Swizzle sendEvent: instead of subclassing NSApplication (#4036)

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

@@ -185,6 +185,7 @@ changelog entry.
 - Updated `windows-sys` to `v0.59`.
   - To match the corresponding changes in `windows-sys`, the `HWND`, `HMONITOR`, and `HMENU` types
     now alias to `*mut c_void` instead of `isize`.
+- On macOS, no longer need control of the main `NSApplication` class (which means you can now override it yourself).
 
 ### Removed
 

src/platform_impl/apple/appkit/app.rs

@@ -1,44 +1,101 @@
 #![allow(clippy::unnecessary_cast)]
 
+use std::cell::Cell;
+use std::mem;
 use std::rc::Rc;
 
-use objc2::{define_class, msg_send, MainThreadMarker};
-use objc2_app_kit::{NSApplication, NSEvent, NSEventModifierFlags, NSEventType, NSResponder};
-use objc2_foundation::NSObject;
+use dispatch2::MainThreadBound;
+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};
 
-define_class!(
-    #[unsafe(super(NSApplication, NSResponder, NSObject))]
-    #[name = "WinitApplication"]
-    pub(super) struct WinitApplication;
+type SendEvent = extern "C-unwind" fn(&NSApplication, Sel, &NSEvent);
 
-    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)
-        #[unsafe(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::Command)
-            {
-                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] }
-            }
+static ORIGINAL: MainThreadBound<Cell<Option<SendEvent>>> = {
+    // SAFETY: Creating in a `const` context, where there is no concept of the main thread.
+    MainThreadBound::new(Cell::new(None), unsafe { MainThreadMarker::new_unchecked() })
+};
+
+extern "C-unwind" 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::Command) {
+        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 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!
+    ORIGINAL.get(mtm).set(Some(original));
+}
 
 fn maybe_dispatch_device_event(app_state: &Rc<AppState>, event: &NSEvent) {
     let event_type = unsafe { event.r#type() };
@@ -80,3 +137,52 @@ fn maybe_dispatch_device_event(app_state: &Rc<AppState>, event: &NSEvent) {
         _ => (),
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use objc2::rc::Retained;
+    use objc2::{define_class, msg_send, ClassType};
+    use objc2_app_kit::NSResponder;
+    use objc2_foundation::NSObject;
+
+    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 };
+
+        define_class!(
+            #[unsafe(super(NSApplication, NSResponder, NSObject))]
+            #[name = "TestApplication"]
+            pub(super) struct TestApplication;
+
+            impl TestApplication {
+                #[unsafe(method(sendEvent:))]
+                fn send_event(&self, _event: &NSEvent) {
+                    todo!()
+                }
+            }
+        );
+
+        let app: Retained<TestApplication> = unsafe { msg_send![TestApplication::class(), new] };
+        override_send_event(&app);
+    }
+}

src/platform_impl/apple/appkit/event_loop.rs

@@ -10,7 +10,7 @@ use std::time::{Duration, Instant};
 
 use objc2::rc::{autoreleasepool, Retained};
 use objc2::runtime::ProtocolObject;
-use objc2::{available, msg_send, ClassType, MainThreadMarker};
+use objc2::{available, MainThreadMarker};
 use objc2_app_kit::{
     NSApplication, NSApplicationActivationPolicy, NSApplicationDidFinishLaunchingNotification,
     NSApplicationWillTerminateNotification, NSWindow,
@@ -24,7 +24,7 @@ use objc2_foundation::{NSNotificationCenter, NSObjectProtocol};
 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;
@@ -209,16 +209,6 @@ impl EventLoop {
         let mtm = MainThreadMarker::new()
             .expect("on macOS, `EventLoop` must be created on the main thread!");
 
-        let app: Retained<NSApplication> =
-            unsafe { msg_send![WinitApplication::class(), sharedApplication] };
-
-        if !app.isKindOfClass(WinitApplication::class()) {
-            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),
@@ -233,6 +223,12 @@ 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);