some kind of signaling

Author: maniwani

crates/bevy_app/src/app.rs

@@ -1,6 +1,7 @@
-use crate::{FixedUpdate, Main, MainSchedulePlugin, Plugin, Plugins, StateTransition};
+use crate::{First, FixedUpdate, Main, MainSchedulePlugin, Plugin, Plugins, StateTransition};
 pub use bevy_derive::AppLabel;
 use bevy_ecs::{
+    event::EventUpdateSignal,
     prelude::*,
     schedule::{
         apply_state_transition, common_conditions::run_once as run_once_condition,
@@ -485,11 +486,18 @@ impl App {
         T: Event,
     {
         if !self.world.contains_resource::<Events<T>>() {
-            self.init_resource::<Events<T>>().add_systems(
-                FixedUpdate,
-                bevy_ecs::event::event_update_system::<T>
-                    .run_if(bevy_ecs::event::event_update_condition::<T>),
-            );
+            self.init_resource::<Events<T>>()
+                .init_resource::<EventUpdateSignal<T>>()
+                .add_systems(
+                    First,
+                    bevy_ecs::event::event_update_system::<T>
+                        .run_if(bevy_ecs::event::event_update_condition::<T>),
+                )
+                .add_systems(
+                    FixedUpdate,
+                    bevy_ecs::event::event_queue_update_system::<T>
+                        .run_if(bevy_ecs::event::event_update_condition::<T>),
+                );
         }
         self
     }

crates/bevy_ecs/src/event.rs

@@ -749,8 +749,34 @@ impl<'a, E: Event> ExactSizeIterator for EventIteratorWithId<'a, E> {
     }
 }
 
+#[doc(hidden)]
+#[derive(Resource)]
+pub struct EventUpdateSignal<T>(bool, PhantomData<T>);
+
+impl<T> Default for EventUpdateSignal<T> {
+    fn default() -> Self {
+        Self(false, PhantomData)
+    }
+}
+
+/// A system that queues a call to [`Events::update`].
+pub fn event_queue_update_system<T: Event>(signal: Option<ResMut<EventUpdateSignal<T>>>) {
+    if let Some(mut s) = signal {
+        s.0 = true;
+    }
+}
+
 /// A system that calls [`Events::update`].
-pub fn event_update_system<T: Event>(mut events: ResMut<Events<T>>) {
+pub fn event_update_system<T: Event>(
+    signal: Option<ResMut<EventUpdateSignal<T>>>,
+    mut events: ResMut<Events<T>>,
+) {
+    if let Some(mut s) = signal {
+        if !std::mem::replace(&mut s.0, false) {
+            return;
+        }
+    }
+
     events.update();
 }