bevy_winit(emit raw winit events) (#15884)

# Objective

- Exposes raw winit events making Bevy even more modular and powerful
for custom plugin developers (e.g. a custom render plugin).

XRef: #5977
It doesn't quite close the issue as sending events is not supported (or
not very useful to be precise). I would think that supporting that
requires some extra considerations by someone a bit more familiar with
the `bevy_winit` crate. That said, this PR could be a nice step forward.

## Solution

Emit `RawWinitWindowEvent` objects for each received event.

## Testing

I verified that the events are emitted using a basic test app. I don't
think it makes sense to solidify this behavior in one of the examples.

---

## Showcase

My example usage for a custom `egui_winit` integration:

```rust
for ev in winit_events.read() {
    if ev.window_id == window.id {
        let _ = egui_winit.on_window_event(&window, &ev.event);
    }
}
```

---------

Co-authored-by: IceSentry <[email protected]>

crates/bevy_winit/src/lib.rs

@@ -19,7 +19,7 @@ use bevy_reflect::prelude::ReflectDefault;
 use bevy_reflect::Reflect;
 use bevy_window::{RawHandleWrapperHolder, WindowEvent};
 use core::marker::PhantomData;
-use winit::event_loop::EventLoop;
+use winit::{event_loop::EventLoop, window::WindowId};
 
 use bevy_a11y::AccessibilityRequested;
 use bevy_app::{App, Last, Plugin};
@@ -121,6 +121,7 @@ impl<T: Event> Plugin for WinitPlugin<T> {
         app.init_non_send_resource::<WinitWindows>()
             .init_resource::<WinitMonitors>()
             .init_resource::<WinitSettings>()
+            .add_event::<RawWinitWindowEvent>()
             .set_runner(winit_runner::<T>)
             .add_systems(
                 Last,
@@ -153,6 +154,21 @@ impl<T: Event> Plugin for WinitPlugin<T> {
 #[reflect(Debug, Default)]
 pub struct WakeUp;
 
+/// The original window event as produced by Winit. This is meant as an escape
+/// hatch for power users that wish to add custom Winit integrations.
+/// If you want to process events for your app or game, you should instead use
+/// `bevy::window::WindowEvent`, or one of its sub-events.
+///
+/// When you receive this event it has already been handled by Bevy's main loop.
+/// Sending these events will NOT cause them to be processed by Bevy.
+#[derive(Debug, Clone, Event)]
+pub struct RawWinitWindowEvent {
+    /// The window for which the event was fired.
+    pub window_id: WindowId,
+    /// The raw winit window event.
+    pub event: winit::event::WindowEvent,
+}
+
 /// A wrapper type around [`winit::event_loop::EventLoopProxy`] with the specific
 /// [`winit::event::Event::UserEvent`] used in the [`WinitPlugin`].
 ///

crates/bevy_winit/src/state.rs

@@ -46,8 +46,8 @@ use crate::{
     accessibility::AccessKitAdapters,
     converters, create_windows,
     system::{create_monitors, CachedWindow},
-    AppSendEvent, CreateMonitorParams, CreateWindowParams, EventLoopProxyWrapper, UpdateMode,
-    WinitSettings, WinitWindows,
+    AppSendEvent, CreateMonitorParams, CreateWindowParams, EventLoopProxyWrapper,
+    RawWinitWindowEvent, UpdateMode, WinitSettings, WinitWindows,
 };
 
 /// Persistent state that is used to run the [`App`] according to the current
@@ -80,6 +80,8 @@ struct WinitAppRunnerState<T: Event> {
     previous_lifecycle: AppLifecycle,
     /// Bevy window events to send
     bevy_window_events: Vec<bevy_window::WindowEvent>,
+    /// Raw Winit window events to send
+    raw_winit_events: Vec<RawWinitWindowEvent>,
     _marker: PhantomData<T>,
 
     event_writer_system_state: SystemState<(
@@ -121,6 +123,7 @@ impl<T: Event> WinitAppRunnerState<T> {
             // 3 seems to be enough, 5 is a safe margin
             startup_forced_updates: 5,
             bevy_window_events: Vec::new(),
+            raw_winit_events: Vec::new(),
             _marker: PhantomData,
             event_writer_system_state,
         }
@@ -250,6 +253,12 @@ impl<T: Event> ApplicationHandler<T> for WinitAppRunnerState<T> {
             return;
         };
 
+        // Store a copy of the event to send to an EventWriter later.
+        self.raw_winit_events.push(RawWinitWindowEvent {
+            window_id,
+            event: event.clone(),
+        });
+
         // Allow AccessKit to respond to `WindowEvent`s before they reach
         // the engine.
         if let Some(adapter) = access_kit_adapters.get_mut(&window) {
@@ -690,14 +699,16 @@ impl<T: Event> WinitAppRunnerState<T> {
     }
 
     fn forward_bevy_events(&mut self) {
+        let raw_winit_events = self.raw_winit_events.drain(..).collect::<Vec<_>>();
         let buffered_events = self.bevy_window_events.drain(..).collect::<Vec<_>>();
+        let world = self.world_mut();
 
-        if buffered_events.is_empty() {
-            return;
+        if !raw_winit_events.is_empty() {
+            world
+                .resource_mut::<Events<RawWinitWindowEvent>>()
+                .send_batch(raw_winit_events);
         }
 
-        let world = self.world_mut();
-
         for winit_event in buffered_events.iter() {
             match winit_event.clone() {
                 BevyWindowEvent::AppLifecycle(e) => {
@@ -784,9 +795,11 @@ impl<T: Event> WinitAppRunnerState<T> {
             }
         }
 
-        world
-            .resource_mut::<Events<BevyWindowEvent>>()
-            .send_batch(buffered_events);
+        if !buffered_events.is_empty() {
+            world
+                .resource_mut::<Events<BevyWindowEvent>>()
+                .send_batch(buffered_events);
+        }
     }
 
     fn update_cursors(&mut self, #[cfg(feature = "custom_cursor")] event_loop: &ActiveEventLoop) {