, DeviceId, FingerId, Force, Ime, RawKeyEvent, SurfaceSizeWriter, TabletToolButton,
@@ -70,8 +73,8 @@ use winit_core::event::{
 };
 use winit_core::event_loop::pump_events::PumpStatus;
 use winit_core::event_loop::{
-    ActiveEventLoop as RootActiveEventLoop, ControlFlow, DeviceEvents,
-    EventLoopProxy as RootEventLoopProxy, EventLoopProxyProvider,
+    ActiveEventLoop as RootActiveEventLoop, AsyncRequestSerial, ControlFlow, DeviceEvents,
+    DndAction, DragIcon, EventLoopProxy as RootEventLoopProxy, EventLoopProxyProvider,
     OwnedDisplayHandle as CoreOwnedDisplayHandle,
 };
 use winit_core::keyboard::ModifiersState;
@@ -82,8 +85,9 @@ pub(super) use self::runner::{Event, EventLoopRunner};
 use super::SelectedCursor;
 use super::window::set_skip_taskbar;
 use crate::dark_mode::try_theme;
+use crate::dnd::{DropSource, FileDropHandler, SourceDataObject, WinDataTransfer, WinTypedData};
 use crate::dpi::{become_dpi_aware, dpi_to_scale_factor};
-use crate::drop_handler::FileDropHandler;
+use crate::event_loop::runner::PendingDrag;
 use crate::icon::WinCursor;
 use crate::ime::ImeContext;
 use crate::keyboard::KeyEventBuilder;
@@ -478,6 +482,105 @@ impl RootActiveEventLoop for ActiveEventLoop {
     fn rwh_06_handle(&self) -> &dyn rwh_06::HasDisplayHandle {
         self
     }
+
+    fn fetch_data_transfer(
+        &self,
+        id: DataTransferId,
+        type_: &dyn TransferType,
+    ) -> Result<AsyncRequestSerial, RequestError> {
+        let Some(state) = self.0.drag_state(id) else {
+            return Err(os_error!(UnknownDataTransfer(id)).into());
+        };
+        let hint = type_.hint().ok_or(RequestError::Ignored)?;
+        let typed_data = WinTypedData::new(state.data.clone(), hint)
+            .map(|value| Arc::new(value) as Arc<dyn TypedData>)
+            .ok_or(RequestError::Ignored)?;
+
+        let serial = AsyncRequestSerial::get();
+
+        self.0.send_event(Event::Window {
+            window_id: state.window_id,
+            event: WindowEvent::DataTransferReceived { id, serial, value: typed_data },
+        });
+
+        Ok(serial)
+    }
+
+    fn data_transfer(&self, id: DataTransferId) -> Result<Box<dyn DataTransfer>, RequestError> {
+        let Some(state) = self.0.drag_state(id) else {
+            return Err(os_error!(UnknownDataTransfer(id)).into());
+        };
+
+        Ok(Box::new(WinDataTransfer::new(state.data.clone())))
+    }
+
+    fn set_valid_dnd_actions(
+        &self,
+        id: DataTransferId,
+        actions: &[DndAction],
+    ) -> Result<(), RequestError> {
+        let mut state = self.0.drag_state.borrow_mut();
+        let Some(state) = state.as_mut().filter(|s| s.id == id) else {
+            return Err(os_error!(UnknownDataTransfer(id)).into());
+        };
+        state.actions = actions.to_vec();
+        Ok(())
+    }
+
+    fn start_drag(
+        &self,
+        source: WindowId,
+        send_data: Box<dyn DataTransferSend>,
+        allowed_actions: &[DndAction],
+        icon: Option<DragIcon>,
+    ) -> Result<DataTransferId, RequestError> {
+        let allowed_effects = crate::dnd::dnd_actions_to_dropeffect_mask(allowed_actions);
+        // Win32 would happily run a modal `DoDragDrop` with `allowed_effects == 0`, but every
+        // target would see "no action allowed" and the drag would end in a guaranteed cancel
+        // after burning a full modal pump. Fail fast instead - the caller asked for a drag
+        // they explicitly refuse to allow.
+        if allowed_effects == 0 {
+            return Err(
+                NotSupportedError::new("start_drag called with an empty action mask").into()
+            );
+        }
+
+        let id = crate::dnd::next_data_transfer_id();
+        let data_object = SourceDataObject::new(send_data);
+        let drop_source = DropSource::new();
+
+        // Attach a drag preview if the app supplied one. Cosmetic failures must not abort the
+        // drag - the gesture still works, just without a custom image - so log and move on.
+        if let Some(icon) = icon {
+            if let Some(rgba) = icon.icon.cast_ref::<winit_core::icon::RgbaIcon>() {
+                let result = unsafe {
+                    crate::dnd::apply_drag_image(
+                        data_object.interface_ptr() as *mut _,
+                        rgba.width(),
+                        rgba.height(),
+                        rgba.buffer(),
+                        icon.offset_x,
+                        icon.offset_y,
+                    )
+                };
+                if let Err(hr) = result {
+                    tracing::warn!("Failed to attach drag image: hr=0x{hr:08x}");
+                }
+            } else {
+                tracing::warn!("DragIcon::icon must be an RgbaIcon on win32; ignoring");
+            }
+        }
+
+        self.0.pending_drag.replace(Some(PendingDrag {
+            window_id: source,
+            id,
+            data_object,
+            drop_source,
+            allowed_effects,
+        }));
+
+        Ok(id)
+    }
 }
 
 impl rwh_06::HasDisplayHandle for ActiveEventLoop {
@@ -487,6 +590,19 @@ impl rwh_06::HasDisplayHandle for ActiveEventLoop {
     }
 }
 
+/// An operation was attempted on a data transfer ID, but that ID was invalid.
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+pub struct UnknownDataTransfer(pub DataTransferId);
+
+impl fmt::Display for UnknownDataTransfer {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let id = self.0.into_raw();
+        write!(f, "Unknown data transfer with ID {id}")
+    }
+}
+
+impl std::error::Error for UnknownDataTransfer {}
+
 #[derive(Clone)]
 pub(crate) struct OwnedDisplayHandle;
 
@@ -1177,6 +1293,14 @@ unsafe fn public_window_callback_inner(
             result = ProcResult::Value(0);
         },
 
+        WM_PAINT if userdata.event_loop_runner.source_drag.get().is_some() => {
+            // While a source-side drag is in flight, the app handler is on the stack (we're
+            // inside `start_drag` -> `DoDragDrop`), so we can neither dispatch `RedrawRequested`
+            // nor keep re-arming via `RDW_INTERNALPAINT` (that would spin in OLE's modal loop).
+            // Let `DefWindowProcW` validate the region and show stale content for the duration of
+            // the drag; the next real paint happens once `DoDragDrop` returns.
+            result = ProcResult::Value(unsafe { DefWindowProcW(window, msg, wparam, lparam) });
+        },
         WM_PAINT => {
             userdata.window_state_lock().redraw_requested =
                 userdata.event_loop_runner.should_buffer();
@@ -2352,6 +2476,13 @@ unsafe fn public_window_callback_inner(
         .catch_unwind(callback)
         .unwrap_or_else(|| result = ProcResult::Value(-1));
 
+    // We execute a new drag operation here instead of immediately starting it in
+    // `ActiveEventLoop::start_drag`. `DoDragDrop` is blocking and synchronous, so if we started
+    // it inside the event loop then an internal drag operation would be re-entrant and the
+    // application would not be able to handle the incoming messages. This is after the application
+    // has had a chance to handle mouse events.
+    userdata.event_loop_runner.try_execute_drag_drop();
+
     match result {
         ProcResult::DefWindowProc(wparam) => unsafe { DefWindowProcW(window, msg, wparam, lparam) },
         ProcResult::Value(val) => val,

winit-win32/src/event_loop/runner.rs

@@ -1,5 +1,5 @@
 use std::any::Any;
-use std::cell::{Cell, RefCell};
+use std::cell::{Cell, Ref, RefCell};
 use std::collections::VecDeque;
 use std::rc::Rc;
 use std::sync::{Arc, Mutex};
@@ -7,18 +7,49 @@ use std::time::Instant;
 use std::{fmt, mem, panic};
 
 use dpi::PhysicalSize;
-use windows_sys::Win32::Foundation::HWND;
+use windows_sys::Win32::Foundation::{DRAGDROP_S_CANCEL, DRAGDROP_S_DROP, HWND};
+use windows_sys::Win32::System::Ole::{
+    DROPEFFECT_COPY, DROPEFFECT_LINK, DROPEFFECT_MOVE, DROPEFFECT_NONE, DoDragDrop,
+};
 use winit_core::application::ApplicationHandler;
+use winit_core::data_transfer::DataTransferId;
 use winit_core::event::{DeviceEvent, DeviceId, StartCause, SurfaceSizeWriter, WindowEvent};
-use winit_core::event_loop::ActiveEventLoop as RootActiveEventLoop;
+use winit_core::event_loop::{ActiveEventLoop as RootActiveEventLoop, DndAction};
 use winit_core::window::WindowId;
 
 use super::{ActiveEventLoop, ControlFlow, EventLoopThreadExecutor};
+use crate::dnd::{DataObject, DropEffect, DropSource, SourceDataObject, drop_effect_to_dnd_action};
 use crate::event_loop::{GWL_USERDATA, WindowData};
 use crate::util::get_window_long;
 
 type EventHandler = Cell<Option<&'static mut (dyn ApplicationHandler + 'static)>>;
 
+/// State for the single drag-and-drop transfer currently in flight (OLE guarantees at most one
+/// active drag per process).
+#[derive(Debug)]
+pub(super) struct DragState {
+    pub(super) id: DataTransferId,
+    pub(super) window_id: WindowId,
+    pub(super) data: Arc<DataObject>,
+    pub(super) actions: Vec<DndAction>,
+}
+
+pub(super) struct PendingDrag {
+    pub(super) window_id: WindowId,
+    pub(super) data_object: SourceDataObject,
+    pub(super) drop_source: DropSource,
+    pub(super) allowed_effects: DropEffect,
+    pub(super) id: DataTransferId,
+}
+
+/// Set while `DoDragDrop` is on the call stack - i.e., this process is the source of an active
+/// drag. The target-side `IDropTarget` checks this to recognize self-drops and reuse the source's
+/// id + allowed actions instead of waiting for the (buffered) app `DragEntered` handler.
+#[derive(Copy, Clone)]
+pub(crate) struct SourceDrag {
+    pub(crate) id: DataTransferId,
+}
+
 pub(crate) struct EventLoopRunner {
     pub(super) thread_id: u32,
 
@@ -37,6 +68,29 @@ pub(crate) struct EventLoopRunner {
     event_handler: Rc<EventHandler>,
     event_buffer: RefCell<VecDeque<Event>>,
 
+    /// The currently in-flight drag transfer, if any, alive between `DragEntered` and
+    /// `DragLeft`/`DragDropped`.
+    pub(super) drag_state: RefCell<Option<DragState>>,
+
+    /// `Some(_)` while `start_drag` has `DoDragDrop` on the call stack.
+    pub(crate) source_drag: Cell<Option<SourceDrag>>,
+
+    /// `DoDragDrop` is blocking and synchronous, so we wait until after the application returns
+    /// control to winit before actually calling into the OS to initiate the drag. This prevents
+    /// the event loop from being re-entrant if we are doing an internal drag operation, since if
+    /// we handled this inside `ActiveEventLoop::start_drag` then all the `WindowEvent::Drag*`
+    /// events would be buffered until `DoDragDrop` returns, preventing the application from
+    /// handling those messages.
+    pub(super) pending_drag: RefCell<Option<PendingDrag>>,
+
+    /// For self-drops, target-side `IDropTarget::Drop` can't release the cached `DragState`
+    /// before its `DragDropped` `WindowEvent` is delivered - the event is buffered (the outer app
+    /// handler holds `event_handler` for the duration of `DoDragDrop`) and `data_transfer(id)`
+    /// would return `UnknownDataTransfer` if cleanup ran synchronously. So we stash the id here
+    /// and drain it at the end of `dispatch_buffered_events`, after the app's buffered handler
+    /// has had its chance to read the data.
+    pending_source_drag_cleanup: Cell<Option<DataTransferId>>,
+
     panic_error: Cell<Option<PanicError>>,
 }
 
@@ -87,9 +141,122 @@ impl EventLoopRunner {
             last_events_cleared: Cell::new(Instant::now()),
             event_handler: Rc::new(Cell::new(None)),
             event_buffer: RefCell::new(VecDeque::new()),
+            drag_state: RefCell::new(None),
+            source_drag: Cell::new(None),
+            pending_drag: RefCell::new(None),
+            pending_source_drag_cleanup: Cell::new(None),
         }
     }
 
+    pub(super) fn try_execute_drag_drop(self: &Rc<Self>) {
+        let Some(PendingDrag { data_object, drop_source, id, allowed_effects, window_id }) =
+            self.pending_drag.take()
+        else {
+            return;
+        };
+
+        // Make the drag visible to our own target-side `IDropTarget` so it can recognize
+        // self-drops and reuse this id + action mask without going through the (buffered) app
+        // handler. The guard ensures the flag is cleared on any exit path - if anything between
+        // here and `DoDragDrop`'s return panics, the stale flag would otherwise permanently
+        // disable `WM_PAINT` dispatch and misclassify all future external drags as self-drops.
+        struct ClearOnDrop<'a>(&'a Cell<Option<SourceDrag>>);
+        impl Drop for ClearOnDrop<'_> {
+            fn drop(&mut self) {
+                self.0.set(None);
+            }
+        }
+        self.source_drag.set(Some(SourceDrag { id }));
+        let _guard = ClearOnDrop(&self.source_drag);
+
+        let mut effect_out: u32 = DROPEFFECT_NONE;
+        let hr = unsafe {
+            DoDragDrop(
+                data_object.interface_ptr(),
+                drop_source.interface_ptr(),
+                allowed_effects,
+                &mut effect_out,
+            )
+        };
+
+        if hr == DRAGDROP_S_DROP {
+            let action = drop_effect_to_dnd_action(effect_out);
+
+            self.send_event(Event::Window {
+                window_id,
+                event: WindowEvent::OutgoingDragDropped { id, action },
+            });
+        } else if hr == DRAGDROP_S_CANCEL {
+            self.send_event(Event::Window {
+                window_id,
+                event: WindowEvent::OutgoingDragCanceled { id },
+            });
+        } else {
+            tracing::error!("DoDragDrop failed: 0x{hr:08x}");
+            return;
+        }
+
+        // Both `DRAGDROP_S_DROP` and `DRAGDROP_S_CANCEL` are success codes for us - the app
+        // will hear about the outcome via the buffered `DragDropped`/`DragLeft` events
+        // (target-side translates `effect_out == DROPEFFECT_NONE` to `DragLeft`).
+        // Log the negotiated effect so cross-process drops, which have no target-side event
+        // in this process, leave a debuggable trace of what action the remote target performed.
+        tracing::trace!(
+            "DoDragDrop completed: hr=0x{hr:08x} effect_out={effect_out} (COPY={DROPEFFECT_COPY}, \
+             MOVE={DROPEFFECT_MOVE}, LINK={DROPEFFECT_LINK})",
+        );
+    }
+
+    pub(crate) fn defer_source_drag_cleanup(&self, id: DataTransferId) {
+        self.pending_source_drag_cleanup.set(Some(id));
+    }
+
+    pub(crate) fn register_data_transfer(
+        &self,
+        id: DataTransferId,
+        window_id: WindowId,
+        data: Arc<DataObject>,
+    ) {
+        // By default, no actions have been set as valid by the target.
+        *self.drag_state.borrow_mut() =
+            Some(DragState { id, window_id, data, actions: Default::default() });
+    }
+
+    pub(crate) fn remove_data_transfer(&self, id: DataTransferId) {
+        let mut state = self.drag_state.borrow_mut();
+        if state.as_ref().is_some_and(|s| s.id == id) {
+            *state = None;
+        }
+    }
+
+    pub(super) fn drag_state(&self, id: DataTransferId) -> Option<Ref<'_, DragState>> {
+        Ref::filter_map(self.drag_state.borrow(), |state| state.as_ref().filter(|s| s.id == id))
+            .ok()
+    }
+
+    pub(crate) fn current_drag_actions(&self, id: DataTransferId) -> Ref<'_, [DndAction]> {
+        Ref::map(self.drag_state.borrow(), |state| {
+            state.as_ref().filter(|s| s.id == id).map(|s| &s.actions[..]).unwrap_or_default()
+        })
+    }
+
+    pub(crate) fn proposed_dnd_action(
+        &self,
+        id: DataTransferId,
+        effects: DropEffect,
+    ) -> Option<DndAction> {
+        self.current_drag_actions(id).iter().copied().find(|action| {
+            let effect = match action {
+                DndAction::Move => DROPEFFECT_MOVE,
+                DndAction::Copy => DROPEFFECT_COPY,
+                DndAction::Link => DROPEFFECT_LINK,
+                _ => return false,
+            };
+
+            (effects & effect) != 0
+        })
+    }
+
     /// Associate the application's event handler with the runner.
     ///
     /// # Safety
@@ -138,12 +305,20 @@ impl EventLoopRunner {
             last_events_cleared: _,
             event_handler,
             event_buffer: _,
+            drag_state,
+            source_drag,
+            pending_drag,
+            pending_source_drag_cleanup,
         } = self;
         interrupt_msg_dispatch.set(false);
         runner_state.set(RunnerState::Uninitialized);
         panic_error.set(None);
         exit.set(None);
         event_handler.set(None);
+        drag_state.take();
+        source_drag.set(None);
+        pending_drag.take();
+        pending_source_drag_cleanup.set(None);
     }
 }
 
@@ -285,6 +460,11 @@ impl EventLoopRunner {
                 None => break,
             }
         }
+        // The app's buffered `DragDropped` handler (if any) has now had its chance to call
+        // `data_transfer(id)`; safe to release the cached `DragState` for a deferred self-drop.
+        if let Some(id) = self.pending_source_drag_cleanup.take() {
+            self.remove_data_transfer(id);
+        }
     }
 
     /// Dispatch control flow events (`NewEvents`, `AboutToWait`, and

winit-win32/src/lib.rs

@@ -8,8 +8,8 @@
 mod util;
 mod dark_mode;
 mod definitions;
+mod dnd;
 mod dpi;
-mod drop_handler;
 mod event_loop;
 mod icon;
 mod ime;

winit-win32/src/window.rs

@@ -61,8 +61,8 @@ use crate::dark_mode::try_theme;
 use crate::definitions::{
     CLSID_TaskbarList, IID_ITaskbarList, IID_ITaskbarList2, ITaskbarList, ITaskbarList2,
 };
+use crate::dnd::FileDropHandler;
 use crate::dpi::{dpi_to_scale_factor, enable_non_client_dpi_scaling, hwnd_dpi};
-use crate::drop_handler::FileDropHandler;
 use crate::event_loop::{self, ActiveEventLoop, DESTROY_MSG_ID, Event, EventLoopRunner};
 use crate::icon::{IconType, WinCursor};
 use crate::ime::ImeContext;
@@ -1191,6 +1191,7 @@ impl InitData<'_> {
         let window_state = {
             let window_state = WindowState::new(
                 &self.attributes,
+                &self.win_attributes,
                 scale_factor,
                 current_theme,
                 self.attributes.preferred_theme,
@@ -1230,15 +1231,16 @@ impl InitData<'_> {
 
             let file_drop_runner = self.runner.clone();
             let window_id = win.id();
-            let file_drop_handler = FileDropHandler::new(
+            let mut file_drop_handler = FileDropHandler::new(
                 win.window.hwnd(),
+                self.runner.clone(),
                 Box::new(move |event| {
                     file_drop_runner.send_event(Event::Window { window_id, event })
                 }),
             );
 
             let handler_interface_ptr =
-                unsafe { &mut (*file_drop_handler.data).interface as *mut _ as *mut c_void };
+                unsafe { file_drop_handler.interface_unchecked_mut() as *mut _ as *mut c_void };
 
             assert_eq!(unsafe { RegisterDragDrop(win.window.hwnd(), handler_interface_ptr) }, S_OK);
             Some(file_drop_handler)

winit-win32/src/window_state.rs

@@ -22,7 +22,7 @@ use winit_core::keyboard::ModifiersState;
 use winit_core::monitor::Fullscreen;
 use winit_core::window::{ImeCapabilities, Theme, WindowAttributes};
 
-use crate::{SelectedCursor, event_loop, util};
+use crate::{SelectedCursor, WindowAttributesWindows, event_loop, util};
 
 /// Contains information about states and the window that the callback is going to use.
 #[derive(Debug)]
@@ -154,6 +154,7 @@ pub enum ImeState {
 impl WindowState {
     pub(crate) fn new(
         attributes: &WindowAttributes,
+        _win_attributes: &WindowAttributesWindows,
         scale_factor: f64,
         current_theme: Theme,
         preferred_theme: Option<Theme>,

winit-x11/src/atoms.rs

@@ -13,7 +13,7 @@ macro_rules! atom_manager {
 
         /// Indices into the `Atoms` struct.
         #[derive(Copy, Clone, Debug)]
-        #[allow(non_camel_case_types)]
+        #[allow(non_camel_case_types, clippy::upper_case_acronyms)]
         pub enum AtomName {
             $($name,)*
         }
@@ -34,6 +34,7 @@ macro_rules! atom_manager {
 atom_manager! {
     // General Use Atoms
     CARD32,
+    STRING,
     UTF8_STRING,
     WM_CHANGE_STATE,
     WM_CLIENT_MACHINE,
@@ -87,11 +88,39 @@ atom_manager! {
     XdndDrop,
     XdndPosition,
     XdndStatus,
-    XdndActionPrivate,
+    XdndActionCopy,
     XdndSelection,
     XdndFinished,
     XdndTypeList,
+
+    // MIME types for reading selections
     TextUriList: b"text/uri-list",
+    TextPlain: b"text/plain",
+    TextPlainCharsetUtf8: b"text/plain;charset=utf-8",
+    TextHtml: b"text/html",
+    TextHtmlCharsetUtf8: b"text/html;charset=utf-8",
+    ApplicationRtf: b"application/rtf",
+    AudioAac: b"audio/aac",
+    AudioAiff: b"audio/aiff",
+    AudioFlac: b"audio/flac",
+    AudioWav: b"audio/wav",
+    AudioWave: b"audio/wave",
+    AudioXWav: b"audio/x-wav",
+    AudioVndWav: b"audio/vnd.wav",
+    AudioVndWave: b"audio/vnd.wave",
+    AudioMpeg: b"audio/mpeg",
+    AudioOgg: b"audio/ogg",
+    ImageBmp: b"image/bmp",
+    ImageGif: b"image/gif",
+    ImageJpeg: b"image/jpeg",
+    ImagePjpeg: b"image/pjpeg",
+    ImagePng: b"image/png",
+    ImageSvg: b"image/svg+xml",
+    ImageTiff: b"image/tiff",
+    ImageWebp: b"image/webp",
+    ImageXIcon: b"image/x-icon",
+    ImageRaw: b"image/x-panasonic-raw",
+
     None: b"None",
 
     // Miscellaneous Atoms

winit-x11/src/dnd.rs

@@ -1,18 +1,19 @@
+use std::collections::VecDeque;
 use std::io;
 use std::os::raw::*;
-use std::path::{Path, PathBuf};
 use std::str::Utf8Error;
 use std::sync::Arc;
+use std::sync::atomic::{AtomicI64, Ordering};
 
-use dpi::PhysicalPosition;
-use percent_encoding::percent_decode;
+use winit_core::data_transfer::{DataTransfer, DataTransferId, TransferType, TypeHint, TypedData};
+use winit_core::event_loop::AsyncRequestSerial;
 use x11rb::protocol::xproto::{self, ConnectionExt};
 
 use crate::atoms::AtomName::None as DndNone;
 use crate::atoms::*;
 use crate::event_loop::{CookieResultExt, X11Error};
-use crate::util;
 use crate::xdisplay::XConnection;
+use crate::{XWindow, util};
 
 #[derive(Debug, Clone, Copy)]
 pub enum DndState {
@@ -21,98 +22,264 @@ pub enum DndState {
 }
 
 #[derive(Debug)]
-pub enum DndDataParseError {
+pub enum UriListParseError {
     EmptyData,
     InvalidUtf8(#[allow(dead_code)] Utf8Error),
     HostnameSpecified(#[allow(dead_code)] String),
     UnexpectedProtocol(#[allow(dead_code)] String),
     UnresolvablePath(#[allow(dead_code)] io::Error),
+    Io(#[allow(dead_code)] io::Error),
 }
 
-impl From<Utf8Error> for DndDataParseError {
+impl From<Utf8Error> for UriListParseError {
     fn from(e: Utf8Error) -> Self {
-        DndDataParseError::InvalidUtf8(e)
+        UriListParseError::InvalidUtf8(e)
     }
 }
 
-impl From<io::Error> for DndDataParseError {
+impl From<io::Error> for UriListParseError {
     fn from(e: io::Error) -> Self {
-        DndDataParseError::UnresolvablePath(e)
+        UriListParseError::UnresolvablePath(e)
+    }
+}
+
+#[derive(Debug)]
+pub struct SelectionReader {
+    type_: SelectionType,
+    data: Vec<u8>,
+}
+
+impl TypedData for SelectionReader {
+    fn try_read(&self) -> Option<Box<dyn io::BufRead>> {
+        Some(Box::new(io::Cursor::new(self.data.clone())))
+    }
+
+    fn type_(&self) -> &dyn TransferType {
+        &self.type_
+    }
+
+    fn try_as_string(&self) -> io::Result<String> {
+        fn invalid_data<E>(err: E) -> io::Error
+        where
+            E: Into<Box<dyn std::error::Error + Send + Sync>>,
+        {
+            io::Error::new(io::ErrorKind::InvalidData, err)
+        }
+
+        fn decode_utf16_bytes(bytes: &[u8]) -> io::Result<String> {
+            let utf16 = bytes
+                .chunks_exact(2)
+                .map(|chunk| {
+                    let bytes: &[u8; 2] = chunk.try_into().unwrap();
+                    u16::from_ne_bytes(*bytes)
+                })
+                .collect::<Vec<_>>();
+            String::from_utf16(&utf16).map_err(invalid_data)
+        }
+
+        match self.type_.hint() {
+            Some(TypeHint::Plaintext) | Some(TypeHint::Html) => std::str::from_utf8(&self.data)
+                .map(|str| str.to_owned())
+                .map_err(invalid_data)
+                .or_else(|_| decode_utf16_bytes(&self.data)),
+            Some(TypeHint::UriList) => String::from_utf8(self.data.clone()).map_err(invalid_data),
+            _ => Err(io::ErrorKind::InvalidData.into()),
+        }
+    }
+
+    fn try_as_uris(&self) -> io::Result<Vec<String>> {
+        if self.type_().hint() != Some(TypeHint::UriList) {
+            return Err(io::ErrorKind::InvalidData.into());
+        }
+
+        Ok(self
+            .try_as_string()?
+            .split(['\n', '\r'])
+            .filter(|s| !s.is_empty())
+            .map(ToOwned::to_owned)
+            .collect())
+    }
+}
+
+#[derive(Debug)]
+pub struct DragState {
+    // Populated by XdndEnter event handler
+    pub version: c_long,
+    pub transfer_id: DataTransferId,
+    pub types: Arc<[SelectionType]>,
+    // Populated by Xdnd* event handlers
+    pub source_window: xproto::Window,
+    // Populated by Xdnd* event handlers
+    pub target_window: xproto::Window,
+    // Populated by `fetch_data_transfer`
+    pub pending_fetch_types: VecDeque<(AsyncRequestSerial, SelectionType)>,
+    pub finished: Option<(XWindow, XWindow)>,
+    /// Whether the drag operation is accepted (or `None` if the user never indicated that it's
+    /// accepted or rejected)
+    // Populated by `Window::accept_drag`/`Window::reject_drag`.
+    pub accepted: bool,
+}
+
+impl Default for DragState {
+    fn default() -> Self {
+        static DATA_TRANSFER_ID: AtomicI64 = AtomicI64::new(0);
+
+        Self {
+            version: Default::default(),
+            transfer_id: DataTransferId::from_raw(DATA_TRANSFER_ID.fetch_add(1, Ordering::Relaxed)),
+            types: Default::default(),
+            source_window: Default::default(),
+            target_window: Default::default(),
+            pending_fetch_types: Default::default(),
+            finished: None,
+            accepted: Default::default(),
+        }
     }
 }
 
 #[derive(Debug)]
 pub struct Dnd {
     xconn: Arc<XConnection>,
-    // Populated by XdndEnter event handler
-    pub version: Option<c_long>,
-    pub type_list: Option<Vec<xproto::Atom>>,
-    // Populated by XdndPosition event handler
-    pub source_window: Option<xproto::Window>,
-    // Populated by XdndPosition event handler
-    pub position: PhysicalPosition<f64>,
-    // Populated by SelectionNotify event handler (triggered by XdndPosition event handler)
-    pub result: Option<Result<Vec<PathBuf>, DndDataParseError>>,
-    // Populated by SelectionNotify event handler (triggered by XdndPosition event handler)
-    pub dragging: bool,
+    // If `None`, no drag operation is in progress.
+    state: Option<DragState>,
+}
+
+#[derive(Debug)]
+pub struct Selection {
+    types: Arc<[SelectionType]>,
+}
+
+impl Selection {
+    pub(crate) fn new(types: Arc<[SelectionType]>) -> Selection {
+        Selection { types }
+    }
+}
+
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub struct SelectionType {
+    hint: Option<TypeHint>,
+    atom: xproto::Atom,
+}
+
+impl SelectionType {
+    pub(crate) fn new(atoms: &Atoms, atom: xproto::Atom) -> Self {
+        let atom_to_hint = [
+            // Files
+            (atoms[TextUriList], TypeHint::UriList),
+            // Plaintext
+            (atoms[STRING], TypeHint::Plaintext),
+            (atoms[UTF8_STRING], TypeHint::Plaintext),
+            (atoms[TextPlain], TypeHint::Plaintext),
+            (atoms[TextPlainCharsetUtf8], TypeHint::Plaintext),
+            // HTML
+            (atoms[TextHtml], TypeHint::Html),
+            (atoms[TextHtmlCharsetUtf8], TypeHint::Html),
+            // RTF
+            (atoms[ApplicationRtf], TypeHint::Rtf),
+            // Audio
+            (atoms[AudioAac], TypeHint::Audio { extension_hint: Some("aac") }),
+            (atoms[AudioAiff], TypeHint::Audio { extension_hint: Some("aif") }),
+            (atoms[AudioFlac], TypeHint::Audio { extension_hint: Some("flac") }),
+            (atoms[AudioVndWav], TypeHint::Audio { extension_hint: Some("wav") }),
+            (atoms[AudioVndWave], TypeHint::Audio { extension_hint: Some("wav") }),
+            (atoms[AudioWav], TypeHint::Audio { extension_hint: Some("wav") }),
+            (atoms[AudioWave], TypeHint::Audio { extension_hint: Some("wav") }),
+            (atoms[AudioXWav], TypeHint::Audio { extension_hint: Some("wav") }),
+            (atoms[AudioOgg], TypeHint::Audio { extension_hint: Some("ogg") }),
+            (atoms[AudioMpeg], TypeHint::Audio { extension_hint: Some("mp3") }),
+            // Image
+            (atoms[ImageBmp], TypeHint::Image { extension_hint: Some("bmp") }),
+            (atoms[ImageGif], TypeHint::Image { extension_hint: Some("gif") }),
+            (atoms[ImageJpeg], TypeHint::Image { extension_hint: Some("jpg") }),
+            (atoms[ImagePjpeg], TypeHint::Image { extension_hint: Some("jpg") }),
+            (atoms[ImagePng], TypeHint::Image { extension_hint: Some("png") }),
+            (atoms[ImageRaw], TypeHint::Image { extension_hint: Some("raw") }),
+            (atoms[ImageSvg], TypeHint::Image { extension_hint: Some("svg") }),
+            (atoms[ImageTiff], TypeHint::Image { extension_hint: Some("tiff") }),
+            (atoms[ImageWebp], TypeHint::Image { extension_hint: Some("webp") }),
+            (atoms[ImageXIcon], TypeHint::Image { extension_hint: Some("ico") }),
+        ];
+        let hint =
+            atom_to_hint.iter().find_map(|(haystack, hint)| (*haystack == atom).then_some(*hint));
+
+        Self { hint, atom }
+    }
+
+    pub fn atom(&self) -> xproto::Atom {
+        self.atom
+    }
+}
+
+impl TransferType for SelectionType {
+    fn hint(&self) -> Option<TypeHint> {
+        self.hint
+    }
+
+    fn matches(&self, other: &dyn TransferType) -> bool {
+        if let Some(other_mime) = other.cast_ref::<Self>() {
+            *self == *other_mime
+        } else {
+            // If either hint is `None`, return false
+            self.hint().is_some_and(|hint| other.hint() == Some(hint))
+        }
+    }
+}
+
+impl DataTransfer for Selection {
+    fn for_each_available_type<'this>(
+        &'this self,
+        func: &'_ mut dyn FnMut(&'this dyn TransferType) -> std::ops::ControlFlow<()>,
+    ) {
+        let _ = self.types.iter().map(|mime| mime as &dyn TransferType).try_for_each(func);
+    }
 }
 
 impl Dnd {
-    pub fn new(xconn: Arc<XConnection>) -> Result<Self, X11Error> {
-        Ok(Dnd {
-            xconn,
-            version: None,
-            type_list: None,
-            source_window: None,
-            position: PhysicalPosition::default(),
-            result: None,
-            dragging: false,
-        })
+    pub fn new(xconn: Arc<XConnection>) -> Self {
+        Dnd { xconn, state: None }
     }
 
-    pub fn reset(&mut self) {
-        self.version = None;
-        self.type_list = None;
-        self.source_window = None;
-        self.result = None;
-        self.dragging = false;
+    pub fn state(&self) -> Option<&DragState> {
+        self.state.as_ref()
     }
 
-    pub unsafe fn send_status(
-        &self,
-        this_window: xproto::Window,
+    pub fn state_mut(&mut self) -> Option<&mut DragState> {
+        self.state.as_mut()
+    }
+
+    pub fn find_type_by_hint(&self, hint: TypeHint) -> Option<&SelectionType> {
+        self.state.as_ref()?.types.iter().find(|haystack| haystack.hint() == Some(hint))
+    }
+
+    pub fn init_state(
+        &mut self,
+        version: c_long,
+        source_window: xproto::Window,
         target_window: xproto::Window,
-        state: DndState,
-    ) -> Result<(), X11Error> {
-        let atoms = self.xconn.atoms();
-        let (accepted, action) = match state {
-            DndState::Accepted => (1, atoms[XdndActionPrivate]),
-            DndState::Rejected => (0, atoms[DndNone]),
-        };
-        self.xconn
-            .send_client_msg(target_window, target_window, atoms[XdndStatus] as _, None, [
-                this_window,
-                accepted,
-                0,
-                0,
-                action as _,
-            ])?
-            .ignore_error();
-
-        Ok(())
+        types: Arc<[SelectionType]>,
+    ) -> &DragState {
+        self.state.get_or_insert(DragState {
+            version,
+            types,
+            source_window,
+            target_window,
+            ..Default::default()
+        })
     }
 
     pub unsafe fn send_finished(
         &self,
         this_window: xproto::Window,
         target_window: xproto::Window,
-        state: DndState,
     ) -> Result<(), X11Error> {
         let atoms = self.xconn.atoms();
-        let (accepted, action) = match state {
-            DndState::Accepted => (1, atoms[XdndActionPrivate]),
-            DndState::Rejected => (0, atoms[DndNone]),
+        let Some(state) = &self.state else {
+            return Err(X11Error::UnexpectedNull(
+                "Drag-and-drop state was not initialized (called `send_finished` before XdndEnter",
+            ));
         };
+        let (accepted, action) =
+            if state.accepted { (1, atoms[XdndActionCopy]) } else { (0, atoms[DndNone]) };
         self.xconn
             .send_client_msg(target_window, target_window, atoms[XdndFinished] as _, None, [
                 this_window,
@@ -138,54 +305,54 @@ impl Dnd {
         )
     }
 
-    pub unsafe fn convert_selection(&self, window: xproto::Window, time: xproto::Timestamp) {
+    pub fn convert_selection(
+        &self,
+        window: xproto::Window,
+        time: xproto::Timestamp,
+        new_type: xproto::Atom,
+    ) {
         let atoms = self.xconn.atoms();
         self.xconn
             .xcb_connection()
-            .convert_selection(
-                window,
-                atoms[XdndSelection],
-                atoms[TextUriList],
-                atoms[XdndSelection],
-                time,
-            )
+            // TODO: We store the converted selection back to `XdndSelection`. We should store to
+            // some new place so that `XdndSelection` remains untouched.
+            .convert_selection(window, atoms[XdndSelection], new_type, atoms[XdndSelection], time)
             .expect_then_ignore_error("Failed to send XdndSelection event")
     }
 
-    pub unsafe fn read_data(
+    pub unsafe fn send_status(
+        &self,
+        this_window: xproto::Window,
+        target_window: xproto::Window,
+        status: DndState,
+    ) -> Result<(), X11Error> {
+        let atoms = self.xconn.atoms();
+        let (accepted, action) = match status {
+            DndState::Accepted => (1, atoms[XdndActionCopy]),
+            DndState::Rejected => (0, atoms[DndNone]),
+        };
+        self.xconn
+            .send_client_msg(target_window, target_window, atoms[XdndStatus] as _, None, [
+                this_window,
+                accepted,
+                0,
+                0,
+                action as _,
+            ])?
+            .ignore_error();
+
+        Ok(())
+    }
+
+    pub fn read_data(
         &self,
         window: xproto::Window,
-    ) -> Result<Vec<c_uchar>, util::GetPropertyError> {
+        type_: SelectionType,
+    ) -> Result<SelectionReader, util::GetPropertyError> {
         let atoms = self.xconn.atoms();
-        self.xconn.get_property(window, atoms[XdndSelection], atoms[TextUriList])
-    }
+        let type_atom = type_.atom();
+        let bytes = self.xconn.get_property(window, atoms[XdndSelection], type_atom)?;
 
-    pub fn parse_data(&self, data: &mut [c_uchar]) -> Result<Vec<PathBuf>, DndDataParseError> {
-        if !data.is_empty() {
-            let mut path_list = Vec::new();
-            let decoded = percent_decode(data).decode_utf8()?.into_owned();
-            for uri in decoded.split("\r\n").filter(|u| !u.is_empty()) {
-                // The format is specified as protocol://host/path
-                // However, it's typically simply protocol:///path
-                let path_str = if uri.starts_with("file://") {
-                    let path_str = uri.replace("file://", "");
-                    if !path_str.starts_with('/') {
-                        // A hostname is specified
-                        // Supporting this case is beyond the scope of my mental health
-                        return Err(DndDataParseError::HostnameSpecified(path_str));
-                    }
-                    path_str
-                } else {
-                    // Only the file protocol is supported
-                    return Err(DndDataParseError::UnexpectedProtocol(uri.to_owned()));
-                };
-
-                let path = Path::new(&path_str).canonicalize()?;
-                path_list.push(path);
-            }
-            Ok(path_list)
-        } else {
-            Err(DndDataParseError::EmptyData)
-        }
+        Ok(SelectionReader { type_, data: bytes })
     }
 }

winit-x11/src/event_loop.rs

@@ -19,12 +19,13 @@ use tracing::warn;
 use winit_common::xkb::Context;
 use winit_core::application::ApplicationHandler;
 use winit_core::cursor::{CustomCursor as CoreCustomCursor, CustomCursorSource};
-use winit_core::error::{EventLoopError, RequestError};
+use winit_core::data_transfer::{DataTransfer, DataTransferId, TransferType};
+use winit_core::error::{EventLoopError, NotSupportedError, RequestError};
 use winit_core::event::{DeviceId, StartCause, WindowEvent};
 use winit_core::event_loop::pump_events::PumpStatus;
 use winit_core::event_loop::{
-    ActiveEventLoop as RootActiveEventLoop, ControlFlow, DeviceEvents,
-    EventLoopProxy as CoreEventLoopProxy, EventLoopProxyProvider,
+    ActiveEventLoop as RootActiveEventLoop, AsyncRequestSerial, ControlFlow, DeviceEvents,
+    DndAction, EventLoopProxy as CoreEventLoopProxy, EventLoopProxyProvider,
     OwnedDisplayHandle as CoreOwnedDisplayHandle,
 };
 use winit_core::monitor::MonitorHandle as CoreMonitorHandle;
@@ -36,14 +37,17 @@ use x11rb::protocol::{xkb, xproto};
 use x11rb::x11_utils::X11Error as LogicalError;
 use x11rb::xcb_ffi::ReplyOrIdError;
 
-use crate::atoms::*;
+use crate::atoms::{
+    _NET_WM_PING, _NET_WM_SYNC_REQUEST, ABS_PRESSURE, ABS_TILT_X, ABS_TILT_Y, ABS_X, ABS_Y, Atoms,
+    WM_DELETE_WINDOW,
+};
 use crate::dnd::Dnd;
 use crate::event_processor::{EventProcessor, MAX_MOD_REPLAY_LEN};
 use crate::ime::{self, Ime, ImeCreationError, ImeSender};
 use crate::util::{self, CustomCursor};
 use crate::window::{UnownedWindow, Window};
 use crate::xdisplay::{XConnection, XError, XNotSupported};
-use crate::{XlibErrorHook, ffi, xsettings};
+use crate::{Selection, SelectionType, XlibErrorHook, ffi, xsettings};
 
 // Xinput constants not defined in x11rb
 pub(crate) const ALL_DEVICES: u16 = 0;
@@ -168,6 +172,7 @@ impl<T> PeekableReceiver<T> {
 #[derive(Debug)]
 pub struct ActiveEventLoop {
     pub(crate) xconn: Arc<XConnection>,
+    pub(crate) dnd: RefCell<Dnd>,
     pub(crate) wm_delete_window: xproto::Atom,
     pub(crate) net_wm_ping: xproto::Atom,
     pub(crate) net_wm_sync_request: xproto::Atom,
@@ -226,8 +231,7 @@ impl EventLoop {
         let net_wm_ping = atoms[_NET_WM_PING];
         let net_wm_sync_request = atoms[_NET_WM_SYNC_REQUEST];
 
-        let dnd = Dnd::new(Arc::clone(&xconn))
-            .expect("Failed to call XInternAtoms when initializing drag and drop");
+        let dnd = Dnd::new(Arc::clone(&xconn)).into();
 
         let (ime_sender, ime_receiver) = mpsc::channel();
         let (ime_event_sender, ime_event_receiver) = mpsc::channel();
@@ -342,6 +346,7 @@ impl EventLoop {
 
         let window_target = ActiveEventLoop {
             ime,
+            dnd,
             root,
             control_flow: Cell::new(ControlFlow::default()),
             exit: Cell::new(None),
@@ -368,7 +373,6 @@ impl EventLoop {
 
         let event_processor = EventProcessor {
             target: window_target,
-            dnd,
             devices: Default::default(),
             randr_event_offset,
             ime_receiver,
@@ -621,9 +625,8 @@ impl EventLoop {
     fn drain_events<A: ApplicationHandler>(&mut self, app: &mut A) {
         let mut xev = MaybeUninit::uninit();
 
-        while unsafe { self.event_processor.poll_one_event(xev.as_mut_ptr()) } {
-            let mut xev = unsafe { xev.assume_init() };
-            self.event_processor.process_event(&mut xev, app);
+        while let Some(xev) = self.event_processor.poll_one_event(&mut xev) {
+            self.event_processor.process_event(xev, app);
         }
     }
 
@@ -759,6 +762,90 @@ impl RootActiveEventLoop for ActiveEventLoop {
     fn rwh_06_handle(&self) -> &dyn rwh_06::HasDisplayHandle {
         self
     }
+
+    fn data_transfer(&self, id: DataTransferId) -> Result<Box<dyn DataTransfer>, RequestError> {
+        let dnd = self.dnd.borrow();
+
+        if dnd.state().is_none_or(|state| state.transfer_id != id) {
+            return Err(RequestError::Ignored);
+        }
+
+        let Some(state) = dnd.state() else {
+            return Err(RequestError::Ignored);
+        };
+
+        Ok(Box::new(Selection::new(state.types.clone())))
+    }
+
+    fn fetch_data_transfer(
+        &self,
+        id: DataTransferId,
+        type_: &dyn TransferType,
+    ) -> Result<AsyncRequestSerial, RequestError> {
+        let mut dnd = self.dnd.borrow_mut();
+
+        let serial = AsyncRequestSerial::get();
+
+        let type_ = type_
+            .cast_ref::<SelectionType>()
+            .or_else(|| dnd.find_type_by_hint(type_.hint()?))
+            .cloned()
+            .ok_or(RequestError::NotSupported(NotSupportedError::new("Unknown type hint")))?;
+
+        let new_convert_selection = {
+            let Some(state) = dnd.state_mut() else {
+                return Err(RequestError::Ignored);
+            };
+
+            if state.transfer_id != id {
+                return Err(RequestError::NotSupported(NotSupportedError::new(
+                    "Unknown data transfer",
+                )));
+            }
+
+            // If it's non-empty, assume that we're still waiting on some other fetch operation.
+            // The `SelectionNotify` handler will send a new `convert_selection` event if any
+            // more are on the stack.
+            let should_emit_convert_selection = state.pending_fetch_types.is_empty();
+
+            let atom = type_.atom();
+
+            state.pending_fetch_types.push_back((serial, type_));
+
+            should_emit_convert_selection.then_some((
+                state.target_window,
+                self.xconn.timestamp(),
+                atom,
+            ))
+        };
+
+        if let Some((window, time, new_type)) = new_convert_selection {
+            // This results in the `SelectionNotify` event
+            dnd.convert_selection(window, time, new_type);
+        }
+
+        Ok(serial)
+    }
+
+    fn set_valid_dnd_actions(
+        &self,
+        id: DataTransferId,
+        actions: &[DndAction],
+    ) -> Result<(), RequestError> {
+        let mut dnd = self.dnd.borrow_mut();
+
+        let Some(state) = dnd.state_mut() else {
+            return Err(os_error!(UnknownDataTransfer(id)).into());
+        };
+
+        if state.transfer_id != id {
+            return Err(os_error!(UnknownDataTransfer(id)).into());
+        }
+
+        state.accepted = !actions.is_empty();
+
+        Ok(())
+    }
 }
 
 impl rwh_06::HasDisplayHandle for ActiveEventLoop {
@@ -767,6 +854,19 @@ impl rwh_06::HasDisplayHandle for ActiveEventLoop {
     }
 }
 
+/// An operation was attempted on a data transfer ID, but that ID was invalid.
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+pub struct UnknownDataTransfer(pub DataTransferId);
+
+impl fmt::Display for UnknownDataTransfer {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let id = self.0.into_raw();
+        write!(f, "Unknown data transfer with ID {id}")
+    }
+}
+
+impl std::error::Error for UnknownDataTransfer {}
+
 pub(crate) struct DeviceInfo<'a> {
     xconn: &'a XConnection,
     info: *const ffi::XIDeviceInfo,

winit-x11/src/event_processor.rs

@@ -1,10 +1,12 @@
 use std::cell::{Cell, RefCell};
 use std::collections::{HashMap, VecDeque};
+use std::mem::MaybeUninit;
 use std::os::raw::{c_char, c_int, c_long, c_ulong};
 use std::slice;
 use std::sync::{Arc, Mutex};
 
 use dpi::{PhysicalPosition, PhysicalSize};
+use tracing::warn;
 use winit_common::xkb::{self, Context, XkbState};
 use winit_core::application::ApplicationHandler;
 use winit_core::event::{
@@ -12,6 +14,7 @@ use winit_core::event::{
     MouseScrollDelta, PointerKind, PointerSource, RawKeyEvent, SurfaceSizeWriter, TouchPhase,
     WindowEvent,
 };
+use winit_core::event_loop::DndAction;
 use winit_core::keyboard::ModifiersState;
 use winit_core::window::WindowId;
 use x11_dl::xinput2::{
@@ -31,7 +34,7 @@ use x11rb::x11_utils::{ExtensionInformation, Serialize};
 use xkbcommon_dl::xkb_mod_mask_t;
 
 use crate::atoms::*;
-use crate::dnd::{Dnd, DndState};
+use crate::dnd::{DndState, SelectionType};
 use crate::event_loop::{
     ALL_DEVICES, ActiveEventLoop, CookieResultExt, Device, DeviceInfo, DeviceType,
     ScrollOrientation, mkdid, mkwid,
@@ -49,7 +52,6 @@ const KEYCODE_OFFSET: u8 = 8;
 
 #[derive(Debug)]
 pub struct EventProcessor {
-    pub dnd: Dnd,
     pub ime_receiver: ImeReceiver,
     pub ime_event_receiver: ImeEventReceiver,
     pub randr_event_offset: u8,
@@ -294,7 +296,10 @@ impl EventProcessor {
         unsafe { (self.target.xconn.xlib.XPending)(self.target.xconn.display) != 0 }
     }
 
-    pub unsafe fn poll_one_event(&mut self, event_ptr: *mut XEvent) -> bool {
+    pub fn poll_one_event<'a>(
+        &mut self,
+        event_ptr: &'a mut MaybeUninit<XEvent>,
+    ) -> Option<&'a mut XEvent> {
         // This function is used to poll and remove a single event
         // from the Xlib event queue in a non-blocking, atomic way.
         // XCheckIfEvent is non-blocking and removes events from queue.
@@ -304,20 +309,21 @@ impl EventProcessor {
         unsafe extern "C" fn predicate(
             _display: *mut XDisplay,
             _event: *mut XEvent,
-            _arg: *mut c_char,
+            _filter: *mut c_char,
         ) -> c_int {
-            // This predicate always returns "true" (1) to accept all events
             1
         }
 
-        unsafe {
+        let event_initialized = unsafe {
             (self.target.xconn.xlib.XCheckIfEvent)(
                 self.target.xconn.display,
-                event_ptr,
+                event_ptr.as_mut_ptr(),
                 Some(predicate),
                 std::ptr::null_mut(),
             ) != 0
-        }
+        };
+
+        event_initialized.then(|| unsafe { event_ptr.assume_init_mut() })
     }
 
     pub fn init_device(&self, device: xinput::DeviceId) {
@@ -423,21 +429,41 @@ impl EventProcessor {
         }
 
         if xev.message_type == atoms[XdndEnter] as c_ulong {
+            // Cautiously limit the scope of the `dnd` lock so we don't rely on `app.window_event`
+            // never contending the lock.
+            let transfer_id = {
+                let mut dnd = self.target.dnd.borrow_mut();
                 let source_window = xev.data.get_long(0) as xproto::Window;
                 let flags = xev.data.get_long(1);
+
                 let version = flags >> 24;
-            self.dnd.version = Some(version);
+
                 let has_more_types = flags - (flags & (c_long::MAX - 1)) == 1;
-            if !has_more_types {
-                let type_list = vec![
+                let types: Vec<_> = if !has_more_types {
+                    [
                         xev.data.get_long(2) as xproto::Atom,
                         xev.data.get_long(3) as xproto::Atom,
                         xev.data.get_long(4) as xproto::Atom,
-                ];
-                self.dnd.type_list = Some(type_list);
-            } else if let Ok(more_types) = unsafe { self.dnd.get_type_list(source_window) } {
-                self.dnd.type_list = Some(more_types);
-            }
+                    ]
+                    .map(|ty_atom| SelectionType::new(atoms, ty_atom))
+                    .into_iter()
+                    .collect()
+                } else if let Ok(more_types) = unsafe { dnd.get_type_list(source_window) } {
+                    more_types
+                        .into_iter()
+                        .map(|ty_atom| SelectionType::new(atoms, ty_atom))
+                        .collect()
+                } else {
+                    Default::default()
+                };
+
+                dnd.init_state(version, source_window, window, types.into()).transfer_id
+            };
+
+            app.window_event(&self.target, window_id, WindowEvent::DragEntered {
+                id: transfer_id,
+                position: None,
+            });
             return;
         }
 
@@ -464,31 +490,17 @@ impl EventProcessor {
                 .xconn
                 .translate_coords(self.target.root, window, x, y)
                 .expect("Failed to translate window coordinates");
-            self.dnd.position = PhysicalPosition::new(coords.dst_x as f64, coords.dst_y as f64);
 
-            // By our own state flow, `version` should never be `None` at this point.
-            let version = self.dnd.version.unwrap_or(5);
-
-            // Action is specified in versions 2 and up, though we don't need it anyway.
-            // let action = xev.data.get_long(4);
-
-            let accepted = if let Some(ref type_list) = self.dnd.type_list {
-                type_list.contains(&atoms[TextUriList])
-            } else {
-                false
-            };
-
-            if !accepted {
-                unsafe {
-                    self.dnd
-                        .send_status(window, source_window, DndState::Rejected)
-                        .expect("Failed to send `XdndStatus` message.");
-                }
-                self.dnd.reset();
+            // Cautiously limit the scope of the `dnd` lock so we don't rely on `app.window_event`
+            // never contending the lock.
+            let transfer_id = {
+                let dnd = self.target.dnd.borrow();
+                let Some(state) = dnd.state() else {
                     return;
-            }
+                };
+                // By our own state flow, `state` should never be `None` at this point.
+                let version = state.version;
 
-            self.dnd.source_window = Some(source_window);
                 let time = if version == 0 {
                     // In version 0, time isn't specified
                     x11rb::CURRENT_TIME
@@ -499,86 +511,183 @@ impl EventProcessor {
                 // Log this timestamp.
                 self.target.xconn.set_timestamp(time);
 
-            // This results in the `SelectionNotify` event below
                 unsafe {
-                self.dnd.convert_selection(window, time);
-            }
-
-            unsafe {
-                self.dnd
-                    .send_status(window, source_window, DndState::Accepted)
+                    dnd.send_status(
+                        window,
+                        source_window,
+                        if state.accepted { DndState::Accepted } else { DndState::Rejected },
+                    )
                     .expect("Failed to send `XdndStatus` message.");
                 }
+
+                state.transfer_id
+            };
+
+            app.window_event(&self.target, window_id, WindowEvent::DragPosition {
+                id: transfer_id,
+                position: PhysicalPosition::new(coords.dst_x as f64, coords.dst_y as f64),
+                // `Copy` is the default. Other actions are possible in X11, but the specification
+                // does not properly explain how to implement them (only giving a vague description
+                // of `XdndMove`). For simplicity's sake, we simply do not implement non-copy drag
+                // on X11.
+                // See https://www.freedesktop.org/wiki/Specifications/XDND/
+                proposed_action: Some(DndAction::Copy),
+            });
+
             return;
         }
 
         if xev.message_type == atoms[XdndDrop] as c_ulong {
-            let (source_window, state) = if let Some(source_window) = self.dnd.source_window {
-                if let Some(Ok(ref path_list)) = self.dnd.result {
-                    let event = WindowEvent::DragDropped {
-                        paths: path_list.iter().map(Into::into).collect(),
-                        position: self.dnd.position,
+            let (source_window, transfer_id) = {
+                let dnd = self.target.dnd.borrow();
+                let Some(state) = dnd.state() else {
+                    warn!("Received `XdndDrop` without `XdndEnter`");
+                    return;
                 };
-                    app.window_event(&self.target, window_id, event);
-                }
-                (source_window, DndState::Accepted)
-            } else {
-                // `source_window` won't be part of our DND state if we already rejected the drop in
-                // our `XdndPosition` handler.
-                let source_window = xev.data.get_long(0) as xproto::Window;
-                (source_window, DndState::Rejected)
+                let source_window = state.source_window;
+
+                (source_window, state.transfer_id)
             };
 
+            app.window_event(
+                &self.target,
+                window_id,
+                // TODO
+                WindowEvent::DragDropped {
+                    id: transfer_id,
+                    // `Copy` is the default. Other actions are possible in X11, but the
+                    // specification does not properly explain how to implement
+                    // them (only giving a vague description of `XdndMove`). For
+                    // simplicity's sake, we simply do not implement non-copy drag
+                    // on X11.
+                    // See https://www.freedesktop.org/wiki/Specifications/XDND/
+                    proposed_action: Some(DndAction::Copy),
+                },
+            );
+
+            let mut dnd = self.target.dnd.borrow_mut();
+
+            if let Some(state) =
+                dnd.state_mut().filter(|state| !state.pending_fetch_types.is_empty())
+            {
+                state.finished = Some((window, source_window));
+            } else {
                 unsafe {
-                self.dnd
-                    .send_finished(window, source_window, state)
+                    dnd.send_finished(window, source_window)
                         .expect("Failed to send `XdndFinished` message.");
                 }
+            }
 
-            self.dnd.reset();
             return;
         }
 
         if xev.message_type == atoms[XdndLeave] as c_ulong {
-            if self.dnd.dragging {
-                let event = WindowEvent::DragLeft { position: Some(self.dnd.position) };
-                app.window_event(&self.target, window_id, event);
-            }
-            self.dnd.reset();
+            let dnd = self.target.dnd.borrow();
+            let Some(state) = dnd.state() else {
+                return;
+            };
+            app.window_event(&self.target, window_id, WindowEvent::DragLeft {
+                id: state.transfer_id,
+            });
         }
     }
 
     fn selection_notify(&mut self, xev: &XSelectionEvent, app: &mut dyn ApplicationHandler) {
         let atoms = self.target.xconn.atoms();
 
-        let window = xev.requestor as xproto::Window;
-        let window_id = mkwid(window);
+        let xwindow = xev.requestor as xproto::Window;
 
         // Set the timestamp.
         self.target.xconn.set_timestamp(xev.time as xproto::Timestamp);
 
+        // For now, winit only supports selections for drag-and-drop. This should be changed
+        // when clipboard support is implemented.
         if xev.property != atoms[XdndSelection] as c_ulong {
             return;
         }
 
-        // This is where we receive data from drag and drop
-        self.dnd.result = None;
-        if let Ok(mut data) = unsafe { self.dnd.read_data(window) } {
-            let parse_result = self.dnd.parse_data(&mut data);
-
-            if let Ok(ref path_list) = parse_result {
-                let event = if self.dnd.dragging {
-                    WindowEvent::DragMoved { position: self.dnd.position }
-                } else {
-                    let paths = path_list.iter().map(Into::into).collect();
-                    self.dnd.dragging = true;
-                    WindowEvent::DragEntered { paths, position: self.dnd.position }
+        let (transfer_id, serial, type_) = {
+            let Some(state) = self.target.dnd.get_mut().state_mut() else {
+                return;
             };
 
-                app.window_event(&self.target, window_id, event);
+            let Some((serial, type_)) = state.pending_fetch_types.pop_front() else {
+                return;
+            };
+
+            // Annoyingly, `xproto::Atom` and `x11_dl::Atom` are different on 64-bit
+            // but the same on 32-bit, so just casting to `u32` will cause "casting
+            // to same type" clippy warnings when compiled as 32-bit.
+            #[cfg(target_pointer_width = "32")]
+            let target_type = xev.target;
+            #[cfg(target_pointer_width = "64")]
+            let target_type = xev.target as u32;
+
+            if target_type != type_.atom() {
+                let get_name = |atom| {
+                    self.target
+                        .xconn
+                        .xcb_connection()
+                        .get_atom_name(atom)
+                        .ok()
+                        .and_then(|cookie| cookie.reply().ok())
+                        .and_then(|reply| String::from_utf8(reply.name).ok())
+                };
+
+                let expected_type_name = get_name(type_.atom());
+                let found_type_name = get_name(xev.type_ as _);
+
+                let extra_context = match (expected_type_name, found_type_name) {
+                    (Some(expected), Some(found)) => {
+                        format!(" (expected {expected}, found {found})")
+                    },
+                    (Some(expected), None) => format!(" (expected {expected})"),
+                    (None, Some(found)) => format!(" (found {found})"),
+                    (None, None) => "".to_string(),
+                };
+                warn!(
+                    "Received `SelectionNotify` with unexpected type{extra_context}. Continuing, \
+                     but this may be a bug."
+                );
             }
 
-            self.dnd.result = Some(parse_result);
+            (state.transfer_id, serial, type_)
+        };
+
+        let value = match self.target.dnd.borrow().read_data(xwindow, type_) {
+            Ok(value) => Arc::new(value),
+            Err(err) => {
+                warn!("Failed to read selection: {err}");
+                return;
+            },
+        };
+
+        let window_id = mkwid(xwindow);
+
+        app.window_event(&self.target, window_id, WindowEvent::DataTransferReceived {
+            id: transfer_id,
+            serial,
+            value,
+        });
+
+        let dnd = self.target.dnd.borrow();
+
+        // If we have another fetch pending, request it from the drag source window
+        if let Some((window, type_)) = dnd.state().and_then(|state| {
+            state
+                .pending_fetch_types
+                .front()
+                .cloned()
+                .map(|(_, type_)| (state.target_window, type_))
+        }) {
+            dnd.convert_selection(window, self.target.xconn.timestamp(), type_.atom());
+        } else if let Some((this_window, target_window)) =
+            dnd.state().and_then(|state| state.finished)
+        {
+            unsafe {
+                dnd.send_finished(this_window, target_window)
+                    .expect("Failed to send `XdndFinished` message.");
+            }
         }
     }
 

winit-x11/src/lib.rs

@@ -26,6 +26,8 @@ mod window;
 mod xdisplay;
 mod xsettings;
 
+pub use dnd::{Selection, SelectionReader, SelectionType, UriListParseError};
+
 /// X window type. Maps directly to
 /// [`_NET_WM_WINDOW_TYPE`](https://specifications.freedesktop.org/wm-spec/wm-spec-1.5.html).
 #[derive(Debug, Default, Copy, Clone, PartialEq, Eq, Hash)]

winit-x11/src/util/window_property.rs

@@ -17,6 +17,7 @@ pub enum GetPropertyError {
     X11rbError(Arc<ReplyError>),
     TypeMismatch(xproto::Atom),
     FormatMismatch(c_int),
+    Unknown,
 }
 
 impl GetPropertyError {
@@ -41,6 +42,7 @@ impl fmt::Display for GetPropertyError {
             GetPropertyError::X11rbError(err) => err.fmt(f),
             GetPropertyError::TypeMismatch(err) => write!(f, "type mismatch: {err}"),
             GetPropertyError::FormatMismatch(err) => write!(f, "format mismatch: {err}"),
+            GetPropertyError::Unknown => write!(f, "internal error"),
         }
     }
 }

winit-x11/src/window.rs

@@ -31,7 +31,14 @@ use x11rb::protocol::sync::{ConnectionExt as _, Int64};
 use x11rb::protocol::xproto::{self, ClipOrdering, ConnectionExt as _, Rectangle};
 use x11rb::protocol::{randr, xinput};
 
-use crate::atoms::*;
+use crate::atoms::{
+    _GTK_THEME_VARIANT, _NET_ACTIVE_WINDOW, _NET_WM_ICON, _NET_WM_MOVERESIZE, _NET_WM_NAME,
+    _NET_WM_PID, _NET_WM_PING, _NET_WM_STATE, _NET_WM_STATE_ABOVE, _NET_WM_STATE_BELOW,
+    _NET_WM_STATE_FULLSCREEN, _NET_WM_STATE_HIDDEN, _NET_WM_STATE_MAXIMIZED_HORZ,
+    _NET_WM_STATE_MAXIMIZED_VERT, _NET_WM_SYNC_REQUEST, _NET_WM_SYNC_REQUEST_COUNTER,
+    _NET_WM_WINDOW_TYPE, _XEMBED, AtomName, CARD32, UTF8_STRING, WM_CHANGE_STATE,
+    WM_CLIENT_MACHINE, WM_DELETE_WINDOW, WM_PROTOCOLS, WM_STATE, XdndAware,
+};
 use crate::event_loop::{
     ALL_MASTER_DEVICES, ActivationItem, ActiveEventLoop, CookieResultExt, ICONIC_STATE, VoidCookie,
     WakeSender, X11Error, xinput_fp1616_to_float,

winit/build.rs

@@ -1,7 +1,14 @@
 use cfg_aliases::cfg_aliases;
 
-// Only relevant for examples and Winit, our usage of println! is fine here.
-#[allow(clippy::disallowed_macros)]
+#[allow(
+    clippy::disallowed_macros,
+    reason = "Only relevant for examples and Winit, our usage of println! is fine here."
+)]
+#[allow(
+    semicolon_in_expressions_from_macros,
+    reason = "This is a future incompatibility lint and we currently use cfg_aliases 0.2.1, which \
+              has not been updated to resolve the latest lints"
+)]
 fn main() {
     // Dummy invocation to enable change-tracking in build scripts.
     println!("cargo:rerun-if-changed=build.rs");

winit/examples/dnd.rs

@@ -1,10 +1,18 @@
 use std::error::Error;
+use std::path::PathBuf;
+use std::sync::Arc;
 
+use image::imageops::FilterType;
+use image::{DynamicImage, GenericImageView, RgbImage};
 use softbuffer::{Context, Surface};
-use tracing::info;
+use tracing::{error, info, warn};
 use winit::application::ApplicationHandler;
-use winit::event::WindowEvent;
-use winit::event_loop::{ActiveEventLoop, EventLoop, OwnedDisplayHandle};
+use winit::data_transfer::{DataTransferId, DataTransferSendBuilder, SendData, TypeHint};
+use winit::event::{ButtonSource, MouseButton, WindowEvent};
+use winit::event_loop::{
+    ActiveEventLoop, AsyncRequestSerial, DndAction, DragIcon, EventLoop, OwnedDisplayHandle,
+};
+use winit::icon::{Icon, RgbaIcon};
 use winit::window::{Window, WindowAttributes, WindowId};
 
 #[path = "util/fill.rs"]
@@ -17,20 +25,55 @@ fn main() -> Result<(), Box<dyn Error>> {
 
     let event_loop = EventLoop::new()?;
 
-    let app = Application::default();
+    let app = Application::new();
     Ok(event_loop.run_app(app)?)
 }
 
 /// Application state and event handling.
-#[derive(Default, Debug)]
+#[derive(Debug)]
 struct Application {
     surface: Option<Surface<OwnedDisplayHandle, Box<dyn Window>>>,
+    last_dnd_fetch: Option<AsyncRequestSerial>,
+    last_drag_start: Option<DataTransferId>,
+    drag_icon: (Icon, i32, i32),
+    drag_image_data: Arc<RgbImage>,
+}
+
+const DRAG_IMAGE: &[u8] = include_bytes!("data/icon.png");
+
+impl Application {
+    fn new() -> Self {
+        let drag_icon = load_icon(DRAG_IMAGE);
+        let drag_image_data = Arc::new(image::load_from_memory(DRAG_IMAGE).unwrap().into_rgb8());
+        Self {
+            surface: None,
+            last_dnd_fetch: None,
+            last_drag_start: None,
+            drag_icon,
+            drag_image_data,
+        }
+    }
+}
+
+fn load_icon(bytes: &[u8]) -> (Icon, i32, i32) {
+    let (icon_rgba, icon_width, icon_height) = {
+        let image = image::load_from_memory(bytes).unwrap().into_rgba8();
+        let (width, height) = image.dimensions();
+        let rgba = image.into_raw();
+        (rgba, width, height)
+    };
+    (
+        RgbaIcon::new(icon_rgba, icon_width, icon_height).expect("Failed to open icon").into(),
+        -(icon_width as i32) / 2,
+        -(icon_height as i32) / 2,
+    )
 }
 
 impl ApplicationHandler for Application {
     fn can_create_surfaces(&mut self, event_loop: &dyn ActiveEventLoop) {
         let window_attributes =
-            WindowAttributes::default().with_title("Drag and drop files on me!");
+            WindowAttributes::default().with_title("Drag and drop files, text or HTML onto me!");
+
         let window = event_loop.create_window(window_attributes).unwrap();
         let context = Context::new(event_loop.owned_display_handle()).unwrap();
         let surface = Surface::new(&context, window).unwrap();
@@ -40,14 +83,168 @@ impl ApplicationHandler for Application {
     fn window_event(
         &mut self,
         event_loop: &dyn ActiveEventLoop,
-        _window_id: WindowId,
+        window_id: WindowId,
         event: WindowEvent,
     ) {
         match event {
-            WindowEvent::DragLeft { .. }
-            | WindowEvent::DragEntered { .. }
-            | WindowEvent::DragMoved { .. }
-            | WindowEvent::DragDropped { .. } => {
+            WindowEvent::PointerButton { button: ButtonSource::Mouse(button), state, .. }
+                if button == MouseButton::Left && state.is_pressed() =>
+            {
+                let (icon, offset_x, offset_y) = self.drag_icon.clone();
+
+                // In a real application, you probably wouldn't advertise so many types.
+                // Depending on platform and destination application, different options may be
+                // chosen.
+                let result = event_loop.start_drag(
+                    window_id,
+                    DataTransferSendBuilder::new(self.drag_image_data.clone())
+                        .with_type(TypeHint::UriList, |_, _| {
+                            let manifest_dir = PathBuf::from(env!("CARGO_MANIFEST_DIR"));
+                            let root = manifest_dir.parent().unwrap();
+                            let this_file = root.join(file!());
+                            let icon_file = this_file.parent().unwrap().join("data/icon.png");
+
+                            SendData::from_file_paths([icon_file])
+                        })
+                        .with_type(TypeHint::Plaintext, |_, _| Some("Winit example".to_string()))
+                        .with_type(TypeHint::Html, |_, _| {
+                            Some("<span><strong>Winit</strong> example</span>".to_string())
+                        })
+                        // You can advertise a `TypeHint` that can match many types, and switch
+                        // inside the callback. For example, this will match any image type.
+                        // This may be desirable on some platforms which restrict the set of
+                        // image types that can be sent.
+                        .with_type(TypeHint::Image { extension_hint: None }, |image, ty| {
+                            let hint = ty.hint()?;
+                            match hint {
+                                TypeHint::Image { extension_hint } => {
+                                    let image = DynamicImage::from((**image).clone());
+                                    let (w, h) = image.dimensions();
+                                    let image = image
+                                        .resize(w * 8, h * 8, FilterType::Gaussian)
+                                        .into_rgb8();
+                                    let ext = extension_hint.unwrap_or("png");
+                                    info!("Destination requested image as {ext}, converting...");
+                                    let format = image::ImageFormat::from_extension(ext)?;
+                                    let mut out_buf = Vec::new();
+                                    let mut out_writer = std::io::Cursor::new(&mut out_buf);
+
+                                    image.write_to(&mut out_writer, format).ok()?;
+
+                                    Some(out_buf)
+                                },
+                                _ => None,
+                            }
+                        })
+                        .build(),
+                    &[DndAction::Move, DndAction::Copy],
+                    Some(DragIcon { icon, offset_x, offset_y }),
+                );
+
+                self.last_drag_start = result.ok();
+            },
+            WindowEvent::DragLeft { .. } => {
+                info!("{event:?}");
+            },
+            WindowEvent::DataTransferReceived { ref value, serial, .. } => {
+                assert_eq!(self.last_dnd_fetch, Some(serial));
+
+                match value.type_().hint() {
+                    Some(TypeHint::Plaintext | TypeHint::Html) => {
+                        let Ok(text) = value.try_as_string() else {
+                            return;
+                        };
+                        info!("{text:?}");
+                    },
+                    Some(TypeHint::UriList) => {
+                        let Ok(uris) = value.try_as_uris() else {
+                            return;
+                        };
+                        info!("URIs: {uris:#?}");
+
+                        // If you only want to support dropping files, rather than arbitrary URIs,
+                        // you can use the `try_as_file_paths` helper method.
+                        let Ok(uris_as_paths) = value.try_as_file_paths() else {
+                            return;
+                        };
+                        info!("URIs as file paths: {uris_as_paths:#?}");
+                    },
+                    Some(TypeHint::Image { extension_hint: ext }) => {
+                        let Ok(bytes) = value.try_as_bytes() else {
+                            return;
+                        };
+                        let format = ext.and_then(image::ImageFormat::from_extension);
+
+                        let reader = std::io::Cursor::new(&bytes[..]);
+                        let reader = match format {
+                            Some(fmt) => image::ImageReader::with_format(reader, fmt),
+                            None => image::ImageReader::new(reader),
+                        };
+
+                        match reader.decode() {
+                            Ok(image) => {
+                                let width = image.width();
+                                let height = image.height();
+                                info!("Received image ({width}x{height})");
+                            },
+                            Err(err) => {
+                                warn!("Failed to decode image: {err}");
+                            },
+                        }
+                    },
+                    _ => {
+                        unreachable!("Received a type we didn't ask for!");
+                    },
+                }
+            },
+            WindowEvent::DragPosition { .. } => {
+                info!("{event:?}");
+            },
+            WindowEvent::DragDropped { .. } => {
+                info!("{event:?}");
+            },
+            WindowEvent::DragEntered { id, .. } => {
+                info!("{event:?}");
+
+                let data_transfer = match event_loop.data_transfer(id) {
+                    Ok(dt) => dt,
+                    Err(e) => {
+                        error!("{e}");
+                        return;
+                    },
+                };
+
+                info!("Types: {:#?}", data_transfer.available_types());
+
+                let readable_image_types = image::ImageFormat::all()
+                    .filter(|fmt| fmt.reading_enabled())
+                    .filter_map(|fmt| {
+                        let ext = fmt.extensions_str().first()?;
+
+                        Some(TypeHint::Image { extension_hint: Some(ext) })
+                    });
+
+                let mut valid_types = readable_image_types.chain([
+                    TypeHint::Html,
+                    TypeHint::UriList,
+                    TypeHint::Plaintext,
+                ]);
+
+                let valid_type = valid_types.find(|ty| data_transfer.has_type(ty));
+
+                let Some(type_) = valid_type else {
+                    event_loop.set_valid_dnd_actions(id, &[]).unwrap();
+                    return;
+                };
+
+                event_loop.set_valid_dnd_actions(id, &[DndAction::Move, DndAction::Copy]).unwrap();
+
+                self.last_dnd_fetch = event_loop.fetch_data_transfer(id, &type_).ok();
+            },
+            WindowEvent::OutgoingDragDropped { .. } => {
+                info!("{event:?}");
+            },
+            WindowEvent::OutgoingDragCanceled { .. } => {
                 info!("{event:?}");
             },
             WindowEvent::RedrawRequested => {

winit/src/lib.rs

@@ -292,7 +292,9 @@ pub use rwh_06 as raw_window_handle;
 #[cfg(any(doc, doctest, test))]
 pub mod changelog;
 pub mod event_loop;
-pub use winit_core::{application, cursor, error, event, icon, keyboard, monitor, window};
+pub use winit_core::{
+    application, cursor, data_transfer, error, event, icon, keyboard, monitor, window,
+};
 #[macro_use]
 mod os_error;
 mod platform_impl;