Application lifetime events (suspend audio on Android) (#10158)

# Objective

- Handle pausing audio when Android app is suspended

## Solution

- This is the start of application lifetime events. They are mostly
useful on mobile
- Next version of winit should add a few more
- When application is suspended, send an event to notify the
application, and run the schedule one last time before actually
suspending the app
- Audio is now suspended too 🎉 



https://github.com/bevyengine/bevy/assets/8672791/d74e2e09-ee29-4f40-adf2-36a0c064f94e

---------

Co-authored-by: Marco Buono <418473+coreh@users.noreply.github.com>

crates/bevy_window/src/event.rs

@@ -330,3 +330,22 @@ pub struct WindowThemeChanged {
     /// The new system theme.
     pub theme: WindowTheme,
 }
+
+/// Application lifetime events
+#[derive(Event, Debug, Clone, Copy, PartialEq, Eq, Reflect)]
+#[reflect(Debug, PartialEq)]
+#[cfg_attr(
+    feature = "serialize",
+    derive(serde::Serialize, serde::Deserialize),
+    reflect(Serialize, Deserialize)
+)]
+pub enum ApplicationLifetime {
+    /// The application just started.
+    Started,
+    /// The application was suspended.
+    ///
+    /// On Android, applications have one frame to react to this event before being paused in the background.
+    Suspended,
+    /// The application was resumed.
+    Resumed,
+}

crates/bevy_window/src/lib.rs

@@ -98,7 +98,8 @@ impl Plugin for WindowPlugin {
             .add_event::<WindowBackendScaleFactorChanged>()
             .add_event::<FileDragAndDrop>()
             .add_event::<WindowMoved>()
-            .add_event::<WindowThemeChanged>();
+            .add_event::<WindowThemeChanged>()
+            .add_event::<ApplicationLifetime>();
 
         if let Some(primary_window) = &self.primary_window {
             let initial_focus = app
@@ -141,7 +142,8 @@ impl Plugin for WindowPlugin {
             .register_type::<WindowBackendScaleFactorChanged>()
             .register_type::<FileDragAndDrop>()
             .register_type::<WindowMoved>()
-            .register_type::<WindowThemeChanged>();
+            .register_type::<WindowThemeChanged>()
+            .register_type::<ApplicationLifetime>();
 
         // Register window descriptor and related types
         app.register_type::<Window>()

crates/bevy_winit/src/lib.rs

@@ -38,10 +38,10 @@ use bevy_utils::{
     Duration, Instant,
 };
 use bevy_window::{
-    exit_on_all_closed, CursorEntered, CursorLeft, CursorMoved, FileDragAndDrop, Ime,
-    ReceivedCharacter, RequestRedraw, Window, WindowBackendScaleFactorChanged,
-    WindowCloseRequested, WindowCreated, WindowDestroyed, WindowFocused, WindowMoved,
-    WindowResized, WindowScaleFactorChanged, WindowThemeChanged,
+    exit_on_all_closed, ApplicationLifetime, CursorEntered, CursorLeft, CursorMoved,
+    FileDragAndDrop, Ime, ReceivedCharacter, RequestRedraw, Window,
+    WindowBackendScaleFactorChanged, WindowCloseRequested, WindowCreated, WindowDestroyed,
+    WindowFocused, WindowMoved, WindowResized, WindowScaleFactorChanged, WindowThemeChanged,
 };
 #[cfg(target_os = "android")]
 use bevy_window::{PrimaryWindow, RawHandleWrapper};
@@ -279,6 +279,7 @@ struct WindowAndInputEventWriters<'w> {
     window_moved: EventWriter<'w, WindowMoved>,
     window_theme_changed: EventWriter<'w, WindowThemeChanged>,
     window_destroyed: EventWriter<'w, WindowDestroyed>,
+    lifetime: EventWriter<'w, ApplicationLifetime>,
     keyboard_input: EventWriter<'w, KeyboardInput>,
     character_input: EventWriter<'w, ReceivedCharacter>,
     mouse_button_input: EventWriter<'w, MouseButtonInput>,
@@ -298,8 +299,8 @@ struct WindowAndInputEventWriters<'w> {
 /// Persistent state that is used to run the [`App`] according to the current
 /// [`UpdateMode`].
 struct WinitAppRunnerState {
-    /// Is `true` if the app is running and not suspended.
-    is_active: bool,
+    /// Current active state of the app.
+    active: ActiveState,
     /// Is `true` if a new [`WindowEvent`] has been received since the last update.
     window_event_received: bool,
     /// Is `true` if the app has requested a redraw since the last update.
@@ -312,10 +313,28 @@ struct WinitAppRunnerState {
     scheduled_update: Option<Instant>,
 }
 
+#[derive(PartialEq, Eq)]
+enum ActiveState {
+    NotYetStarted,
+    Active,
+    Suspended,
+    WillSuspend,
+}
+
+impl ActiveState {
+    #[inline]
+    fn should_run(&self) -> bool {
+        match self {
+            ActiveState::NotYetStarted | ActiveState::Suspended => false,
+            ActiveState::Active | ActiveState::WillSuspend => true,
+        }
+    }
+}
+
 impl Default for WinitAppRunnerState {
     fn default() -> Self {
         Self {
-            is_active: false,
+            active: ActiveState::NotYetStarted,
             window_event_received: false,
             redraw_requested: false,
             wait_elapsed: false,
@@ -700,19 +719,23 @@ pub fn winit_runner(mut app: App) {
                 });
             }
             event::Event::Suspended => {
-                runner_state.is_active = false;
-                #[cfg(target_os = "android")]
-                {
-                    // Remove the `RawHandleWrapper` from the primary window.
-                    // This will trigger the surface destruction.
-                    let mut query = app.world.query_filtered::<Entity, With<PrimaryWindow>>();
-                    let entity = query.single(&app.world);
-                    app.world.entity_mut(entity).remove::<RawHandleWrapper>();
-                    *control_flow = ControlFlow::Wait;
-                }
+                let (mut event_writers, _, _) = event_writer_system_state.get_mut(&mut app.world);
+                event_writers.lifetime.send(ApplicationLifetime::Suspended);
+                // Mark the state as `WillSuspend`. This will let the schedule run one last time
+                // before actually suspending to let the application react
+                runner_state.active = ActiveState::WillSuspend;
             }
             event::Event::Resumed => {
-                runner_state.is_active = true;
+                let (mut event_writers, _, _) = event_writer_system_state.get_mut(&mut app.world);
+                match runner_state.active {
+                    ActiveState::NotYetStarted => {
+                        event_writers.lifetime.send(ApplicationLifetime::Started);
+                    }
+                    _ => {
+                        event_writers.lifetime.send(ApplicationLifetime::Resumed);
+                    }
+                }
+                runner_state.active = ActiveState::Active;
                 #[cfg(target_os = "android")]
                 {
                     // Get windows that are cached but without raw handles. Those window were already created, but got their
@@ -754,7 +777,20 @@ pub fn winit_runner(mut app: App) {
                 }
             }
             event::Event::MainEventsCleared => {
-                if runner_state.is_active {
+                if runner_state.active.should_run() {
+                    if runner_state.active == ActiveState::WillSuspend {
+                        runner_state.active = ActiveState::Suspended;
+                        #[cfg(target_os = "android")]
+                        {
+                            // Remove the `RawHandleWrapper` from the primary window.
+                            // This will trigger the surface destruction.
+                            let mut query =
+                                app.world.query_filtered::<Entity, With<PrimaryWindow>>();
+                            let entity = query.single(&app.world);
+                            app.world.entity_mut(entity).remove::<RawHandleWrapper>();
+                            *control_flow = ControlFlow::Wait;
+                        }
+                    }
                     let (config, windows) = focused_windows_state.get(&app.world);
                     let focused = windows.iter().any(|window| window.focused);
                     let should_update = match config.update_mode(focused) {

examples/mobile/src/lib.rs

@@ -2,7 +2,11 @@
 // type aliases tends to obfuscate code while offering no improvement in code cleanliness.
 #![allow(clippy::type_complexity)]
 
-use bevy::{input::touch::TouchPhase, prelude::*, window::WindowMode};
+use bevy::{
+    input::touch::TouchPhase,
+    prelude::*,
+    window::{ApplicationLifetime, WindowMode},
+};
 
 // the `bevy_main` proc_macro generates the required boilerplate for iOS and Android
 #[bevy_main]
@@ -17,7 +21,7 @@ fn main() {
         ..default()
     }))
     .add_systems(Startup, (setup_scene, setup_music))
-    .add_systems(Update, (touch_camera, button_handler));
+    .add_systems(Update, (touch_camera, button_handler, handle_lifetime));
 
     // MSAA makes some Android devices panic, this is under investigation
     // https://github.com/bevyengine/bevy/issues/8229
@@ -161,3 +165,18 @@ fn setup_music(asset_server: Res<AssetServer>, mut commands: Commands) {
         settings: PlaybackSettings::LOOP,
     });
 }
+
+// Pause audio when app goes into background and resume when it returns.
+// This is handled by the OS on iOS, but not on Android.
+fn handle_lifetime(
+    mut lifetime_events: EventReader<ApplicationLifetime>,
+    music_controller: Query<&AudioSink>,
+) {
+    for event in lifetime_events.read() {
+        match event {
+            ApplicationLifetime::Suspended => music_controller.single().pause(),
+            ApplicationLifetime::Resumed => music_controller.single().play(),
+            ApplicationLifetime::Started => (),
+        }
+    }
+}