Optimize Event Updates

crates/bevy_app/src/app.rs

@@ -1,8 +1,10 @@
 use crate::{
-    Main, MainSchedulePlugin, PlaceholderPlugin, Plugin, Plugins, PluginsState, SubApp, SubApps,
+    First, Main, MainSchedulePlugin, PlaceholderPlugin, Plugin, Plugins, PluginsState, SubApp,
+    SubApps,
 };
 pub use bevy_derive::AppLabel;
 use bevy_ecs::{
+    event::event_update_system,
     intern::Interned,
     prelude::*,
     schedule::{ScheduleBuildSettings, ScheduleLabel},
@@ -89,7 +91,12 @@ impl Default for App {
         #[cfg(feature = "bevy_reflect")]
         app.init_resource::<AppTypeRegistry>();
         app.add_plugins(MainSchedulePlugin);
-
+        app.add_systems(
+            First,
+            event_update_system
+                .in_set(bevy_ecs::event::EventUpdates)
+                .run_if(bevy_ecs::event::event_update_condition),
+        );
         app.add_event::<AppExit>();
 
         app
@@ -369,8 +376,6 @@ 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,

crates/bevy_app/src/sub_app.rs

@@ -1,5 +1,6 @@
-use crate::{App, First, InternedAppLabel, Plugin, Plugins, PluginsState, StateTransition};
+use crate::{App, InternedAppLabel, Plugin, Plugins, PluginsState, StateTransition};
 use bevy_ecs::{
+    event::EventRegistry,
     prelude::*,
     schedule::{
         common_conditions::run_once as run_once_condition, run_enter_schedule,
@@ -362,12 +363,7 @@ impl SubApp {
         T: Event,
     {
         if !self.world.contains_resource::<Events<T>>() {
-            self.init_resource::<Events<T>>().add_systems(
-                First,
-                bevy_ecs::event::event_update_system::<T>
-                    .in_set(bevy_ecs::event::EventUpdates)
-                    .run_if(bevy_ecs::event::event_update_condition::<T>),
-            );
+            EventRegistry::register_event::<T>(self.world_mut());
         }
 
         self

crates/bevy_ecs/examples/events.rs

@@ -1,12 +1,14 @@
 //! In this example a system sends a custom event with a 50/50 chance during any frame.
 //! If an event was send, it will be printed by the console in a receiving system.
 
-use bevy_ecs::prelude::*;
+use bevy_ecs::{event::EventRegistry, prelude::*};
 
 fn main() {
     // Create a new empty world and add the event as a resource
     let mut world = World::new();
-    world.insert_resource(Events::<MyEvent>::default());
+    // The event registry is stored as a resource, and allows us to quickly update all events at once.
+    // This call adds both the registry resource and the events resource into the world.
+    EventRegistry::register_event::<MyEvent>(&mut world);
 
     // Create a schedule to store our systems
     let mut schedule = Schedule::default();
@@ -17,7 +19,7 @@ fn main() {
     #[derive(SystemSet, Debug, Clone, PartialEq, Eq, Hash)]
     pub struct FlushEvents;
 
-    schedule.add_systems(bevy_ecs::event::event_update_system::<MyEvent>.in_set(FlushEvents));
+    schedule.add_systems(bevy_ecs::event::event_update_system.in_set(FlushEvents));
 
     // Add systems sending and receiving events after the events are flushed.
     schedule.add_systems((

crates/bevy_ecs/src/change_detection.rs

@@ -833,6 +833,12 @@ impl<'w> MutUntyped<'w> {
         }
     }
 
+    /// Returns `true` if this value was changed or mutably dereferenced
+    /// either since a specific change tick.
+    pub fn has_changed_since(&self, tick: Tick) -> bool {
+        self.ticks.changed.is_newer_than(tick, self.ticks.this_run)
+    }
+
     /// Returns a pointer to the value without taking ownership of this smart pointer, marking it as changed.
     ///
     /// In order to avoid marking the value as changed, you need to call [`bypass_change_detection`](DetectChangesMut::bypass_change_detection).

crates/bevy_ecs/src/component.rs

@@ -765,7 +765,7 @@ impl Components {
 
 /// A value that tracks when a system ran relative to other systems.
 /// This is used to power change detection.
-#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+#[derive(Copy, Clone, Default, Debug, Eq, PartialEq)]
 #[cfg_attr(feature = "bevy_reflect", derive(Reflect), reflect(Debug, PartialEq))]
 pub struct Tick {
     tick: u32,

crates/bevy_ecs/src/event.rs

@@ -1,7 +1,13 @@
 //! Event handling types.
 
 use crate as bevy_ecs;
-use crate::system::{Local, Res, ResMut, Resource, SystemParam};
+use crate::change_detection::MutUntyped;
+use crate::{
+    change_detection::{DetectChangesMut, Mut},
+    component::{ComponentId, Tick},
+    system::{Local, Res, ResMut, Resource, SystemParam},
+    world::World,
+};
 pub use bevy_ecs_macros::Event;
 use bevy_ecs_macros::SystemSet;
 use bevy_utils::detailed_trace;
@@ -253,7 +259,13 @@ impl<E: Event> Events<E> {
     ///
     /// If you need access to the events that were removed, consider using [`Events::update_drain`].
     pub fn update(&mut self) {
-        let _ = self.update_drain();
+        std::mem::swap(&mut self.events_a, &mut self.events_b);
+        self.events_b.clear();
+        self.events_b.start_event_count = self.event_count;
+        debug_assert_eq!(
+            self.events_a.start_event_count + self.events_a.len(),
+            self.events_b.start_event_count
+        );
     }
 
     /// Swaps the event buffers and drains the oldest event buffer, returning an iterator
@@ -798,48 +810,90 @@ impl<'a, E: Event> ExactSizeIterator for EventIteratorWithId<'a, E> {
     }
 }
 
-#[doc(hidden)]
+struct RegisteredEvent {
+    component_id: ComponentId,
+    // Required to flush the secondary buffer and drop events even if left unchanged.
+    previously_updated: bool,
+    // SAFETY: The component ID and the function must be used to fetch the Events<T> resource
+    // of the same type initialized in `register_event`, or improper type casts will occur.
+    update: unsafe fn(MutUntyped),
+}
+
+/// A registry of all of the [`Events`] in the [`World`], used by [`event_update_system`]
+/// to update all of the events.
 #[derive(Resource, Default)]
-pub struct EventUpdateSignal(bool);
+pub struct EventRegistry {
+    needs_update: bool,
+    event_updates: Vec<RegisteredEvent>,
+}
+
+impl EventRegistry {
+    /// Registers an event type to be updated.
+    pub fn register_event<T: Event>(world: &mut World) {
+        // By initializing the resource here, we can be sure that it is present,
+        // and receive the correct, up-to-date `ComponentId` even if it was previously removed.
+        let component_id = world.init_resource::<Events<T>>();
+        let mut registry = world.get_resource_or_insert_with(Self::default);
+        registry.event_updates.push(RegisteredEvent {
+            component_id,
+            previously_updated: false,
+            update: |ptr| {
+                // SAFETY: The resource was initialized with the type Events<T>.
+                unsafe { ptr.with_type::<Events<T>>() }
+                    .bypass_change_detection()
+                    .update();
+            },
+        });
+    }
+
+    /// Updates all of the registered events in the World.
+    pub fn run_updates(&mut self, world: &mut World, last_change_tick: Tick) {
+        for registered_event in &mut self.event_updates {
+            // Bypass the type ID -> Component ID lookup with the cached component ID.
+            if let Some(events) = world.get_resource_mut_by_id(registered_event.component_id) {
+                let has_changed = events.has_changed_since(last_change_tick);
+                if registered_event.previously_updated || has_changed {
+                    // SAFETY: The update function pointer is called with the resource
+                    // fetched from the same component ID.
+                    unsafe { (registered_event.update)(events) };
+                    // Always set to true if the events have changed, otherwise disable running on the second invocation
+                    // to wait for more changes.
+                    registered_event.previously_updated =
+                        has_changed || !registered_event.previously_updated;
+                }
+            }
+        }
+    }
+}
 
 #[doc(hidden)]
 #[derive(SystemSet, Clone, Debug, PartialEq, Eq, Hash)]
 pub struct EventUpdates;
 
 /// Signals the [`event_update_system`] to run after `FixedUpdate` systems.
-pub fn signal_event_update_system(signal: Option<ResMut<EventUpdateSignal>>) {
-    if let Some(mut s) = signal {
-        s.0 = true;
+pub fn signal_event_update_system(signal: Option<ResMut<EventRegistry>>) {
+    if let Some(mut registry) = signal {
+        registry.needs_update = true;
     }
 }
 
-/// Resets the `EventUpdateSignal`
-pub fn reset_event_update_signal_system(signal: Option<ResMut<EventUpdateSignal>>) {
-    if let Some(mut s) = signal {
-        s.0 = false;
-    }
-}
-
-/// A system that calls [`Events::update`].
-pub fn event_update_system<T: Event>(
-    update_signal: Option<Res<EventUpdateSignal>>,
-    mut events: ResMut<Events<T>>,
-) {
-    if let Some(signal) = update_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 !signal.0 {
-            return;
-        }
+/// A system that calls [`Events::update`] on all registered [`Events`] in the world.
+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;
+        });
     }
-
-    events.update();
+    *last_change_tick = world.change_tick();
 }
 
-/// 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()
+/// 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)
 }
 
 /// [`Iterator`] over sent [`EventIds`](`EventId`) from a batch.

crates/bevy_ecs/src/schedule/condition.rs

@@ -837,7 +837,7 @@ pub mod common_conditions {
     /// # let mut world = World::new();
     /// # world.init_resource::<Counter>();
     /// # world.init_resource::<Events<MyEvent>>();
-    /// # app.add_systems(bevy_ecs::event::event_update_system::<MyEvent>.before(my_system));
+    /// # app.add_systems(bevy_ecs::event::event_update_system.before(my_system));
     ///
     /// app.add_systems(
     ///     my_system.run_if(on_event::<MyEvent>()),

crates/bevy_time/src/lib.rs

@@ -30,7 +30,7 @@ pub mod prelude {
 }
 
 use bevy_app::{prelude::*, RunFixedMainLoop};
-use bevy_ecs::event::{signal_event_update_system, EventUpdateSignal, EventUpdates};
+use bevy_ecs::event::signal_event_update_system;
 use bevy_ecs::prelude::*;
 use bevy_utils::{tracing::warn, Duration, Instant};
 pub use crossbeam_channel::TrySendError;
@@ -57,19 +57,11 @@ impl Plugin for TimePlugin {
             .register_type::<Time<Virtual>>()
             .register_type::<Time<Fixed>>()
             .register_type::<Timer>()
-            .add_systems(
-                First,
-                (time_system, virtual_time_system.after(time_system)).in_set(TimeSystem),
-            )
+            .add_systems(First, time_system.in_set(TimeSystem))
             .add_systems(RunFixedMainLoop, run_fixed_main_schedule);
 
         // ensure the events are not dropped until `FixedMain` systems can observe them
-        app.init_resource::<EventUpdateSignal>()
-            .add_systems(
-                First,
-                bevy_ecs::event::reset_event_update_signal_system.after(EventUpdates),
-            )
-            .add_systems(FixedPostUpdate, signal_event_update_system);
+        app.add_systems(FixedPostUpdate, signal_event_update_system);
     }
 }
 
@@ -111,7 +103,9 @@ pub fn create_time_channels() -> (TimeSender, TimeReceiver) {
 /// The system used to update the [`Time`] used by app logic. If there is a render world the time is
 /// sent from there to this system through channels. Otherwise the time is updated in this system.
 fn time_system(
-    mut time: ResMut<Time<Real>>,
+    mut real_time: ResMut<Time<Real>>,
+    mut virtual_time: ResMut<Time<Virtual>>,
+    mut time: ResMut<Time>,
     update_strategy: Res<TimeUpdateStrategy>,
     time_recv: Option<Res<TimeReceiver>>,
     mut has_received_time: Local<bool>,
@@ -132,10 +126,12 @@ fn time_system(
     };
 
     match update_strategy.as_ref() {
-        TimeUpdateStrategy::Automatic => time.update_with_instant(new_time),
-        TimeUpdateStrategy::ManualInstant(instant) => time.update_with_instant(*instant),
-        TimeUpdateStrategy::ManualDuration(duration) => time.update_with_duration(*duration),
+        TimeUpdateStrategy::Automatic => real_time.update_with_instant(new_time),
+        TimeUpdateStrategy::ManualInstant(instant) => real_time.update_with_instant(*instant),
+        TimeUpdateStrategy::ManualDuration(duration) => real_time.update_with_duration(*duration),
     }
+
+    update_virtual_time(&mut time, &mut virtual_time, &real_time);
 }
 
 #[cfg(test)]

crates/bevy_time/src/virt.rs

@@ -1,4 +1,3 @@
-use bevy_ecs::system::{Res, ResMut};
 use bevy_reflect::Reflect;
 use bevy_utils::{tracing::debug, Duration};
 
@@ -268,11 +267,7 @@ impl Default for Virtual {
 /// Advances [`Time<Virtual>`] and [`Time`] based on the elapsed [`Time<Real>`].
 ///
 /// The virtual time will be advanced up to the provided [`Time::max_delta`].
-pub fn virtual_time_system(
-    mut current: ResMut<Time>,
-    mut virt: ResMut<Time<Virtual>>,
-    real: Res<Time<Real>>,
-) {
+pub fn update_virtual_time(current: &mut Time, virt: &mut Time<Virtual>, real: &Time<Real>) {
     let raw_delta = real.delta();
     virt.advance_with_raw_delta(raw_delta);
     *current = virt.as_generic();