Swizzle sendEvent: instead of subclassing NSApplication

This is done to avoid order-dependent behavior that you'd otherwise
encounter where `EventLoop::new` had to be called at the beginning of
`fn main` to ensure that Winit's application was the one being
registered as the main application by calling `sharedApplication`.

Fixes https://github.com/rust-windowing/winit/issues/3772.

This should also make it (more) possible to use multiple versions of
Winit in the same application (though that's still untested).

Finally, it should allow the user to override `NSApplication` themselves
if they need to do that for some reason.

src/changelog/unreleased.md

@@ -46,6 +46,10 @@ changelog entry.
 - On Windows, add `CursorGrabMode::Locked`.
 - On Wayland, add `WindowExtWayland::xdg_toplevel`.
 
+### Changed
+
+- On macOS, no longer need control of the main `NSApplication` class (which means you can now override it yourself).
+
 ### Fixed
 
 - On Windows, fixed ~500 ms pause when clicking the title bar during continuous redraw.

src/platform_impl/macos/app.rs

@@ -1,49 +1,104 @@
 #![allow(clippy::unnecessary_cast)]
+#![allow(unknown_lints)] // New lint below
+#![allow(static_mut_refs)] // Uses `MainThreadBound` in new version.
 
-use objc2::{declare_class, msg_send, mutability, ClassType, DeclaredClass};
-use objc2_app_kit::{NSApplication, NSEvent, NSEventModifierFlags, NSEventType, NSResponder};
-use objc2_foundation::{MainThreadMarker, NSObject};
+use std::cell::Cell;
+use std::mem;
+
+use objc2::runtime::{Imp, Sel};
+use objc2::sel;
+use objc2_app_kit::{NSApplication, NSEvent, NSEventModifierFlags, NSEventType};
+use objc2_foundation::MainThreadMarker;
 
 use super::app_state::ApplicationDelegate;
 use crate::event::{DeviceEvent, ElementState};
 
-declare_class!(
-    pub(super) struct WinitApplication;
+type SendEvent = extern "C" fn(&NSApplication, Sel, &NSEvent);
 
-    unsafe impl ClassType for WinitApplication {
-        #[inherits(NSResponder, NSObject)]
-        type Super = NSApplication;
-        type Mutability = mutability::MainThreadOnly;
-        const NAME: &'static str = "WinitApplication";
-    }
+// NOTE: Only used on the main thread. Ideally, we'd use `MainThreadBound`, but that isn't
+// constructible from `const` with this `objc2` version.
+static mut ORIGINAL: Cell<Option<SendEvent>> = Cell::new(None);
 
-    impl DeclaredClass for WinitApplication {}
+extern "C" fn send_event(app: &NSApplication, sel: Sel, event: &NSEvent) {
+    let mtm = MainThreadMarker::from(app);
 
-    unsafe impl WinitApplication {
-        // Normally, holding Cmd + any key never sends us a `keyUp` event for that key.
-        // Overriding `sendEvent:` like this fixes that. (https://stackoverflow.com/a/15294196)
-        // Fun fact: Firefox still has this bug! (https://bugzilla.mozilla.org/show_bug.cgi?id=1299553)
-        #[method(sendEvent:)]
-        fn send_event(&self, event: &NSEvent) {
-            // For posterity, there are some undocumented event types
-            // (https://github.com/servo/cocoa-rs/issues/155)
-            // but that doesn't really matter here.
-            let event_type = unsafe { event.r#type() };
-            let modifier_flags = unsafe { event.modifierFlags() };
-            if event_type == NSEventType::KeyUp
-                && modifier_flags.contains(NSEventModifierFlags::NSEventModifierFlagCommand)
-            {
-                if let Some(key_window) = self.keyWindow() {
-                    key_window.sendEvent(event);
-                }
-            } else {
-                let delegate = ApplicationDelegate::get(MainThreadMarker::from(self));
-                maybe_dispatch_device_event(&delegate, event);
-                unsafe { msg_send![super(self), sendEvent: event] }
-            }
+    // Normally, holding Cmd + any key never sends us a `keyUp` event for that key.
+    // Overriding `sendEvent:` fixes that. (https://stackoverflow.com/a/15294196)
+    // Fun fact: Firefox still has this bug! (https://bugzilla.mozilla.org/show_bug.cgi?id=1299553)
+    //
+    // For posterity, there are some undocumented event types
+    // (https://github.com/servo/cocoa-rs/issues/155)
+    // but that doesn't really matter here.
+    let event_type = unsafe { event.r#type() };
+    let modifier_flags = unsafe { event.modifierFlags() };
+    if event_type == NSEventType::KeyUp
+        && modifier_flags.contains(NSEventModifierFlags::NSEventModifierFlagCommand)
+    {
+        if let Some(key_window) = app.keyWindow() {
+            key_window.sendEvent(event);
         }
+        return;
     }
-);
+
+    // Events are generally scoped to the window level, so the best way
+    // to get device events is to listen for them on NSApplication.
+    let delegate = ApplicationDelegate::get(mtm);
+    maybe_dispatch_device_event(&delegate, event);
+
+    let _ = mtm;
+    let original = unsafe { ORIGINAL.get().expect("no existing sendEvent: handler set") };
+    original(app, sel, event)
+}
+
+/// Override the [`sendEvent:`][NSApplication::sendEvent] method on the given application class.
+///
+/// The previous implementation created a subclass of [`NSApplication`], however we would like to
+/// give the user full control over their `NSApplication`, so we override the method here using
+/// method swizzling instead.
+///
+/// This _should_ also allow two versions of Winit to exist in the same application.
+///
+/// See the following links for more info on method swizzling:
+/// - <https://nshipster.com/method-swizzling/>
+/// - <https://spin.atomicobject.com/method-swizzling-objective-c/>
+/// - <https://web.archive.org/web/20130308110627/http://cocoadev.com/wiki/MethodSwizzling>
+///
+/// NOTE: This function assumes that the passed in application object is the one returned from
+/// [`NSApplication::sharedApplication`], i.e. the one and only global shared application object.
+/// For testing though, we allow it to be a different object.
+pub(crate) fn override_send_event(global_app: &NSApplication) {
+    let mtm = MainThreadMarker::from(global_app);
+    let class = global_app.class();
+
+    let method =
+        class.instance_method(sel!(sendEvent:)).expect("NSApplication must have sendEvent: method");
+
+    // SAFETY: Converting our `sendEvent:` implementation to an IMP.
+    let overridden = unsafe { mem::transmute::<SendEvent, Imp>(send_event) };
+
+    // If we've already overridden the method, don't do anything.
+    // FIXME(madsmtm): Use `std::ptr::fn_addr_eq` (Rust 1.85) once available in MSRV.
+    #[allow(unknown_lints, unpredictable_function_pointer_comparisons)]
+    if overridden == method.implementation() {
+        return;
+    }
+
+    // SAFETY: Our implementation has:
+    // 1. The same signature as `sendEvent:`.
+    // 2. Does not impose extra safety requirements on callers.
+    let original = unsafe { method.set_implementation(overridden) };
+
+    // SAFETY: This is the actual signature of `sendEvent:`.
+    let original = unsafe { mem::transmute::<Imp, SendEvent>(original) };
+
+    // NOTE: If NSApplication was safe to use from multiple threads, then this would potentially be
+    // a (checked) race-condition, since one could call `sendEvent:` before the original had been
+    // stored here.
+    //
+    // It is only usable from the main thread, however, so we're good!
+    let _ = mtm;
+    unsafe { ORIGINAL.set(Some(original)) };
+}
 
 fn maybe_dispatch_device_event(delegate: &ApplicationDelegate, event: &NSEvent) {
     let event_type = unsafe { event.r#type() };
@@ -85,3 +140,56 @@ fn maybe_dispatch_device_event(delegate: &ApplicationDelegate, event: &NSEvent)
         _ => (),
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use objc2::rc::Retained;
+    use objc2::{declare_class, msg_send_id, mutability, ClassType, DeclaredClass};
+
+    use super::*;
+
+    #[test]
+    fn test_override() {
+        // FIXME(madsmtm): Ensure this always runs (maybe use cargo-nextest or `--test-threads=1`?)
+        let Some(mtm) = MainThreadMarker::new() else { return };
+
+        // Create a new application, without making it the shared application.
+        let app = unsafe { NSApplication::new(mtm) };
+        override_send_event(&app);
+        // Test calling twice works.
+        override_send_event(&app);
+
+        // FIXME(madsmtm): Can't test this yet, need some way to mock AppState.
+        // unsafe {
+        //     let event = super::super::event::dummy_event().unwrap();
+        //     app.sendEvent(&event)
+        // }
+    }
+
+    #[test]
+    fn test_custom_class() {
+        let Some(_mtm) = MainThreadMarker::new() else { return };
+
+        declare_class!(
+            struct TestApplication;
+
+            unsafe impl ClassType for TestApplication {
+                type Super = NSApplication;
+                type Mutability = mutability::MainThreadOnly;
+                const NAME: &'static str = "TestApplication";
+            }
+
+            impl DeclaredClass for TestApplication {}
+
+            unsafe impl TestApplication {
+                #[method(sendEvent:)]
+                fn send_event(&self, _event: &NSEvent) {
+                    todo!()
+                }
+            }
+        );
+
+        let app: Retained<TestApplication> = unsafe { msg_send_id![TestApplication::class(), new] };
+        override_send_event(&app);
+    }
+}

src/platform_impl/macos/event_loop.rs

@@ -16,11 +16,11 @@ use core_foundation::runloop::{
 };
 use objc2::rc::{autoreleasepool, Retained};
 use objc2::runtime::ProtocolObject;
-use objc2::{msg_send_id, sel, ClassType};
+use objc2::sel;
 use objc2_app_kit::{NSApplication, NSApplicationActivationPolicy, NSWindow};
 use objc2_foundation::{MainThreadMarker, NSObjectProtocol};
 
-use super::app::WinitApplication;
+use super::app::override_send_event;
 use super::app_state::{ApplicationDelegate, HandlePendingUserEvents};
 use super::event::dummy_event;
 use super::monitor::{self, MonitorHandle};
@@ -220,15 +220,8 @@ impl<T> EventLoop<T> {
         let mtm = MainThreadMarker::new()
             .expect("on macOS, `EventLoop` must be created on the main thread!");
 
-        let app: Retained<NSApplication> =
-            unsafe { msg_send_id![WinitApplication::class(), sharedApplication] };
-
-        if !app.is_kind_of::<WinitApplication>() {
-            panic!(
-                "`winit` requires control over the principal class. You must create the event \
-                 loop before other parts of your application initialize NSApplication"
-            );
-        }
+        // Initialize the application (if it has not already been).
+        let app = NSApplication::sharedApplication(mtm);
 
         let activation_policy = match attributes.activation_policy {
             None => None,
@@ -247,6 +240,9 @@ impl<T> EventLoop<T> {
             app.setDelegate(Some(ProtocolObject::from_ref(&*delegate)));
         });
 
+        // Override `sendEvent:` on the application to forward to our application state.
+        override_send_event(&app);
+
         let panic_info: Rc<PanicInfo> = Default::default();
         setup_control_flow_observers(mtm, Rc::downgrade(&panic_info));