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use std::{marker::PhantomData, mem};
use bevy_ecs::{
event::{Event, EventReader, EventWriter},
schedule::{
InternedScheduleLabel, IntoSystemSetConfigs, Schedule, ScheduleLabel, Schedules, SystemSet,
},
system::{Commands, In, ResMut},
world::World,
};
use super::{resources::State, states::States};
/// The label of a [`Schedule`] that **only** runs whenever [`State<S>`] enters the provided state.
///
/// This schedule ignores identity transitions.
#[derive(ScheduleLabel, Clone, Debug, PartialEq, Eq, Hash)]
pub struct OnEnter<S: States>(pub S);
/// The label of a [`Schedule`] that **only** runs whenever [`State<S>`] exits the provided state.
///
/// This schedule ignores identity transitions.
#[derive(ScheduleLabel, Clone, Debug, PartialEq, Eq, Hash)]
pub struct OnExit<S: States>(pub S);
/// The label of a [`Schedule`] that **only** runs whenever [`State<S>`]
/// exits AND enters the provided `exited` and `entered` states.
///
/// Systems added to this schedule are always ran *after* [`OnExit`], and *before* [`OnEnter`].
///
/// This schedule will run on identity transitions.
#[derive(ScheduleLabel, Clone, Debug, PartialEq, Eq, Hash)]
pub struct OnTransition<S: States> {
/// The state being exited.
pub exited: S,
/// The state being entered.
pub entered: S,
}
/// Runs [state transitions](States).
///
/// By default, it will be triggered after `PreUpdate`, but
/// you can manually trigger it at arbitrary times by creating an exclusive
/// system to run the schedule.
///
/// ```rust
/// use bevy_state::prelude::*;
/// use bevy_ecs::prelude::*;
///
/// fn run_state_transitions(world: &mut World) {
/// let _ = world.try_run_schedule(StateTransition);
/// }
/// ```
#[derive(ScheduleLabel, Clone, Debug, PartialEq, Eq, Hash)]
pub struct StateTransition;
/// Event sent when any state transition of `S` happens.
/// This includes identity transitions, where `exited` and `entered` have the same value.
///
/// If you know exactly what state you want to respond to ahead of time, consider [`OnEnter`], [`OnTransition`], or [`OnExit`]
#[derive(Debug, Copy, Clone, PartialEq, Eq, Event)]
pub struct StateTransitionEvent<S: States> {
/// The state being exited.
pub exited: Option<S>,
/// The state being entered.
pub entered: Option<S>,
}
/// Applies state transitions and runs transitions schedules in order.
///
/// These system sets are run sequentially, in the order of the enum variants.
#[derive(SystemSet, Clone, Debug, PartialEq, Eq, Hash)]
pub(crate) enum StateTransitionSteps {
/// States apply their transitions from [`NextState`] and compute functions based on their parent states.
DependentTransitions,
/// Exit schedules are executed in leaf to root order
ExitSchedules,
/// Transition schedules are executed in arbitrary order.
TransitionSchedules,
/// Enter schedules are executed in root to leaf order.
EnterSchedules,
}
#[derive(SystemSet, Clone, Debug, PartialEq, Eq, Hash)]
/// System set that runs exit schedule(s) for state `S`.
pub struct ExitSchedules<S: States>(PhantomData<S>);
impl<S: States> Default for ExitSchedules<S> {
fn default() -> Self {
Self(Default::default())
}
}
#[derive(SystemSet, Clone, Debug, PartialEq, Eq, Hash)]
/// System set that runs transition schedule(s) for state `S`.
pub struct TransitionSchedules<S: States>(PhantomData<S>);
impl<S: States> Default for TransitionSchedules<S> {
fn default() -> Self {
Self(Default::default())
}
}
#[derive(SystemSet, Clone, Debug, PartialEq, Eq, Hash)]
/// System set that runs enter schedule(s) for state `S`.
pub struct EnterSchedules<S: States>(PhantomData<S>);
impl<S: States> Default for EnterSchedules<S> {
fn default() -> Self {
Self(Default::default())
}
}
/// System set that applies transitions for state `S`.
#[derive(SystemSet, Clone, Debug, PartialEq, Eq, Hash)]
pub(crate) struct ApplyStateTransition<S: States>(PhantomData<S>);
impl<S: States> Default for ApplyStateTransition<S> {
fn default() -> Self {
Self(Default::default())
}
}
/// This function actually applies a state change, and registers the required
/// schedules for downstream computed states and transition schedules.
///
/// The `new_state` is an option to allow for removal - `None` will trigger the
/// removal of the `State<S>` resource from the [`World`].
pub(crate) fn internal_apply_state_transition<S: States>(
mut event: EventWriter<StateTransitionEvent<S>>,
mut commands: Commands,
current_state: Option<ResMut<State<S>>>,
new_state: Option<S>,
) {
match new_state {
Some(entered) => {
match current_state {
// If the [`State<S>`] resource exists, and the state is not the one we are
// entering - we need to set the new value, compute dependant states, send transition events
// and register transition schedules.
Some(mut state_resource) => {
let exited = match *state_resource == entered {
true => entered.clone(),
false => mem::replace(&mut state_resource.0, entered.clone()),
};
// Transition events are sent even for same state transitions
// Although enter and exit schedules are not run by default.
event.send(StateTransitionEvent {
exited: Some(exited.clone()),
entered: Some(entered.clone()),
});
}
None => {
// If the [`State<S>`] resource does not exist, we create it, compute dependant states, send a transition event and register the `OnEnter` schedule.
commands.insert_resource(State(entered.clone()));
event.send(StateTransitionEvent {
exited: None,
entered: Some(entered.clone()),
});
}
};
}
None => {
// We first remove the [`State<S>`] resource, and if one existed we compute dependant states, send a transition event and run the `OnExit` schedule.
if let Some(resource) = current_state {
commands.remove_resource::<State<S>>();
event.send(StateTransitionEvent {
exited: Some(resource.get().clone()),
entered: None,
});
}
}
}
}
/// Sets up the schedules and systems for handling state transitions
/// within a [`World`].
///
/// Runs automatically when using `App` to insert states, but needs to
/// be added manually in other situations.
pub fn setup_state_transitions_in_world(
world: &mut World,
startup_label: Option<InternedScheduleLabel>,
) {
let mut schedules = world.get_resource_or_insert_with(Schedules::default);
if schedules.contains(StateTransition) {
return;
}
let mut schedule = Schedule::new(StateTransition);
schedule.configure_sets(
(
StateTransitionSteps::DependentTransitions,
StateTransitionSteps::ExitSchedules,
StateTransitionSteps::TransitionSchedules,
StateTransitionSteps::EnterSchedules,
)
.chain(),
);
schedules.insert(schedule);
if let Some(startup) = startup_label {
schedules.add_systems(startup, |world: &mut World| {
let _ = world.try_run_schedule(StateTransition);
});
}
}
/// 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>(
transition: In<Option<StateTransitionEvent<S>>>,
world: &mut World,
) {
let Some(transition) = transition.0 else {
return;
};
if transition.entered == transition.exited {
return;
}
let Some(entered) = transition.entered else {
return;
};
let _ = world.try_run_schedule(OnEnter(entered));
}
pub(crate) fn run_exit<S: States>(
transition: In<Option<StateTransitionEvent<S>>>,
world: &mut World,
) {
let Some(transition) = transition.0 else {
return;
};
if transition.entered == transition.exited {
return;
}
let Some(exited) = transition.exited else {
return;
};
let _ = world.try_run_schedule(OnExit(exited));
}
pub(crate) fn run_transition<S: States>(
transition: In<Option<StateTransitionEvent<S>>>,
world: &mut World,
) {
let Some(transition) = transition.0 else {
return;
};
let Some(exited) = transition.exited else {
return;
};
let Some(entered) = transition.entered else {
return;
};
let _ = world.try_run_schedule(OnTransition { exited, entered });
}