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Add state scoped events (#15085)
# Objective - Improve robustness of state transitions. Currently events that should be scoped to a specific state can leak between state scopes since events live for two ticks. - See https://github.com/bevyengine/bevy/issues/15072 ## Solution - Allow registering state scoped events that will be automatically cleared when exiting a state. This is *most of the time* not obviously useful, but enables users to write correct code that will avoid/reduce edge conditions (such as systems that aren't state scoped polling for a state scoped event and having unintended side effects outside a specific state instance). ## Testing Did not test. --- ## Showcase Added state scoped events that will be automatically cleared when exiting a state. Useful when you want to guarantee clean state transitions. Normal way to add an event: ```rust fn setup(app: &mut App) { app.add_event::<MyGameEvent>(); } ``` Add a state-scoped event (**NEW**): ```rust fn setup(app: &mut App) { app.add_state_scoped_event::<MyGameEvent>(GameState::Play); } ```
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@ -44,6 +44,10 @@ pub mod state;
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/// Provides [`StateScoped`](crate::state_scoped::StateScoped) and
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/// [`clear_state_scoped_entities`](crate::state_scoped::clear_state_scoped_entities) for managing lifetime of entities.
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pub mod state_scoped;
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#[cfg(feature = "bevy_app")]
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/// Provides [`App`](bevy_app::App) and [`SubApp`](bevy_app::SubApp) with methods for registering
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/// state-scoped events.
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pub mod state_scoped_events;
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#[cfg(feature = "bevy_reflect")]
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/// Provides definitions for the basic traits required by the state system
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@ -71,4 +75,7 @@ pub mod prelude {
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};
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#[doc(hidden)]
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pub use crate::state_scoped::StateScoped;
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#[cfg(feature = "bevy_app")]
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#[doc(hidden)]
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pub use crate::state_scoped_events::StateScopedEventsAppExt;
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}
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109
crates/bevy_state/src/state_scoped_events.rs
Normal file
109
crates/bevy_state/src/state_scoped_events.rs
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@ -0,0 +1,109 @@
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use std::marker::PhantomData;
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use bevy_app::{App, SubApp};
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use bevy_ecs::{
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event::{Event, EventReader, Events},
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system::{Commands, Resource},
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world::World,
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};
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use bevy_utils::HashMap;
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use crate::state::{FreelyMutableState, OnExit, StateTransitionEvent};
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fn clear_event_queue<E: Event>(w: &mut World) {
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if let Some(mut queue) = w.get_resource_mut::<Events<E>>() {
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queue.clear();
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}
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}
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#[derive(Resource)]
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struct StateScopedEvents<S: FreelyMutableState> {
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cleanup_fns: HashMap<S, Vec<fn(&mut World)>>,
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}
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impl<S: FreelyMutableState> StateScopedEvents<S> {
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fn add_event<E: Event>(&mut self, state: S) {
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self.cleanup_fns
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.entry(state)
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.or_default()
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.push(clear_event_queue::<E>);
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}
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fn cleanup(&self, w: &mut World, state: S) {
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let Some(fns) = self.cleanup_fns.get(&state) else {
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return;
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};
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for callback in fns {
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(*callback)(w);
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}
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}
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}
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impl<S: FreelyMutableState> Default for StateScopedEvents<S> {
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fn default() -> Self {
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Self {
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cleanup_fns: HashMap::default(),
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}
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}
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}
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fn cleanup_state_scoped_event<S: FreelyMutableState>(
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mut c: Commands,
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mut transitions: EventReader<StateTransitionEvent<S>>,
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) {
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let Some(transition) = transitions.read().last() else {
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return;
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};
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if transition.entered == transition.exited {
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return;
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}
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let Some(exited) = transition.exited.clone() else {
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return;
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};
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c.add(move |w: &mut World| {
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w.resource_scope::<StateScopedEvents<S>, ()>(|w, events| {
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events.cleanup(w, exited);
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});
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});
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}
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fn add_state_scoped_event_impl<E: Event, S: FreelyMutableState>(
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app: &mut SubApp,
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_p: PhantomData<E>,
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state: S,
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) {
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if !app.world().contains_resource::<StateScopedEvents<S>>() {
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app.init_resource::<StateScopedEvents<S>>();
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}
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app.add_event::<E>();
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app.world_mut()
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.resource_mut::<StateScopedEvents<S>>()
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.add_event::<E>(state.clone());
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app.add_systems(OnExit(state), cleanup_state_scoped_event::<S>);
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}
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/// Extension trait for [`App`] adding methods for registering state scoped events.
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pub trait StateScopedEventsAppExt {
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/// Adds an [`Event`] that is automatically cleaned up when leaving the specified `state`.
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///
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/// Note that event cleanup is ordered ambiguously relative to [`StateScoped`](crate::prelude::StateScoped) entity
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/// cleanup and the [`OnExit`] schedule for the target state. All of these (state scoped
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/// entities and events cleanup, and `OnExit`) occur within schedule [`StateTransition`](crate::prelude::StateTransition)
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/// and system set `StateTransitionSteps::ExitSchedules`.
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fn add_state_scoped_event<E: Event>(&mut self, state: impl FreelyMutableState) -> &mut Self;
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}
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impl StateScopedEventsAppExt for App {
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fn add_state_scoped_event<E: Event>(&mut self, state: impl FreelyMutableState) -> &mut Self {
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add_state_scoped_event_impl(self.main_mut(), PhantomData::<E>, state);
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self
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}
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}
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impl StateScopedEventsAppExt for SubApp {
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fn add_state_scoped_event<E: Event>(&mut self, state: impl FreelyMutableState) -> &mut Self {
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add_state_scoped_event_impl(self, PhantomData::<E>, state);
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self
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}
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}
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