Only run event systems if they have tangible work to do (#7728)

# Objective
Scheduling low cost systems has significant overhead due to task pool
contention and the extra machinery to schedule and run them. Event
update systems are the prime example of a low cost system, requiring a
guaranteed O(1) operation, and there are a *lot* of them.

## Solution
Add a run condition to every event system so they only run when there is
an event in either of it's two internal Vecs.

---

## Changelog
Changed: Event update systems will not run if there are no events to
process.

## Migration Guide
`Events<T>::update_system` has been split off from the the type and can
be found at `bevy_ecs::event::event_update_system`.

---------

Co-authored-by: IceSentry <IceSentry@users.noreply.github.com>

crates/bevy_app/src/app.rs

@@ -424,7 +424,7 @@ impl App {
     /// Setup the application to manage events of type `T`.
     ///
     /// This is done by adding a [`Resource`] of type [`Events::<T>`],
-    /// and inserting an [`update_system`](Events::update_system) into [`First`].
+    /// and inserting an [`event_update_system`] into [`First`].
     ///
     /// See [`Events`] for defining events.
     ///
@@ -440,13 +440,18 @@ impl App {
     /// #
     /// app.add_event::<MyEvent>();
     /// ```
+    ///
+    /// [`event_update_system`]: bevy_ecs::event::event_update_system
     pub fn add_event<T>(&mut self) -> &mut Self
     where
         T: Event,
     {
         if !self.world.contains_resource::<Events<T>>() {
-            self.init_resource::<Events<T>>()
-                .add_systems(First, Events::<T>::update_system);
+            self.init_resource::<Events<T>>().add_systems(
+                First,
+                bevy_ecs::event::event_update_system::<T>
+                    .run_if(bevy_ecs::event::event_update_condition::<T>),
+            );
         }
         self
     }

crates/bevy_ecs/examples/events.rs

@@ -16,7 +16,7 @@ fn main() {
     #[derive(SystemSet, Debug, Clone, PartialEq, Eq, Hash)]
     pub struct FlushEvents;
 
-    schedule.add_systems(Events::<MyEvent>::update_system.in_set(FlushEvents));
+    schedule.add_systems(bevy_ecs::event::event_update_system::<MyEvent>.in_set(FlushEvents));
 
     // Add systems sending and receiving events after the events are flushed.
     schedule.add_systems((

crates/bevy_ecs/src/event.rs

@@ -103,7 +103,7 @@ struct EventInstance<E: Event> {
 /// This collection is meant to be paired with a system that calls
 /// [`Events::update`] exactly once per update/frame.
 ///
-/// [`Events::update_system`] is a system that does this, typically initialized automatically using
+/// [`event_update_system`] is a system that does this, typically initialized automatically using
 /// [`add_event`](https://docs.rs/bevy/*/bevy/app/struct.App.html#method.add_event).
 /// [`EventReader`]s are expected to read events from this collection at least once per loop/frame.
 /// Events will persist across a single frame boundary and so ordering of event producers and
@@ -251,11 +251,6 @@ impl<E: Event> Events<E> {
         iter.map(|e| e.event)
     }
 
-    /// A system that calls [`Events::update`] once per frame.
-    pub fn update_system(mut events: ResMut<Self>) {
-        events.update();
-    }
-
     #[inline]
     fn reset_start_event_count(&mut self) {
         self.events_a.start_event_count = self.event_count;
@@ -754,6 +749,17 @@ 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>>) {
+    events.update();
+}
+
+/// A run condition that checks if the event's [`event_update_system`]
+/// needs to run or not.
+pub fn event_update_condition<T: Event>(events: Res<Events<T>>) -> bool {
+    !events.events_a.is_empty() || !events.events_b.is_empty()
+}
+
 #[cfg(test)]
 mod tests {
     use crate::system::assert_is_read_only_system;

crates/bevy_ecs/src/schedule/condition.rs

@@ -867,7 +867,7 @@ pub mod common_conditions {
     /// # let mut world = World::new();
     /// # world.init_resource::<Counter>();
     /// # world.init_resource::<Events<MyEvent>>();
-    /// # app.add_systems(Events::<MyEvent>::update_system.before(my_system));
+    /// # app.add_systems(bevy_ecs::event::event_update_system::<MyEvent>.before(my_system));
     ///
     /// app.add_systems(
     ///     my_system.run_if(on_event::<MyEvent>()),