Store web_sys::File inside of DroppedFile (#8354)

* Closes #4654
* Related #4667
* [x] I have followed the instructions in the PR template

This PR avoids materializing the contents of a file that was dragged
into an egui application on the web. It does so by storing the
`web_sys::File` handle directly on WASM.

This breaks the existing API of `DroppedFile` on the web, because there
is no way to retrieve the bytes synchronously form a `DroppedFile`
anymore, forcing handling call sites to become asynchronous.

The native API remains the same.

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/eframe/src/web/dropped_file.rs

@@ -0,0 +1,48 @@
+use std::{
+    future::Future,
+    path::{Path, PathBuf},
+    pin::Pin,
+};
+
+#[derive(Debug)]
+pub(crate) struct WebFile {
+    file: web_sys::File,
+    // We store a `PathBuf` here so that we can hand out `Path`s
+    // without allocating each time.
+    path: PathBuf,
+}
+
+impl From<web_sys::File> for WebFile {
+    fn from(file: web_sys::File) -> Self {
+        let path = file.name().into();
+        Self { file, path }
+    }
+}
+
+impl egui::DroppedFile for WebFile {
+    fn path(&self) -> &Path {
+        &self.path
+    }
+
+    fn bytes_async(&self) -> Pin<Box<dyn Future<Output = Result<Vec<u8>, String>> + '_>> {
+        let file = self.file.clone();
+        Box::pin(async move {
+            if file.size() > f64::from(u32::MAX) {
+                return Err(format!(
+                    "File is too large: browser file reads are limited to {} bytes",
+                    u32::MAX
+                ));
+            }
+
+            let array_buffer = file
+                .array_buffer()
+                .await
+                .map_err(|err| crate::web::string_from_js_value(&err))?;
+            Ok(js_sys::Uint8Array::new(&array_buffer).to_vec())
+        })
+    }
+
+    fn web_file(&self) -> Option<&web_sys::File> {
+        Some(&self.file)
+    }
+}

crates/eframe/src/web/events.rs

@@ -973,62 +973,25 @@ fn install_drag_and_drop(runner_ref: &WebRunner, target: &EventTarget) -> Result
         event.prevent_default();
     })?;
 
-    runner_ref.add_event_listener(target, "drop", {
-        let runner_ref = runner_ref.clone();
+    runner_ref.add_event_listener(target, "drop", |event: web_sys::DragEvent, runner| {
+        if let Some(data_transfer) = event.data_transfer() {
+            // TODO(https://github.com/emilk/egui/issues/3702): support dropping folders
+            runner.input.raw.hovered_files.clear();
+            runner.needs_repaint.repaint_asap();
 
-        move |event: web_sys::DragEvent, runner| {
-            if let Some(data_transfer) = event.data_transfer() {
-                // TODO(https://github.com/emilk/egui/issues/3702): support dropping folders
-                runner.input.raw.hovered_files.clear();
-                runner.needs_repaint.repaint_asap();
+            if let Some(files) = data_transfer.files() {
+                for i in 0..files.length() {
+                    if let Some(file) = files.get(i) {
+                        log::debug!("Dropped {:?} ({} bytes)", file.name(), file.size());
 
-                if let Some(files) = data_transfer.files() {
-                    for i in 0..files.length() {
-                        if let Some(file) = files.get(i) {
-                            let name = file.name();
-                            let mime = file.type_();
-                            let last_modified = std::time::UNIX_EPOCH
-                                + std::time::Duration::from_millis(file.last_modified() as u64);
-
-                            log::debug!("Loading {:?} ({} bytes)…", name, file.size());
-
-                            let future = wasm_bindgen_futures::JsFuture::from(file.array_buffer());
-
-                            let runner_ref = runner_ref.clone();
-                            let future = async move {
-                                match future.await {
-                                    Ok(array_buffer) => {
-                                        let bytes = js_sys::Uint8Array::new(&array_buffer).to_vec();
-                                        log::debug!("Loaded {:?} ({} bytes).", name, bytes.len());
-
-                                        if let Some(mut runner_lock) = runner_ref.try_lock() {
-                                            runner_lock.input.raw.dropped_files.push(
-                                                egui::DroppedFile {
-                                                    name,
-                                                    mime,
-                                                    last_modified: Some(last_modified),
-                                                    bytes: Some(bytes.into()),
-                                                    ..Default::default()
-                                                },
-                                            );
-                                            runner_lock.needs_repaint.repaint_asap();
-                                        }
-                                    }
-                                    Err(err) => {
-                                        log::error!(
-                                            "Failed to read file: {}",
-                                            string_from_js_value(&err)
-                                        );
-                                    }
-                                }
-                            };
-                            wasm_bindgen_futures::spawn_local(future);
-                        }
+                        runner.input.raw.dropped_files.push(std::sync::Arc::new(
+                            super::dropped_file::WebFile::from(file),
+                        ));
                     }
                 }
-                event.stop_propagation();
-                event.prevent_default();
             }
+            event.stop_propagation();
+            event.prevent_default();
         }
     })?;
 

crates/eframe/src/web/mod.rs

@@ -5,6 +5,7 @@
 
 mod app_runner;
 mod backend;
+mod dropped_file;
 mod events;
 mod input;
 mod panic_handler;
@@ -207,13 +208,12 @@ fn set_clipboard_text(s: &str) {
             return;
         }
         let promise = window.navigator().clipboard().write_text(s);
-        let future = wasm_bindgen_futures::JsFuture::from(promise);
         let future = async move {
-            if let Err(err) = future.await {
+            if let Err(err) = promise.await {
                 log::error!("Copy/cut action failed: {}", string_from_js_value(&err));
             }
         };
-        wasm_bindgen_futures::spawn_local(future);
+        js_sys::futures::spawn_local(future);
     }
 }
 
@@ -248,16 +248,15 @@ fn set_clipboard_image(image: &egui::ColorImage) {
         };
         let items = js_sys::Array::of1(&item);
         let promise = window.navigator().clipboard().write(&items);
-        let future = wasm_bindgen_futures::JsFuture::from(promise);
         let future = async move {
-            if let Err(err) = future.await {
+            if let Err(err) = promise.await {
                 log::error!(
                     "Copy/cut image action failed: {}",
                     string_from_js_value(&err)
                 );
             }
         };
-        wasm_bindgen_futures::spawn_local(future);
+        js_sys::futures::spawn_local(future);
     }
 }