Wait until FixedUpdate can see events before dropping them

crates/bevy_app/src/app.rs

@@ -190,6 +190,7 @@ impl Default for App {
         app.init_resource::<AppTypeRegistry>();
 
         app.add_plugins(MainSchedulePlugin);
+
         app.add_event::<AppExit>();
 
         #[cfg(feature = "bevy_ci_testing")]

crates/bevy_ecs/src/event.rs

@@ -733,8 +733,30 @@ impl<'a, E: Event> ExactSizeIterator for EventIteratorWithId<'a, E> {
     }
 }
 
-/// A system that calls [`Events::update`] once per frame.
-pub fn event_update_system<T: Event>(mut events: ResMut<Events<T>>) {
+#[doc(hidden)]
+#[derive(Resource, Default)]
+pub struct EventUpdateSignal(bool);
+
+/// A system that queues a call to [`Events::update`].
+pub fn event_queue_update_system(signal: Option<ResMut<EventUpdateSignal>>) {
+    if let Some(mut s) = signal {
+        s.0 = true;
+    }
+}
+
+/// A system that calls [`Events::update`].
+pub fn event_update_system<T: Event>(
+    signal: Option<ResMut<EventUpdateSignal>>,
+    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;
+        }
+    }
+
     events.update();
 }
 

crates/bevy_time/src/lib.rs

@@ -18,19 +18,18 @@ pub use time::*;
 pub use timer::*;
 pub use virt::*;
 
-use bevy_ecs::system::{Res, ResMut};
-use bevy_utils::{tracing::warn, Duration, Instant};
-pub use crossbeam_channel::TrySendError;
-use crossbeam_channel::{Receiver, Sender};
-
 pub mod prelude {
     //! The Bevy Time Prelude.
     #[doc(hidden)]
     pub use crate::{Fixed, Real, Time, Timer, TimerMode, Virtual};
 }
 
 use bevy_app::{prelude::*, RunFixedUpdateLoop};
+use bevy_ecs::event::{event_queue_update_system, EventUpdateSignal};
 use bevy_ecs::prelude::*;
+use bevy_utils::{tracing::warn, Duration, Instant};
+pub use crossbeam_channel::TrySendError;
+use crossbeam_channel::{Receiver, Sender};
 
 /// Adds time functionality to Apps.
 #[derive(Default)]
@@ -60,6 +59,10 @@ impl Plugin for TimePlugin {
             )
             .add_systems(RunFixedUpdateLoop, run_fixed_update_schedule);
 
+        // ensure the events are not dropped until `FixedUpdate` systems can observe them
+        app.init_resource::<EventUpdateSignal>()
+            .add_systems(FixedUpdate, event_queue_update_system);
+
         #[cfg(feature = "bevy_ci_testing")]
         if let Some(ci_testing_config) = app
             .world