Simplify state transitions (#13616)

# Objective Prerequisite to #13579. Make state transition schedule running simpler. ## Solution - Remove `should_run_transition` which read the latest event and fake-fire an event for the startup transitions (e.g. startup `OnEnter()`). - Account for startup event, by actually emitting an event when adding states to `App`. - Replace `should_run_transition` with `last_transition`, which is a light wrapper over `EventReader::read().last()`. --------- Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
bevyengine · Jun 1, 2024 · 5cb4808 · 5cb4808
1 parent 4b996c7
commit 5cb4808
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Showing 4 changed files with 50 additions and 52 deletions.
diff --git a/crates/bevy_app/src/sub_app.rs b/crates/bevy_app/src/sub_app.rs
@@ -308,6 +308,11 @@ impl SubApp {
  .add_event::<StateTransitionEvent<S>>();
  let schedule = self.get_schedule_mut(StateTransition).unwrap();
  S::register_state(schedule);
+ let state = self.world.resource::<State<S>>().get().clone();
+ self.world.send_event(StateTransitionEvent {
+ exited: None,
+ entered: Some(state),
+ });
  }
 
  self
@@ -318,12 +323,15 @@ impl SubApp {
  pub fn insert_state<S: FreelyMutableState>(&mut self, state: S) -> &mut Self {
  if !self.world.contains_resource::<State<S>>() {
  setup_state_transitions_in_world(&mut self.world, Some(Startup.intern()));
- self.insert_resource::<State<S>>(State::new(state))
+ self.insert_resource::<State<S>>(State::new(state.clone()))
  .init_resource::<NextState<S>>()
  .add_event::<StateTransitionEvent<S>>();
-
  let schedule = self.get_schedule_mut(StateTransition).unwrap();
  S::register_state(schedule);
+ self.world.send_event(StateTransitionEvent {
+ exited: None,
+ entered: Some(state),
+ });
  }
 
  self
@@ -340,6 +348,11 @@ impl SubApp {
  self.add_event::<StateTransitionEvent<S>>();
  let schedule = self.get_schedule_mut(StateTransition).unwrap();
  S::register_computed_state_systems(schedule);
+ let state = self.world.resource::<State<S>>().get().clone();
+ self.world.send_event(StateTransitionEvent {
+ exited: None,
+ entered: Some(state),
+ });
  }
 
  self
@@ -357,6 +370,11 @@ impl SubApp {
  self.add_event::<StateTransitionEvent<S>>();
  let schedule = self.get_schedule_mut(StateTransition).unwrap();
  S::register_sub_state_systems(schedule);
+ let state = self.world.resource::<State<S>>().get().clone();
+ self.world.send_event(StateTransitionEvent {
+ exited: None,
+ entered: Some(state),
+ });
  }
 
  self

diff --git a/crates/bevy_state/src/state/freely_mutable_state.rs b/crates/bevy_state/src/state/freely_mutable_state.rs
@@ -17,17 +17,17 @@ pub trait FreelyMutableState: States {
  apply_state_transition::<Self>.in_set(ApplyStateTransition::<Self>::apply()),
  )
  .add_systems(
- should_run_transition::<Self, OnEnter<Self>>
+ last_transition::<Self>
  .pipe(run_enter::<Self>)
  .in_set(StateTransitionSteps::EnterSchedules),
  )
  .add_systems(
- should_run_transition::<Self, OnExit<Self>>
+ last_transition::<Self>
  .pipe(run_exit::<Self>)
  .in_set(StateTransitionSteps::ExitSchedules),
  )
  .add_systems(
- should_run_transition::<Self, OnTransition<Self>>
+ last_transition::<Self>
  .pipe(run_transition::<Self>)
  .in_set(StateTransitionSteps::TransitionSchedules),
  )

diff --git a/crates/bevy_state/src/state/state_set.rs b/crates/bevy_state/src/state/state_set.rs
@@ -9,9 +9,8 @@ use self::sealed::StateSetSealed;
 
 use super::{
  apply_state_transition, computed_states::ComputedStates, internal_apply_state_transition,
- run_enter, run_exit, run_transition, should_run_transition, sub_states::SubStates,
- ApplyStateTransition, OnEnter, OnExit, OnTransition, State, StateTransitionEvent,
- StateTransitionSteps, States,
+ last_transition, run_enter, run_exit, run_transition, sub_states::SubStates,
+ ApplyStateTransition, State, StateTransitionEvent, StateTransitionSteps, States,
 };
 
 mod sealed {
@@ -117,17 +116,17 @@ impl<S: InnerStateSet> StateSet for S {
  schedule
  .add_systems(system.in_set(ApplyStateTransition::<T>::apply()))
  .add_systems(
- should_run_transition::<T, OnEnter<T>>
+ last_transition::<T>
  .pipe(run_enter::<T>)
  .in_set(StateTransitionSteps::EnterSchedules),
  )
  .add_systems(
- should_run_transition::<T, OnExit<T>>
+ last_transition::<T>
  .pipe(run_exit::<T>)
  .in_set(StateTransitionSteps::ExitSchedules),
  )
  .add_systems(
- should_run_transition::<T, OnTransition<T>>
+ last_transition::<T>
  .pipe(run_transition::<T>)
  .in_set(StateTransitionSteps::TransitionSchedules),
  )
@@ -181,17 +180,17 @@ impl<S: InnerStateSet> StateSet for S {
  apply_state_transition::<T>.in_set(StateTransitionSteps::ManualTransitions),
  )
  .add_systems(
- should_run_transition::<T, OnEnter<T>>
+ last_transition::<T>
  .pipe(run_enter::<T>)
  .in_set(StateTransitionSteps::EnterSchedules),
  )
  .add_systems(
- should_run_transition::<T, OnExit<T>>
+ last_transition::<T>
  .pipe(run_exit::<T>)
  .in_set(StateTransitionSteps::ExitSchedules),
  )
  .add_systems(
- should_run_transition::<T, OnTransition<T>>
+ last_transition::<T>
  .pipe(run_transition::<T>)
  .in_set(StateTransitionSteps::TransitionSchedules),
  )
@@ -232,9 +231,9 @@ macro_rules! impl_state_set_sealed_tuples {
 
  schedule
  .add_systems(system.in_set(ApplyStateTransition::<T>::apply()))
- .add_systems(should_run_transition::<T, OnEnter<T>>.pipe(run_enter::<T>).in_set(StateTransitionSteps::EnterSchedules))
- .add_systems(should_run_transition::<T, OnExit<T>>.pipe(run_exit::<T>).in_set(StateTransitionSteps::ExitSchedules))
- .add_systems(should_run_transition::<T, OnTransition<T>>.pipe(run_transition::<T>).in_set(StateTransitionSteps::TransitionSchedules))
+ .add_systems(last_transition::<T>.pipe(run_enter::<T>).in_set(StateTransitionSteps::EnterSchedules))
+ .add_systems(last_transition::<T>.pipe(run_exit::<T>).in_set(StateTransitionSteps::ExitSchedules))
+ .add_systems(last_transition::<T>.pipe(run_transition::<T>).in_set(StateTransitionSteps::TransitionSchedules))
  .configure_sets(
  ApplyStateTransition::<T>::apply()
  .in_set(StateTransitionSteps::DependentTransitions)
@@ -271,9 +270,9 @@ macro_rules! impl_state_set_sealed_tuples {
  schedule
  .add_systems(system.in_set(ApplyStateTransition::<T>::apply()))
  .add_systems(apply_state_transition::<T>.in_set(StateTransitionSteps::ManualTransitions))
- .add_systems(should_run_transition::<T, OnEnter<T>>.pipe(run_enter::<T>).in_set(StateTransitionSteps::EnterSchedules))
- .add_systems(should_run_transition::<T, OnExit<T>>.pipe(run_exit::<T>).in_set(StateTransitionSteps::ExitSchedules))
- .add_systems(should_run_transition::<T, OnTransition<T>>.pipe(run_transition::<T>).in_set(StateTransitionSteps::TransitionSchedules))
+ .add_systems(last_transition::<T>.pipe(run_enter::<T>).in_set(StateTransitionSteps::EnterSchedules))
+ .add_systems(last_transition::<T>.pipe(run_exit::<T>).in_set(StateTransitionSteps::ExitSchedules))
+ .add_systems(last_transition::<T>.pipe(run_transition::<T>).in_set(StateTransitionSteps::TransitionSchedules))
  .configure_sets(
  ApplyStateTransition::<T>::apply()
  .in_set(StateTransitionSteps::DependentTransitions)

diff --git a/crates/bevy_state/src/state/transitions.rs b/crates/bevy_state/src/state/transitions.rs
@@ -1,11 +1,11 @@
-use std::{marker::PhantomData, mem, ops::DerefMut};
+use std::{marker::PhantomData, mem};
 
 use bevy_ecs::{
  event::{Event, EventReader, EventWriter},
  schedule::{
  InternedScheduleLabel, IntoSystemSetConfigs, Schedule, ScheduleLabel, Schedules, SystemSet,
  },
- system::{Commands, In, Local, Res, ResMut},
+ system::{Commands, In, ResMut},
  world::World,
 };
 
@@ -202,34 +202,18 @@ pub fn apply_state_transition<S: FreelyMutableState>(
  *next_state_resource.as_mut() = NextState::<S>::Unchanged;
 }
 
-pub(crate) fn should_run_transition<S: States, T: ScheduleLabel>(
- mut first: Local<bool>,
- res: Option<Res<State<S>>>,
- mut event: EventReader<StateTransitionEvent<S>>,
-) -> (Option<StateTransitionEvent<S>>, PhantomData<T>) {
- let first_mut = first.deref_mut();
- if !*first_mut {
- *first_mut = true;
- if let Some(res) = res {
- event.clear();
-
- return (
- Some(StateTransitionEvent {
- exited: None,
- entered: Some(res.get().clone()),
- }),
- PhantomData,
- );
- }
- }
- (event.read().last().cloned(), PhantomData)
+/// Returns the latest state transition event of type `S`, if any are available.
+pub fn last_transition<S: States>(
+ mut reader: EventReader<StateTransitionEvent<S>>,
+) -> Option<StateTransitionEvent<S>> {
+ reader.read().last().cloned()
 }
 
 pub(crate) fn run_enter<S: States>(
- In((transition, _)): In<(Option<StateTransitionEvent<S>>, PhantomData<OnEnter<S>>)>,
+ transition: In<Option<StateTransitionEvent<S>>>,
  world: &mut World,
 ) {
- let Some(transition) = transition else {
+ let Some(transition) = transition.0 else {
  return;
  };
  let Some(entered) = transition.entered else {
@@ -240,10 +224,10 @@ pub(crate) fn run_enter<S: States>(
 }
 
 pub(crate) fn run_exit<S: States>(
- In((transition, _)): In<(Option<StateTransitionEvent<S>>, PhantomData<OnExit<S>>)>,
+ transition: In<Option<StateTransitionEvent<S>>>,
  world: &mut World,
 ) {
- let Some(transition) = transition else {
+ let Some(transition) = transition.0 else {
  return;
  };
  let Some(exited) = transition.exited else {
@@ -254,13 +238,10 @@ pub(crate) fn run_exit<S: States>(
 }
 
 pub(crate) fn run_transition<S: States>(
- In((transition, _)): In<(
- Option<StateTransitionEvent<S>>,
- PhantomData<OnTransition<S>>,
- )>,
+ transition: In<Option<StateTransitionEvent<S>>>,
  world: &mut World,
 ) {
- let Some(transition) = transition else {
+ let Some(transition) = transition.0 else {
  return;
  };
  let Some(exited) = transition.exited else {