bevy/crates/bevy_ecs/src/schedule/state.rs

use std::fmt::Debug;
use std::hash::Hash;
use std::mem;

use crate as bevy_ecs;
use crate::change_detection::DetectChangesMut;
use crate::schedule::ScheduleLabel;
use crate::system::Resource;
use crate::world::World;

pub use bevy_ecs_macros::States;

/// Types that can define world-wide states in a finite-state machine.
///
/// The [`Default`] trait defines the starting state.
/// Multiple states can be defined for the same world,
/// allowing you to classify the state of the world across orthogonal dimensions.
/// You can access the current state of type `T` with the [`State<T>`] resource,
/// and the queued state with the [`NextState<T>`] resource.
///
/// State transitions typically occur in the [`OnEnter<T::Variant>`] and [`OnExit<T:Variant>`] schedules,
/// which can be run via the [`apply_state_transition::<T>`] system.
///
/// # Example
///
/// ```rust
/// use bevy_ecs::prelude::States;
///
/// #[derive(Clone, Copy, PartialEq, Eq, Hash, Debug, Default, States)]
/// enum GameState {
///  #[default]
///   MainMenu,
///   SettingsMenu,
///   InGame,
/// }
///
/// ```
pub trait States: 'static + Send + Sync + Clone + PartialEq + Eq + Hash + Debug + Default {
    type Iter: Iterator<Item = Self>;

    /// Returns an iterator over all the state variants.
    fn variants() -> Self::Iter;
}

/// The label of a [`Schedule`](super::Schedule) that runs whenever [`State<S>`]
/// enters this state.
#[derive(ScheduleLabel, Clone, Debug, PartialEq, Eq, Hash)]
pub struct OnEnter<S: States>(pub S);

/// The label of a [`Schedule`](super::Schedule) that runs whenever [`State<S>`]
/// exits this state.
#[derive(ScheduleLabel, Clone, Debug, PartialEq, Eq, Hash)]
pub struct OnExit<S: States>(pub S);

/// The label of a [`Schedule`](super::Schedule) that **only** runs whenever [`State<S>`]
/// exits the `from` state, AND enters the `to` state.
///
/// Systems added to this schedule are always ran *after* [`OnExit`], and *before* [`OnEnter`].
#[derive(ScheduleLabel, Clone, Debug, PartialEq, Eq, Hash)]
pub struct OnTransition<S: States> {
    /// The state being exited.
    pub from: S,
    /// The state being entered.
    pub to: S,
}

/// A finite-state machine whose transitions have associated schedules
/// ([`OnEnter(state)`] and [`OnExit(state)`]).
///
/// The current state value can be accessed through this resource. To *change* the state,
/// queue a transition in the [`NextState<S>`] resource, and it will be applied by the next
/// [`apply_state_transition::<S>`] system.
///
/// The starting state is defined via the [`Default`] implementation for `S`.
#[derive(Resource, Default, Debug)]
pub struct State<S: States>(pub S);

/// The next state of [`State<S>`].
///
/// To queue a transition, just set the contained value to `Some(next_state)`.
/// Note that these transitions can be overridden by other systems:
/// only the actual value of this resource at the time of [`apply_state_transition`] matters.
#[derive(Resource, Default, Debug)]
pub struct NextState<S: States>(pub Option<S>);

impl<S: States> NextState<S> {
    /// Tentatively set a planned state transition to `Some(state)`.
    pub fn set(&mut self, state: S) {
        self.0 = Some(state);
    }
}

/// Run the enter schedule (if it exists) for the current state.
pub fn run_enter_schedule<S: States>(world: &mut World) {
    world
        .try_run_schedule(OnEnter(world.resource::<State<S>>().0.clone()))
        .ok();
}

/// If a new state is queued in [`NextState<S>`], this system:
/// - Takes the new state value from [`NextState<S>`] and updates [`State<S>`].
/// - Runs the [`OnExit(exited_state)`] schedule, if it exists.
/// - Runs the [`OnTransition { from: exited_state, to: entered_state }`](OnTransition), if it exists.
/// - Runs the [`OnEnter(entered_state)`] schedule, if it exists.
pub fn apply_state_transition<S: States>(world: &mut World) {
    // We want to take the `NextState` resource,
    // but only mark it as changed if it wasn't empty.
    let mut next_state_resource = world.resource_mut::<NextState<S>>();
    if let Some(entered) = next_state_resource.bypass_change_detection().0.take() {
        next_state_resource.set_changed();

        let exited = mem::replace(&mut world.resource_mut::<State<S>>().0, entered.clone());

        // Try to run the schedules if they exist.
        world.try_run_schedule(OnExit(exited.clone())).ok();
        world
            .try_run_schedule(OnTransition {
                from: exited,
                to: entered.clone(),
            })
            .ok();
        world.try_run_schedule(OnEnter(entered)).ok();
    }
}