Fix bug where events where not being dropped

crates/bevy_app/src/app.rs

@@ -1,4 +1,4 @@
-use crate::{First, Main, MainSchedulePlugin, Plugin, Plugins, StateTransition};
+use crate::{First, FixedPostUpdate, Main, MainSchedulePlugin, Plugin, Plugins, StateTransition};
 pub use bevy_derive::AppLabel;
 use bevy_ecs::{
     prelude::*,
@@ -213,6 +213,7 @@ pub enum PluginsState {
 
 // Dummy plugin used to temporary hold the place in the plugin registry
 struct PlaceholderPlugin;
+
 impl Plugin for PlaceholderPlugin {
     fn build(&self, _app: &mut App) {}
 }
@@ -507,6 +508,11 @@ impl App {
                 bevy_ecs::event::event_update_system::<T>
                     .run_if(bevy_ecs::event::event_update_condition::<T>),
             );
+            self.init_resource::<bevy_ecs::event::EventUpdateSignal<T>>()
+                .add_systems(
+                    FixedPostUpdate,
+                    bevy_ecs::event::event_queue_update_system::<T>,
+                );
         }
         self
     }

crates/bevy_ecs/src/event.rs

@@ -4,6 +4,7 @@ use crate as bevy_ecs;
 use crate::system::{Local, Res, ResMut, Resource, SystemParam};
 pub use bevy_ecs_macros::Event;
 use bevy_utils::detailed_trace;
+use bevy_utils::tracing::error;
 use std::ops::{Deref, DerefMut};
 use std::{
     cmp::Ordering,
@@ -13,6 +14,7 @@ use std::{
     marker::PhantomData,
     slice::Iter,
 };
+
 /// A type that can be stored in an [`Events<E>`] resource
 /// You can conveniently access events using the [`EventReader`] and [`EventWriter`] system parameter.
 ///
@@ -33,6 +35,7 @@ pub struct EventId<E: Event> {
 }
 
 impl<E: Event> Copy for EventId<E> {}
+
 impl<E: Event> Clone for EventId<E> {
     fn clone(&self) -> Self {
         *self
@@ -747,27 +750,51 @@ impl<'a, E: Event> ExactSizeIterator for EventIteratorWithId<'a, E> {
     }
 }
 
+#[doc(hidden)]
+#[derive(Resource)]
+pub struct EventUpdateSignal<T: Event> {
+    signal: bool,
+    _marker: PhantomData<fn() -> T>,
+}
+
+impl<T: Event> Default for EventUpdateSignal<T> {
+    fn default() -> Self {
+        Self {
+            signal: false,
+            _marker: PhantomData,
+        }
+    }
+}
+
 #[doc(hidden)]
 #[derive(Resource, Default)]
-pub struct EventUpdateSignal(bool);
+pub struct EventUpdateShouldWaitForFixedUpdate;
 
 /// A system that queues a call to [`Events::update`].
-pub fn event_queue_update_system(signal: Option<ResMut<EventUpdateSignal>>) {
+pub fn event_queue_update_system<T: Event>(signal: Option<ResMut<EventUpdateSignal<T>>>) {
     if let Some(mut s) = signal {
-        s.0 = true;
+        s.signal = true;
     }
 }
 
 /// A system that calls [`Events::update`].
 pub fn event_update_system<T: Event>(
-    signal: Option<ResMut<EventUpdateSignal>>,
+    should_wait: Option<Res<EventUpdateShouldWaitForFixedUpdate>>,
+    should_update: Option<ResMut<EventUpdateSignal<T>>>,
     mut events: ResMut<Events<T>>,
 ) {
-    if let Some(mut s) = signal {
-        // If we haven't got a signal to update the events, but we *could* get such a signal
-        // return early and update the events later.
-        if !std::mem::replace(&mut s.0, false) {
-            return;
+    if should_wait.is_some() {
+        match should_update {
+            Some(mut should_update) => {
+                // If we haven't got a signal to update the events, but we *could* get such a signal
+                // return early and update the events later.
+                if !std::mem::replace(&mut should_update.signal, false) {
+                    return;
+                }
+            }
+            None => {
+                error!("EventUpdateSignal<{0}> resource not found but fixed update systems are active. Please add EventUpdateSignal<{0}> as a resource", std::any::type_name::<T>());
+            }
         }
     }
 

crates/bevy_time/src/lib.rs

@@ -25,7 +25,7 @@ pub mod prelude {
 }
 
 use bevy_app::{prelude::*, RunFixedMainLoop};
-use bevy_ecs::event::{event_queue_update_system, EventUpdateSignal};
+use bevy_ecs::event::EventUpdateShouldWaitForFixedUpdate;
 use bevy_ecs::prelude::*;
 use bevy_utils::{tracing::warn, Duration, Instant};
 pub use crossbeam_channel::TrySendError;
@@ -60,8 +60,7 @@ impl Plugin for TimePlugin {
             .add_systems(RunFixedMainLoop, run_fixed_main_schedule);
 
         // ensure the events are not dropped until `FixedMain` systems can observe them
-        app.init_resource::<EventUpdateSignal>()
-            .add_systems(FixedPostUpdate, event_queue_update_system);
+        app.init_resource::<EventUpdateShouldWaitForFixedUpdate>();
 
         #[cfg(feature = "bevy_ci_testing")]
         if let Some(ci_testing_config) = app
@@ -142,3 +141,66 @@ fn time_system(
         TimeUpdateStrategy::ManualDuration(duration) => time.update_with_duration(*duration),
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::{Fixed, Time, TimePlugin, TimeUpdateStrategy};
+    use bevy_app::{App, Startup, Update};
+    use bevy_ecs::event::{Event, EventReader, EventWriter};
+    use std::error::Error;
+    use std::time::Duration;
+
+    #[derive(Event)]
+    struct TestEvent<T: Default> {
+        sender: std::sync::mpsc::Sender<T>,
+    }
+
+    impl<T: Default> Drop for TestEvent<T> {
+        fn drop(&mut self) {
+            self.sender
+                .send(T::default())
+                .expect("Failed to send drop signal");
+        }
+    }
+
+    #[test]
+    fn events_get_dropped_regression_test_11528() -> Result<(), impl Error> {
+        let (tx1, rx1) = std::sync::mpsc::channel();
+        let (tx2, rx2) = std::sync::mpsc::channel();
+        let mut app = App::new();
+        app.add_plugins(TimePlugin)
+            .add_event::<TestEvent<i32>>()
+            .add_event::<TestEvent<()>>()
+            .add_systems(Startup, move |mut ev2: EventWriter<TestEvent<()>>| {
+                ev2.send(TestEvent {
+                    sender: tx2.clone(),
+                });
+            })
+            .add_systems(Update, move |mut ev1: EventWriter<TestEvent<i32>>| {
+                // Keep adding events so this event type is processed every update
+                ev1.send(TestEvent {
+                    sender: tx1.clone(),
+                });
+            })
+            .add_systems(
+                Update,
+                |mut ev1: EventReader<TestEvent<i32>>, mut ev2: EventReader<TestEvent<()>>| {
+                    // Read events so they can be dropped
+                    for _ in ev1.read() {}
+                    for _ in ev2.read() {}
+                },
+            )
+            .insert_resource(TimeUpdateStrategy::ManualDuration(
+                Time::<Fixed>::default().timestep(),
+            ));
+
+        for _ in 0..10 {
+            app.update();
+        }
+
+        // Check event type 1 as been dropped at least once
+        let _drop_signal = rx1.recv_timeout(Duration::from_millis(1000));
+        // Check event type 2 has been dropped
+        rx2.recv_timeout(Duration::from_millis(1000))
+    }
+}