Simplify sending empty events (#2935)

# Objective

When using empty events, it can feel redundant to have to specify the type of the event when sending it.

## Solution

Add a new `fire()` function that sends the default value of the event. This requires that the event derives Default.


Co-authored-by: Carter Anderson <mcanders1@gmail.com>

crates/bevy_app/src/app.rs

@@ -915,5 +915,5 @@ fn run_once(mut app: App) {
 }
 
 /// An event that indicates the app should exit. This will fully exit the app process.
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Default)]
 pub struct AppExit;

crates/bevy_ecs/src/event.rs

@@ -122,6 +122,10 @@ enum State {
 /// This complicates consumption and risks ever-expanding memory usage if not cleaned up,
 /// but can be done by adding your event as a resource instead of using
 /// [`add_event`](https://docs.rs/bevy/*/bevy/app/struct.App.html#method.add_event).
+///
+/// [Example usage.](https://github.com/bevyengine/bevy/blob/latest/examples/ecs/event.rs)
+/// [Example usage standalone.](https://github.com/bevyengine/bevy/blob/latest/bevy_ecs/examples/events.rs)
+///
 #[derive(Debug)]
 pub struct Events<T> {
     events_a: Vec<EventInstance<T>>,
@@ -169,6 +173,8 @@ pub struct EventWriter<'w, 's, T: Resource> {
 }
 
 impl<'w, 's, T: Resource> EventWriter<'w, 's, T> {
+    /// Sends an `event`. [`EventReader`]s can then read the event.
+    /// See [`Events`] for details.
     pub fn send(&mut self, event: T) {
         self.events.send(event);
     }
@@ -176,6 +182,14 @@ impl<'w, 's, T: Resource> EventWriter<'w, 's, T> {
     pub fn send_batch(&mut self, events: impl Iterator<Item = T>) {
         self.events.extend(events);
     }
+
+    /// Sends the default value of the event. Useful when the event is an empty struct.
+    pub fn send_default(&mut self)
+    where
+        T: Default,
+    {
+        self.events.send_default();
+    }
 }
 
 pub struct ManualEventReader<T> {
@@ -296,6 +310,14 @@ impl<T: Resource> Events<T> {
         self.event_count += 1;
     }
 
+    /// Sends the default value of the event. Useful when the event is an empty struct.
+    pub fn send_default(&mut self)
+    where
+        T: Default,
+    {
+        self.send(Default::default());
+    }
+
     /// Gets a new [`ManualEventReader`]. This will include all events already in the event buffers.
     pub fn get_reader(&self) -> ManualEventReader<T> {
         ManualEventReader {
@@ -540,11 +562,11 @@ mod tests {
         );
     }
 
-    fn get_events(
-        events: &Events<TestEvent>,
-        reader: &mut ManualEventReader<TestEvent>,
-    ) -> Vec<TestEvent> {
-        reader.iter(events).cloned().collect::<Vec<TestEvent>>()
+    fn get_events<T: Resource + Clone>(
+        events: &Events<T>,
+        reader: &mut ManualEventReader<T>,
+    ) -> Vec<T> {
+        reader.iter(events).cloned().collect::<Vec<T>>()
     }
 
     #[derive(PartialEq, Eq, Debug)]
@@ -652,4 +674,19 @@ mod tests {
         events.update();
         assert!(reader.is_empty(&events));
     }
+
+    #[derive(Clone, PartialEq, Debug, Default)]
+    struct EmptyTestEvent;
+
+    #[test]
+    fn test_firing_empty_event() {
+        let mut events = Events::<EmptyTestEvent>::default();
+        events.send_default();
+
+        let mut reader = events.get_reader();
+        assert_eq!(
+            get_events(&events, &mut reader),
+            vec![EmptyTestEvent::default()]
+        );
+    }
 }

crates/bevy_input/src/system.rs

@@ -13,7 +13,7 @@ pub fn exit_on_esc_system(
     for event in keyboard_input_events.iter() {
         if let Some(key_code) = event.key_code {
             if event.state == ElementState::Pressed && key_code == KeyCode::Escape {
-                app_exit_events.send(AppExit);
+                app_exit_events.send_default();
             }
         }
     }

examples/ecs/event.rs

@@ -6,9 +6,11 @@ fn main() {
     App::new()
         .add_plugins(DefaultPlugins)
         .add_event::<MyEvent>()
+        .add_event::<PlaySound>()
         .init_resource::<EventTriggerState>()
-        .add_system(event_trigger_system)
-        .add_system(event_listener_system)
+        .add_system(event_trigger)
+        .add_system(event_listener)
+        .add_system(sound_player)
         .run();
 }
 
@@ -16,6 +18,9 @@ struct MyEvent {
     pub message: String,
 }
 
+#[derive(Default)]
+struct PlaySound;
+
 struct EventTriggerState {
     event_timer: Timer,
 }
@@ -28,22 +33,30 @@ impl Default for EventTriggerState {
     }
 }
 
-// sends MyEvent every second
-fn event_trigger_system(
+// sends MyEvent and PlaySound every second
+fn event_trigger(
     time: Res<Time>,
     mut state: ResMut<EventTriggerState>,
     mut my_events: EventWriter<MyEvent>,
+    mut play_sound_events: EventWriter<PlaySound>,
 ) {
     if state.event_timer.tick(time.delta()).finished() {
         my_events.send(MyEvent {
             message: "MyEvent just happened!".to_string(),
         });
+        play_sound_events.send_default();
     }
 }
 
 // prints events as they come in
-fn event_listener_system(mut events: EventReader<MyEvent>) {
+fn event_listener(mut events: EventReader<MyEvent>) {
     for my_event in events.iter() {
         info!("{}", my_event.message);
     }
 }
+
+fn sound_player(mut play_sound_events: EventReader<PlaySound>) {
+    for _ in play_sound_events.iter() {
+        info!("Playing a sound");
+    }
+}