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);
}
```
This commit is contained in:
UkoeHB 2024-09-09 11:37:27 -05:00 committed by GitHub
parent 09d2292016
commit adc2cf7dfe
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2 changed files with 116 additions and 0 deletions

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@ -44,6 +44,10 @@ pub mod state;
/// Provides [`StateScoped`](crate::state_scoped::StateScoped) and
/// [`clear_state_scoped_entities`](crate::state_scoped::clear_state_scoped_entities) for managing lifetime of entities.
pub mod state_scoped;
#[cfg(feature = "bevy_app")]
/// Provides [`App`](bevy_app::App) and [`SubApp`](bevy_app::SubApp) with methods for registering
/// state-scoped events.
pub mod state_scoped_events;
#[cfg(feature = "bevy_reflect")]
/// Provides definitions for the basic traits required by the state system
@ -71,4 +75,7 @@ pub mod prelude {
};
#[doc(hidden)]
pub use crate::state_scoped::StateScoped;
#[cfg(feature = "bevy_app")]
#[doc(hidden)]
pub use crate::state_scoped_events::StateScopedEventsAppExt;
}

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@ -0,0 +1,109 @@
use std::marker::PhantomData;
use bevy_app::{App, SubApp};
use bevy_ecs::{
event::{Event, EventReader, Events},
system::{Commands, Resource},
world::World,
};
use bevy_utils::HashMap;
use crate::state::{FreelyMutableState, OnExit, StateTransitionEvent};
fn clear_event_queue<E: Event>(w: &mut World) {
if let Some(mut queue) = w.get_resource_mut::<Events<E>>() {
queue.clear();
}
}
#[derive(Resource)]
struct StateScopedEvents<S: FreelyMutableState> {
cleanup_fns: HashMap<S, Vec<fn(&mut World)>>,
}
impl<S: FreelyMutableState> StateScopedEvents<S> {
fn add_event<E: Event>(&mut self, state: S) {
self.cleanup_fns
.entry(state)
.or_default()
.push(clear_event_queue::<E>);
}
fn cleanup(&self, w: &mut World, state: S) {
let Some(fns) = self.cleanup_fns.get(&state) else {
return;
};
for callback in fns {
(*callback)(w);
}
}
}
impl<S: FreelyMutableState> Default for StateScopedEvents<S> {
fn default() -> Self {
Self {
cleanup_fns: HashMap::default(),
}
}
}
fn cleanup_state_scoped_event<S: FreelyMutableState>(
mut c: Commands,
mut transitions: EventReader<StateTransitionEvent<S>>,
) {
let Some(transition) = transitions.read().last() else {
return;
};
if transition.entered == transition.exited {
return;
}
let Some(exited) = transition.exited.clone() else {
return;
};
c.add(move |w: &mut World| {
w.resource_scope::<StateScopedEvents<S>, ()>(|w, events| {
events.cleanup(w, exited);
});
});
}
fn add_state_scoped_event_impl<E: Event, S: FreelyMutableState>(
app: &mut SubApp,
_p: PhantomData<E>,
state: S,
) {
if !app.world().contains_resource::<StateScopedEvents<S>>() {
app.init_resource::<StateScopedEvents<S>>();
}
app.add_event::<E>();
app.world_mut()
.resource_mut::<StateScopedEvents<S>>()
.add_event::<E>(state.clone());
app.add_systems(OnExit(state), cleanup_state_scoped_event::<S>);
}
/// Extension trait for [`App`] adding methods for registering state scoped events.
pub trait StateScopedEventsAppExt {
/// Adds an [`Event`] that is automatically cleaned up when leaving the specified `state`.
///
/// Note that event cleanup is ordered ambiguously relative to [`StateScoped`](crate::prelude::StateScoped) entity
/// cleanup and the [`OnExit`] schedule for the target state. All of these (state scoped
/// entities and events cleanup, and `OnExit`) occur within schedule [`StateTransition`](crate::prelude::StateTransition)
/// and system set `StateTransitionSteps::ExitSchedules`.
fn add_state_scoped_event<E: Event>(&mut self, state: impl FreelyMutableState) -> &mut Self;
}
impl StateScopedEventsAppExt for App {
fn add_state_scoped_event<E: Event>(&mut self, state: impl FreelyMutableState) -> &mut Self {
add_state_scoped_event_impl(self.main_mut(), PhantomData::<E>, state);
self
}
}
impl StateScopedEventsAppExt for SubApp {
fn add_state_scoped_event<E: Event>(&mut self, state: impl FreelyMutableState) -> &mut Self {
add_state_scoped_event_impl(self, PhantomData::<E>, state);
self
}
}