Only run event systems if they have tangible work to do

crates/bevy_app/src/app.rs

@@ -585,8 +585,11 @@ impl App {
         T: Event,
     {
         if !self.world.contains_resource::<Events<T>>() {
-            self.init_resource::<Events<T>>()
-                .add_system(Events::<T>::update_system.in_base_set(CoreSet::First));
+            self.init_resource::<Events<T>>().add_system(
+                bevy_ecs::events::update_system::<T>
+                    .in_base_set(CoreSet::First)
+                    .run_if(bevy_ecs::events::update_condition::<T>),
+            );
         }
         self
     }

crates/bevy_ecs/src/event.rs

@@ -555,11 +555,6 @@ impl<E: Event> Events<E> {
         );
     }
 
-    /// 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;
@@ -665,6 +660,17 @@ impl<E: Event> std::iter::Extend<E> for Events<E> {
     }
 }
 
+/// A system that calls [`Events::update`] once per frame.
+pub fn update_system<T: Send + Sync + 'static>(mut events: ResMut<Events<T>>) {
+    events.update();
+}
+
+/// A run condition that checks if the event's [`update_system`]
+/// needs to run or not.
+pub fn update_condition<T: Send + Sync + 'static>(events: Res<Events<T>>) -> bool {
+    !events.events_a.is_empty() || !events.events_b.is_empty()
+}
+
 #[cfg(test)]
 mod tests {
     use crate::{prelude::World, system::SystemState};