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 #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

@@ -76,6 +76,7 @@ changelog entry.
 - Added `Window::surface_position`, which is the position of the surface inside the window.
 - Added `Window::safe_area`, which describes the area of the surface that is unobstructed.
 - On X11, Wayland, Windows and macOS, improved scancode conversions for more obscure key codes.
+- On macOS, no longer need control of the main `NSApplication` class (which means you can now override it yourself).
 
 ### Changed
 

src/platform_impl/apple/appkit/app.rs

@@ -1,51 +1,114 @@
 #![allow(clippy::unnecessary_cast)]
 
+use std::cell::Cell;
+use std::mem;
 use std::rc::Rc;
 
-use objc2::{declare_class, msg_send, mutability, ClassType, DeclaredClass};
-use objc2_app_kit::{NSApplication, NSEvent, NSEventModifierFlags, NSEventType, NSResponder};
-use objc2_foundation::{MainThreadMarker, NSObject};
+use objc2::runtime::{Imp, Sel};
+use objc2::sel;
+use objc2_app_kit::{NSApplication, NSEvent, NSEventModifierFlags, NSEventType};
+use objc2_foundation::MainThreadMarker;
 
 use super::app_state::AppState;
 use crate::event::{DeviceEvent, ElementState};
 
-declare_class!(
-    pub(super) struct WinitApplication;
+// TODO(madsmtm): Use `MainThreadBound` once that is possible in `static`s.
+struct StaticMainThreadBound<T>(T);
 
-    unsafe impl ClassType for WinitApplication {
-        #[inherits(NSResponder, NSObject)]
-        type Super = NSApplication;
-        type Mutability = mutability::MainThreadOnly;
-        const NAME: &'static str = "WinitApplication";
+impl<T> StaticMainThreadBound<T> {
+    const fn get(&self, _mtm: MainThreadMarker) -> &T {
+        &self.0
     }
+}
 
-    impl DeclaredClass for WinitApplication {}
-
-    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 app_state = AppState::get(MainThreadMarker::from(self));
-                maybe_dispatch_device_event(&app_state, event);
-                unsafe { msg_send![super(self), sendEvent: event] }
-            }
+unsafe impl<T> Send for StaticMainThreadBound<T> {}
+unsafe impl<T> Sync for StaticMainThreadBound<T> {}
+
+// SAFETY: Creating `StaticMainThreadBound` in a `const` context,
+// where there is no concept of the main thread.
+static ORIGINAL: StaticMainThreadBound<Cell<Option<extern "C" fn(&NSApplication, Sel, &NSEvent)>>> =
+    StaticMainThreadBound(Cell::new(None));
+
+// FIXME(madsmtm): Use `extern "C-unwind"` once `objc2` supports that.
+extern "C" fn send_event(app: &NSApplication, sel: Sel, event: &NSEvent) {
+    let mtm = MainThreadMarker::from(app);
+
+    // 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 app_state = AppState::get(mtm);
+    maybe_dispatch_device_event(&app_state, event);
+
+    let original = ORIGINAL.get(mtm).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 e.g. 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::<extern "C" fn(&NSApplication, Sel, &NSEvent), 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.
+    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, extern "C" fn(&NSApplication, Sel, &NSEvent)>(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!
+    ORIGINAL.get(mtm).set(Some(original));
+}
 
 fn maybe_dispatch_device_event(app_state: &Rc<AppState>, event: &NSEvent) {
     let event_type = unsafe { event.r#type() };
@@ -87,3 +150,57 @@ fn maybe_dispatch_device_event(app_state: &Rc<AppState>, event: &NSEvent) {
         _ => (),
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use objc2::rc::Retained;
+    use objc2::{declare_class, msg_send_id, mutability, ClassType, DeclaredClass};
+    use objc2_app_kit::NSResponder;
+    use objc2_foundation::NSObject;
+
+    use super::*;
+
+    #[test]
+    fn test_override() {
+        // This is a test, so main thread safety doesn't _really_ matter.
+        let mtm = unsafe { MainThreadMarker::new_unchecked() };
+        // 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() {
+        declare_class!(
+            pub(super) struct TestApplication;
+
+            unsafe impl ClassType for TestApplication {
+                #[inherits(NSResponder, NSObject)]
+                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!()
+                }
+            }
+        );
+
+        // This is a test, so main thread safety doesn't _really_ matter.
+        let app: Retained<TestApplication> = unsafe { msg_send_id![TestApplication::class(), new] };
+        override_send_event(&app);
+    }
+}

src/platform_impl/apple/appkit/event_loop.rs

@@ -14,7 +14,7 @@ use core_foundation::runloop::{
     CFRunLoopSourceCreate, CFRunLoopSourceRef, CFRunLoopSourceSignal, CFRunLoopWakeUp,
 };
 use objc2::rc::{autoreleasepool, Retained};
-use objc2::{msg_send_id, sel, ClassType};
+use objc2::sel;
 use objc2_app_kit::{
     NSApplication, NSApplicationActivationPolicy, NSApplicationDidFinishLaunchingNotification,
     NSApplicationWillTerminateNotification, NSWindow,
@@ -23,7 +23,7 @@ use objc2_foundation::{MainThreadMarker, NSNotificationCenter, NSObject, NSObjec
 use rwh_06::HasDisplayHandle;
 
 use super::super::notification_center::create_observer;
-use super::app::WinitApplication;
+use super::app::override_send_event;
 use super::app_state::AppState;
 use super::cursor::CustomCursor;
 use super::event::dummy_event;
@@ -207,16 +207,6 @@ impl EventLoop {
         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"
-            );
-        }
-
         let activation_policy = match attributes.activation_policy {
             None => None,
             Some(ActivationPolicy::Regular) => Some(NSApplicationActivationPolicy::Regular),
@@ -231,6 +221,12 @@ impl EventLoop {
             attributes.activate_ignoring_other_apps,
         );
 
+        // Initialize the application (if it has not already been).
+        let app = NSApplication::sharedApplication(mtm);
+
+        // Override `sendEvent:` on the application to forward to our application state.
+        override_send_event(&app);
+
         let center = unsafe { NSNotificationCenter::defaultCenter() };
 
         let weak_app_state = Rc::downgrade(&app_state);