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. Keep those two owners separate.

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. bevy_state ships in the default bevy app plugins. Install a state with init_state.
• Per-entity mode. Store an enum in a Component and branch in a system. That enum is not a States type.

[dependencies]
bevy = "0.20"

App modes are States. Gameplay systems use in_state. Pause writes NextState.

#[derive(States, Debug, Clone, Copy, Eq, PartialEq, Hash, Default)]
enum AppMode {
    #[default]
    Menu,
    Play,
}

fn show_menu(mut commands: Commands) {
    commands.spawn((Name::new("menu"), DespawnOnExit(AppMode::Menu)));
}

fn spawn_player(mut commands: Commands) {
    commands.spawn((Name::new("player"), Transform::default()));
}

fn request_play(keys: Res<ButtonInput<KeyCode>>, mut next: ResMut<NextState<AppMode>>) {
    if keys.just_pressed(KeyCode::Enter) {
        next.set(AppMode::Play);
    }
}

pub struct ModePlugin;

impl Plugin for ModePlugin {
    fn build(&self, app: &mut App) {
        app.init_state::<AppMode>()
            .add_systems(OnEnter(AppMode::Menu), show_menu)
            .add_systems(OnEnter(AppMode::Play), spawn_player)
            .add_systems(Update, request_play.run_if(in_state(AppMode::Menu)));
    }
}

A pause flag exists only while AppMode::Play is active. SubStates removes it otherwise.

#[derive(States, Debug, Clone, Copy, Eq, PartialEq, Hash, Default)]
enum AppMode {
    #[default]
    Menu,
    Play,
}

#[derive(SubStates, Debug, Clone, Copy, Eq, PartialEq, Hash, Default)]
#[source(AppMode = AppMode::Play)]
enum Session {
    #[default]
    Running,
    Paused,
}

fn toggle_pause(
    keys: Res<ButtonInput<KeyCode>>,
    session: Res<State<Session>>,
    mut next: ResMut<NextState<Session>>,
) {
    if keys.just_pressed(KeyCode::Escape) {
        let flipped = match session.get() {
            Session::Running => Session::Paused,
            Session::Paused => Session::Running,
        };
        next.set(flipped);
    }
}

fn install_pause(app: &mut App) {
    app.add_sub_state::<Session>()
        .add_systems(Update, toggle_pause.run_if(in_state(Session::Running)));
}

One actor's stance is a component. It does not change the app state.

#[derive(Component, Clone, Copy, PartialEq, Eq)]
enum Stance {
    Idle,
    Attack,
}

fn begin_attack(keys: Res<ButtonInput<KeyCode>>, mut actors: Query<&mut Stance>) {
    if keys.just_pressed(KeyCode::KeyF) {
        for mut stance in &mut actors {
            *stance = Stance::Attack;
        }
    }
}

Choose the owner

Need	Owner
App mode (menu, pause, play)	A States type and NextState<S>
Run logic only in one mode	in_state or an OnEnter / OnExit / OnTransition schedule
Child mode that exists only inside a parent mode	SubStates
Mode derived from other modes	ComputedStates
One actor's stance	An enum Component plus a system. Not States unless it is truly app-wide

Scope

• Install app modes with init_state. Install a child mode with add_sub_state.
• OnEnter, OnExit, and in_state cover the schedules shown above.
• A per-entity stance is the Stance component. Animation graphs belong on bevy-animation-graph.

Expert Knowledge Delta

Target: Bevy 0.20+.

Core ECS types: Component, Resource, Query, Commands, Startup, Update, Fixed / FixedMain (bevy::time), .chain(), Plugin, DefaultPlugins, MinimalPlugins. Messages and observers are an ownership slot. See bevy-events-messaging for their APIs. Systems run in parallel unless .chain() orders the ones that conflict.

Who owns state

Piece	Owner	Who may mutate	Schedule
Data on one entity	Component	One system via Query	Update or FixedMain
One value for the whole app	Resource	One system	Startup to insert, then the schedule that reads it
Spawn	Commands	The spawning system	Startup for the initial world; later schedules for runtime spawn
Order that must not race	.chain()	The chained systems only	Same schedule
Plugin boundary	Plugin	build registers systems and resources	Before the app runs

Borrow rule: two systems that both mutably query the same component must be ordered (.chain()) or split so only one writes. Unrelated systems stay parallel. That is the ECS page rule, not a guess at a system-set API.

Decision

If you need	Do this	Do not
Per-entity data	Component + Query	A value that is not a component
App-wide config or clock	Resource	Copy the value onto every entity
Stable simulation step	FixedMain and the Fixed clock	Tie simulation to raw frame Update
Startup spawn	Commands in Startup	Spawn from a render system
Cross-system notice that is not shared state	Message or observer slot in bevy-events-messaging	A new event type invented in this file
An API outside the documented set	Check the official docs first	A made-up crate or API

Load references/ecs-ownership.md for the ownership table.

NEVER

• Do not pin /0.20.0/ in docs URLs. WHY: links stay on latest; the 0.20 target is stated once in prose.

• Do not mutably query the same component from two unordered systems. WHY: the schedule runs systems in parallel unless .chain() orders them.

• Do not invent a physics, navigation, networking, or XR crate here. WHY: this skill only names documented Bevy crates and modules.

• No Godot animation-tree or node state-machine types. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

• No invented App state-setup methods. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

• No third-party crates. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

Reference

The library pin on bevy-master stays 0.20.0.

Official Documentation

• Quick Start: ECS - Component and Query for a per-entity mode flag.
• Examples - Example tree on the latest branch. State examples live in that tree.

Related Skills

Prerequisites

• bevy-master - Schedules versus a scene tree.
• bevy-data-resources - NextState<S> is a resource. Per-entity modes are components.

Complements

• bevy-events-messaging - Messages and observers that are not state transitions.
• godot-state-machine-advanced - Equivalent Godot skill. Do not import Godot types.

Downstream / consumers

• bevy-character-movement - Actor motion. A jump or ground mode stays a component unless a mode needs States.

Master

• bevy-master - Bevy router.