Bevy state machines

When to use

Load this when the app needs a large-scale mode (paused, in a menu, loading) or when one entity needs a small mode flag. Use bevy::state for the first. Use a component for the second. Do not merge them into one Godot-style state graph.

ECS shape

• App states. bevy::state: states are app-wide finite state machines. Three types implement States.
  • Standard states change only by setting the NextState<S> resource.
  • SubStates also use NextState<S>, and are removed from the World when source states are not in the required state.
  • ComputedStates derive from other states through a compute method.
• Transition schedules. OnEnter<S>, OnExit<S>, and OnTransition<S> run systems on matching transitions. StateTransitionEvent<S> fires when state S changes. Run conditions: in_state<S> and state_changed<S>.
• Entity lifetime. DespawnOnEnter<S> and DespawnOnExit<S> despawn entities on those transitions (state_scoped).
• Feature. The bevy 0.20.0 crate page includes bevy_state in the default_app collection. The state module was opened; App installation method names were not. Do not invent init_state or similar.
• Per-entity mode. Store an enum in a Component and branch in a system (Query from the ECS page). No per-entity state-machine type was opened.

Coming from Godot