Ensure that events are updated even when using a bare-bones Bevy App

crates/bevy_app/src/app.rs

@@ -925,7 +925,7 @@ mod tests {
         change_detection::{DetectChanges, ResMut},
         component::Component,
         entity::Entity,
-        event::EventWriter,
+        event::{Event, EventWriter, Events},
         query::With,
         removal_detection::RemovedComponents,
         schedule::{IntoSystemConfigs, ScheduleLabel},
@@ -1274,4 +1274,41 @@ mod tests {
             .init_non_send_resource::<NonSendTestResource>()
             .init_resource::<TestResource>();
     }
+
+    #[test]
+    fn events_should_be_updated_once_per_update() {
+        #[derive(Event, Clone)]
+        struct TestEvent;
+
+        let mut app = App::new();
+        app.add_event::<TestEvent>();
+
+        // Starts empty
+        let test_events = app.world().resource::<Events<TestEvent>>();
+        assert_eq!(test_events.len(), 0);
+        assert_eq!(test_events.iter_current_update_events().count(), 0);
+        app.update();
+
+        // Sending one event
+        app.world_mut().send_event(TestEvent);
+
+        let test_events = app.world().resource::<Events<TestEvent>>();
+        assert_eq!(test_events.len(), 1);
+        assert_eq!(test_events.iter_current_update_events().count(), 1);
+        app.update();
+
+        // Sending two events on the next frame
+        app.world_mut().send_event(TestEvent);
+        app.world_mut().send_event(TestEvent);
+
+        let test_events = app.world().resource::<Events<TestEvent>>();
+        assert_eq!(test_events.len(), 3); // Events are double-buffered, so we see 1 + 2 = 3
+        assert_eq!(test_events.iter_current_update_events().count(), 2);
+        app.update();
+
+        // Sending zero events
+        let test_events = app.world().resource::<Events<TestEvent>>();
+        assert_eq!(test_events.len(), 2); // Events are double-buffered, so we see 2 + 0 = 2
+        assert_eq!(test_events.iter_current_update_events().count(), 0);
+    }
 }

crates/bevy_ecs/src/event/collections.rs

@@ -366,3 +366,41 @@ impl<E: Event> ExactSizeIterator for SendBatchIds<E> {
         self.event_count.saturating_sub(self.last_count)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::{self as bevy_ecs, event::Events};
+    use bevy_ecs_macros::Event;
+
+    #[test]
+    fn iter_current_update_events_iterates_over_current_events() {
+        #[derive(Event, Clone)]
+        struct TestEvent;
+
+        let mut test_events = Events::<TestEvent>::default();
+
+        // Starting empty
+        assert_eq!(test_events.len(), 0);
+        assert_eq!(test_events.iter_current_update_events().count(), 0);
+        test_events.update();
+
+        // Sending one event
+        test_events.send(TestEvent);
+
+        assert_eq!(test_events.len(), 1);
+        assert_eq!(test_events.iter_current_update_events().count(), 1);
+        test_events.update();
+
+        // Sending two events on the next frame
+        test_events.send(TestEvent);
+        test_events.send(TestEvent);
+
+        assert_eq!(test_events.len(), 3); // Events are double-buffered, so we see 1 + 2 = 3
+        assert_eq!(test_events.iter_current_update_events().count(), 2);
+        test_events.update();
+
+        // Sending zero events
+        assert_eq!(test_events.len(), 2); // Events are double-buffered, so we see 2 + 0 = 2
+        assert_eq!(test_events.iter_current_update_events().count(), 0);
+    }
+}

crates/bevy_ecs/src/event/mod.rs

@@ -13,7 +13,7 @@ pub use bevy_ecs_macros::Event;
 pub use collections::{Events, SendBatchIds};
 pub use iterators::{EventIterator, EventIteratorWithId, EventParIter};
 pub use reader::{EventReader, ManualEventReader};
-pub use registry::EventRegistry;
+pub use registry::{EventRegistry, ShouldUpdateEvents};
 pub use update::{
     event_update_condition, event_update_system, signal_event_update_system, EventUpdates,
 };

crates/bevy_ecs/src/event/registry.rs

@@ -17,14 +17,29 @@ struct RegisteredEvent {
     update: unsafe fn(MutUntyped),
 }
 
-/// A registry of all of the [`Events`] in the [`World`], used by [`event_update_system`]
+/// A registry of all of the [`Events`] in the [`World`], used by [`event_update_system`](crate::event::update::event_update_system)
 /// to update all of the events.
 #[derive(Resource, Default)]
 pub struct EventRegistry {
-    pub(super) needs_update: bool,
+    /// Should the events be updated?
+    ///
+    /// This field is generally automatically updated by the [`signal_event_update_system`](crate::event::update::signal_event_update_system).
+    pub should_update: ShouldUpdateEvents,
     event_updates: Vec<RegisteredEvent>,
 }
 
+/// Controls whether or not the events in an [`EventRegistry`] should be updated.
+#[derive(Default, Debug, Clone, Copy, PartialEq, Eq)]
+pub enum ShouldUpdateEvents {
+    /// Without any fixed timestep, events should always be updated each frame.
+    #[default]
+    Always,
+    /// We need to wait until at least one pass of the fixed update schedules to update the events.
+    Waiting,
+    /// At least one pass of the fixed update schedules has occurred, and the events are ready to be updated.
+    Ready,
+}
+
 impl EventRegistry {
     /// Registers an event type to be updated.
     pub fn register_event<T: Event>(world: &mut World) {

crates/bevy_ecs/src/event/update.rs

@@ -10,14 +10,19 @@ use bevy_ecs_macros::SystemSet;
 #[cfg(feature = "bevy_reflect")]
 use std::hash::Hash;
 
+use super::registry::ShouldUpdateEvents;
+
 #[doc(hidden)]
 #[derive(SystemSet, Clone, Debug, PartialEq, Eq, Hash)]
 pub struct EventUpdates;
 
 /// Signals the [`event_update_system`] to run after `FixedUpdate` systems.
+///
+/// This will change the behavior of the [`EventRegistry`] to only run after a fixed update cycle has passed.
+/// Normally, this will simply run every frame.
 pub fn signal_event_update_system(signal: Option<ResMut<EventRegistry>>) {
     if let Some(mut registry) = signal {
-        registry.needs_update = true;
+        registry.should_update = ShouldUpdateEvents::Ready;
     }
 }
 
@@ -26,16 +31,32 @@ pub fn event_update_system(world: &mut World, mut last_change_tick: Local<Tick>)
     if world.contains_resource::<EventRegistry>() {
         world.resource_scope(|world, mut registry: Mut<EventRegistry>| {
             registry.run_updates(world, *last_change_tick);
-            // Disable the system until signal_event_update_system runs again.
-            registry.needs_update = false;
+
+            registry.should_update = match registry.should_update {
+                // If we're always updating, keep doing so.
+                ShouldUpdateEvents::Always => ShouldUpdateEvents::Always,
+                // Disable the system until signal_event_update_system runs again.
+                ShouldUpdateEvents::Waiting | ShouldUpdateEvents::Ready => {
+                    ShouldUpdateEvents::Waiting
+                }
+            };
         });
     }
     *last_change_tick = world.change_tick();
 }
 
 /// A run condition for [`event_update_system`].
-pub fn event_update_condition(signal: Option<Res<EventRegistry>>) -> bool {
-    // 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.
-    signal.map_or(false, |signal| signal.needs_update)
+///
+/// If [`signal_event_update_system`] has been run at least once,
+/// we will wait for it to be run again before updating the events.
+///
+/// Otherwise, we will always update the events.
+pub fn event_update_condition(maybe_signal: Option<Res<EventRegistry>>) -> bool {
+    match maybe_signal {
+        Some(signal) => match signal.should_update {
+            ShouldUpdateEvents::Always | ShouldUpdateEvents::Ready => true,
+            ShouldUpdateEvents::Waiting => false,
+        },
+        None => true,
+    }
 }