Drop application handler on run loop exit

Instead of the explicit `ApplicationHandler::exiting` event.

Author: madsmtm

examples/application.rs

@@ -578,18 +578,18 @@ impl ApplicationHandler for Application {
         }
     }
 
-    #[cfg(not(android_platform))]
-    fn exiting(&mut self, _event_loop: &dyn ActiveEventLoop) {
-        // We must drop the context here.
-        self.context = None;
-    }
-
     #[cfg(target_os = "macos")]
     fn macos_handler(&mut self) -> Option<&mut dyn ApplicationHandlerExtMacOS> {
         Some(self)
     }
 }
 
+impl Drop for Application {
+    fn drop(&mut self) {
+        info!("Application exited");
+    }
+}
+
 #[cfg(target_os = "macos")]
 impl ApplicationHandlerExtMacOS for Application {
     fn standard_key_binding(

examples/window.rs

@@ -11,6 +11,8 @@ use winit::window::{Window, WindowAttributes, WindowId};
 
 #[path = "util/fill.rs"]
 mod fill;
+#[path = "util/tracing.rs"]
+mod tracing;
 
 #[derive(Default)]
 struct App {
@@ -70,9 +72,12 @@ fn main() -> Result<(), Box<dyn Error>> {
     #[cfg(web_platform)]
     console_error_panic_hook::set_once();
 
+    tracing::init();
+
     let event_loop = EventLoop::new()?;
-    let mut app = App::default();
 
     // For alternative loop run options see `pump_events` and `run_on_demand` examples.
-    event_loop.run_app(&mut app).map_err(Into::into)
+    event_loop.run_app(App::default())?;
+
+    Ok(())
 }

src/application.rs

@@ -6,7 +6,17 @@ use crate::event_loop::ActiveEventLoop;
 use crate::platform::macos::ApplicationHandlerExtMacOS;
 use crate::window::WindowId;
 
-/// The handler of the application events.
+/// The handler of application-level events.
+///
+/// See [the top-level docs] for example usage, and [`EventLoop::run_app`] for an overview of when
+/// events are delivered.
+///
+/// This is [dropped] when the event loop is shut down. Note that this only works if you're passing
+/// the entire state to [`EventLoop::run_app`] (passing `&mut app` won't work).
+///
+/// [the top-level docs]: crate
+/// [`EventLoop::run_app`]: crate::event_loop::EventLoop::run_app
+/// [dropped]: std::ops::Drop
 pub trait ApplicationHandler {
     /// Emitted when new events arrive from the OS to be processed.
     ///
@@ -57,7 +67,6 @@ pub trait ApplicationHandler {
     ///
     /// [`resumed()`]: Self::resumed()
     /// [`suspended()`]: Self::suspended()
-    /// [`exiting()`]: Self::exiting()
     fn resumed(&mut self, event_loop: &dyn ActiveEventLoop) {
         let _ = event_loop;
     }
@@ -162,16 +171,14 @@ pub trait ApplicationHandler {
     ///
     ///     let (sender, receiver) = mpsc::channel();
     ///
-    ///     let mut app = MyApp { receiver };
-    ///
     ///     // Send an event in a loop
     ///     let proxy = event_loop.create_proxy();
     ///     let background_thread = thread::spawn(move || {
     ///         let mut i = 0;
     ///         loop {
     ///             println!("sending: {i}");
     ///             if sender.send(i).is_err() {
-    ///                 // Stop sending once `MyApp` is dropped
+    ///                 // Stop sending once the receiver is dropped
     ///                 break;
     ///             }
     ///             // Trigger the wake-up _after_ we placed the event in the channel.
@@ -182,9 +189,8 @@ pub trait ApplicationHandler {
     ///         }
     ///     });
     ///
-    ///     event_loop.run_app(&mut app)?;
+    ///     event_loop.run_app(MyApp { receiver })?;
     ///
-    ///     drop(app);
     ///     background_thread.join().unwrap();
     ///
     ///     Ok(())
@@ -203,6 +209,10 @@ pub trait ApplicationHandler {
     );
 
     /// Emitted when the OS sends an event to a device.
+    ///
+    /// For this to be called, it must be enabled with [`EventLoop::listen_device_events`].
+    ///
+    /// [`EventLoop::listen_device_events`]: crate::event_loop::EventLoop::listen_device_events
     fn device_event(
         &mut self,
         event_loop: &dyn ActiveEventLoop,
@@ -258,7 +268,7 @@ pub trait ApplicationHandler {
     /// to the user. This is a good place to stop refreshing UI, running animations and other visual
     /// things. It is driven by Android's [`onStop()`] method.
     ///
-    /// After this event the application either receives [`resumed()`] again, or [`exiting()`].
+    /// After this event the application either receives [`resumed()`] again, or will exit.
     ///
     /// [`onStop()`]: https://developer.android.com/reference/android/app/Activity#onStop()
     ///
@@ -268,7 +278,6 @@ pub trait ApplicationHandler {
     ///
     /// [`resumed()`]: Self::resumed()
     /// [`suspended()`]: Self::suspended()
-    /// [`exiting()`]: Self::exiting()
     fn suspended(&mut self, event_loop: &dyn ActiveEventLoop) {
         let _ = event_loop;
     }
@@ -310,14 +319,6 @@ pub trait ApplicationHandler {
         let _ = event_loop;
     }
 
-    /// Emitted when the event loop is being shut down.
-    ///
-    /// This is irreversible - if this method is called, it is guaranteed that the event loop
-    /// will exit right after.
-    fn exiting(&mut self, event_loop: &dyn ActiveEventLoop) {
-        let _ = event_loop;
-    }
-
     /// Emitted when the application has received a memory warning.
     ///
     /// ## Platform-specific
@@ -413,11 +414,6 @@ impl<A: ?Sized + ApplicationHandler> ApplicationHandler for &mut A {
         (**self).destroy_surfaces(event_loop);
     }
 
-    #[inline]
-    fn exiting(&mut self, event_loop: &dyn ActiveEventLoop) {
-        (**self).exiting(event_loop);
-    }
-
     #[inline]
     fn memory_warning(&mut self, event_loop: &dyn ActiveEventLoop) {
         (**self).memory_warning(event_loop);
@@ -487,11 +483,6 @@ impl<A: ?Sized + ApplicationHandler> ApplicationHandler for Box<A> {
         (**self).destroy_surfaces(event_loop);
     }
 
-    #[inline]
-    fn exiting(&mut self, event_loop: &dyn ActiveEventLoop) {
-        (**self).exiting(event_loop);
-    }
-
     #[inline]
     fn memory_warning(&mut self, event_loop: &dyn ActiveEventLoop) {
         (**self).memory_warning(event_loop);

src/changelog/unreleased.md

@@ -221,6 +221,8 @@ changelog entry.
  `ButtonSource` as part of the new pointer event overhaul.
 - Remove `Force::altitude_angle`.
 - Removed `Window::inner_position`, use the new `Window::surface_position` instead.
+- Removed `ApplicationHandler::exited`, the event loop being shut down can now be listened to in
+  the `Drop` impl on the application handler.
 
 ### Fixed
 

src/event.rs

@@ -1,39 +1,4 @@
 //! The event enums and assorted supporting types.
-//!
-//! These are sent to the closure given to [`EventLoop::run_app(...)`], where they get
-//! processed and used to modify the program state. For more details, see the root-level
-//! documentation.
-//!
-//! Some of these events represent different "parts" of a traditional event-handling loop. You could
-//! approximate the basic ordering loop of [`EventLoop::run_app(...)`] like this:
-//!
-//! ```rust,ignore
-//! let mut start_cause = StartCause::Init;
-//!
-//! while !elwt.exiting() {
-//!     app.new_events(event_loop, start_cause);
-//!
-//!     for event in (window events, user events, device events) {
-//!         // This will pick the right method on the application based on the event.
-//!         app.handle_event(event_loop, event);
-//!     }
-//!
-//!     for window_id in (redraw windows) {
-//!         app.window_event(event_loop, window_id, RedrawRequested);
-//!     }
-//!
-//!     app.about_to_wait(event_loop);
-//!     start_cause = wait_if_necessary();
-//! }
-//!
-//! app.exiting(event_loop);
-//! ```
-//!
-//! This leaves out timing details like [`ControlFlow::WaitUntil`] but hopefully
-//! describes what happens in what order.
-//!
-//! [`EventLoop::run_app(...)`]: crate::event_loop::EventLoop::run_app
-//! [`ControlFlow::WaitUntil`]: crate::event_loop::ControlFlow::WaitUntil
 use std::path::PathBuf;
 use std::sync::{Mutex, Weak};
 #[cfg(not(web_platform))]
@@ -99,11 +64,6 @@ pub(crate) enum Event {
     /// [`ApplicationHandler::about_to_wait()`]: crate::application::ApplicationHandler::about_to_wait()
     AboutToWait,
 
-    /// See [`ApplicationHandler::exiting()`] for details.
-    ///
-    /// [`ApplicationHandler::exiting()`]: crate::application::ApplicationHandler::exiting()
-    LoopExiting,
-
     /// See [`ApplicationHandler::memory_warning()`] for details.
     ///
     /// [`ApplicationHandler::memory_warning()`]: crate::application::ApplicationHandler::memory_warning()
@@ -701,10 +661,12 @@ impl FingerId {
 ///
 /// Useful for interactions that diverge significantly from a conventional 2D GUI, such as 3D camera
 /// or first-person game controls. Many physical actions, such as mouse movement, can produce both
-/// device and window events. Because window events typically arise from virtual devices
+/// device and [window events]. Because window events typically arise from virtual devices
 /// (corresponding to GUI pointers and keyboard focus) the device IDs may not match.
 ///
 /// Note that these events are delivered regardless of input focus.
+///
+/// [window events]: WindowEvent
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub enum DeviceEvent {
     /// Change in physical position of a pointing device.
@@ -1236,7 +1198,6 @@ mod tests {
                 let wid = WindowId::from_raw(0);
                 x(NewEvents(event::StartCause::Init));
                 x(AboutToWait);
-                x(LoopExiting);
                 x(Suspended);
                 x(Resumed);
 

src/event_loop.rs

@@ -197,7 +197,56 @@ impl EventLoop {
 impl EventLoop {
     /// Run the application with the event loop on the calling thread.
     ///
-    /// See the [`set_control_flow()`] docs on how to change the event loop's behavior.
+    /// ## Event loop flow
+    ///
+    /// This function internally handles the different parts of a traditional event-handling loop.
+    /// You can imagine this method as being implemented like this:
+    ///
+    /// ```rust,ignore
+    /// let mut start_cause = StartCause::Init;
+    ///
+    /// // Run the event loop.
+    /// while !event_loop.exiting() {
+    ///     // Wake up.
+    ///     app.new_events(event_loop, start_cause);
+    ///
+    ///     // Indicate that surfaces can now safely be created.
+    ///     if start_cause == StartCause::Init {
+    ///         app.can_create_surfaces(event_loop);
+    ///     }
+    ///
+    ///     // Handle proxy wake-up event.
+    ///     if event_loop.proxy_wake_up_set() {
+    ///         event_loop.proxy_wake_up_clear();
+    ///         app.proxy_wake_up(event_loop);
+    ///     }
+    ///
+    ///     // Handle actions done by the user / system such as moving the cursor, resizing the
+    ///     // window, changing the window theme, etc.
+    ///     for event in event_loop.events() {
+    ///         match event {
+    ///             window event => app.window_event(event_loop, window_id, event),
+    ///             device event => app.device_event(event_loop, device_id, event),
+    ///         }
+    ///     }
+    ///
+    ///     // Handle redraws.
+    ///     for window_id in event_loop.pending_redraws() {
+    ///         app.window_event(event_loop, window_id, WindowEvent::RedrawRequested);
+    ///     }
+    ///
+    ///     // Done handling events, wait until we're woken up again.
+    ///     app.about_to_wait(event_loop);
+    ///     start_cause = event_loop.wait_if_necessary();
+    /// }
+    ///
+    /// // Finished running, drop application state.
+    /// drop(app);
+    /// ```
+    ///
+    /// This is of course a very coarse-grained overview, and leaves out timing details like
+    /// [`ControlFlow::WaitUntil`] and life-cycle methods like [`ApplicationHandler::resumed`], but
+    /// it should give you an idea of how things fit together.
     ///
     /// ## Platform-specific
     ///
@@ -392,14 +441,21 @@ pub trait ActiveEventLoop: AsAny {
     /// Gets the current [`ControlFlow`].
     fn control_flow(&self) -> ControlFlow;
 
-    /// This exits the event loop.
+    /// Stop the event loop.
     ///
-    /// See [`exiting`][crate::application::ApplicationHandler::exiting].
+    /// ## Platform-specific
+    ///
+    /// ### iOS
+    ///
+    /// It is not possible to programmatically exit/quit an application on iOS, so this function is
+    /// a no-op there. See also [this technical Q&A][qa1561].
+    ///
+    /// [qa1561]: https://developer.apple.com/library/archive/qa/qa1561/_index.html
     fn exit(&self);
 
-    /// Returns if the [`EventLoop`] is about to stop.
+    /// Returns whether the [`EventLoop`] is about to stop.
     ///
-    /// See [`exit()`][Self::exit].
+    /// Set by [`exit()`][Self::exit].
     fn exiting(&self) -> bool;
 
     /// Gets a persistent reference to the underlying platform display.