bevy-ecs-systems

Bevy 0.19 — ECS Systems (params, sets, run conditions)

When to use this skill

• Bundling related state into a single system parameter via #[derive(SystemParam)].
• Grouping systems into a SystemSet so callers can order against the whole group.
• Gating systems with run conditions (run_if, on_message, in_state, resource_exists, any_with_component).
• Hooking into state transitions with OnEnter(S) / OnExit(S) / OnTransition { from, to }.
• Hot-removing systems at runtime (e.g. tearing down a debug overlay) with remove_systems_in_set.
• Hitting ScheduleBuildError ambiguity panics — the executor no longer guesses.

Canonical pattern

use bevy::ecs::system::SystemParam;
use bevy::prelude::*;

#[derive(SystemSet, Hash, PartialEq, Eq, Clone, Debug)]
enum GameLoop { Input, Simulate, Render }

#[derive(States, Default, Hash, PartialEq, Eq, Clone, Debug)]
enum AppState { #[default] Loading, Playing }

#[derive(Resource, Default)]
struct Score(u32);

#[derive(Message)]
struct GoalScored { team: u8 }

// Composite param: pass one argument, get four.
// 'w = world borrow; 's = system-local state borrow.
#[derive(SystemParam)]
struct GameCtx<'w, 's> {
    time:  Res<'w, Time>,
    score: ResMut<'w, Score>,
    goals: MessageReader<'w, 's, GoalScored>,
}

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .init_resource::<Score>()
        .add_message::<GoalScored>()
        .init_state::<AppState>()
        .configure_sets(Update, (GameLoop::Input, GameLoop::Simulate, GameLoop::Render).chain())
        // State schedule: fires once when entering Playing.
        .add_systems(OnEnter(AppState::Playing), spawn_level)
        // State schedule: fires once when leaving Playing.
        .add_systems(OnExit(AppState::Playing), despawn_level)
        .add_systems(Update, read_input.in_set(GameLoop::Input))
        .add_systems(
            Update,
            tally_goals
                .in_set(GameLoop::Simulate)
                .run_if(on_message::<GoalScored>),
        )
        .add_systems(Update, draw_hud.in_set(GameLoop::Render))
        .run();
}

#[derive(Component)]
struct LevelEntity;

fn spawn_level(mut commands: Commands) {
    commands.spawn((LevelEntity, Name::new("level")));
}

fn despawn_level(mut commands: Commands, query: Query<Entity, With<LevelEntity>>) {
    for e in &query { commands.entity(e).despawn(); }
}

fn read_input(mut writer: MessageWriter<GoalScored>) {
    writer.write(GoalScored { team: 0 });
}

fn tally_goals(mut ctx: GameCtx) {
    let _ = ctx.time.delta_secs();
    for goal in ctx.goals.read() {
        ctx.score.0 += 1;
        info!("team {} scored, total {}", goal.team, ctx.score.0);
    }
}

fn draw_hud(score: Res<Score>) { let _ = score.0; }

Run condition cheat sheet

Built-in	Triggers when
on_message::<M>	The Messages<M> buffer has unread items.
resource_exists::<R>	Resource R is in the World.
resource_changed::<R>	R was mutated this tick.
in_state(MyState::X)	Current State<MyState> equals X.
state_changed::<MyState>	The state changed this tick.
any_with_component::<C>	At least one entity has C.

Combine with .and() / .or(): run_if(in_state(GameState::Playing).and(resource_exists::<NetSession>())).

Topics

Topic	Reference
#[derive(SystemParam)], 'w/'s lifetimes, Local<T>, Commands deferred apply	references/system-params.md
#[derive(SystemSet)] requirements, .in_set, .configure_sets, .chain()	references/system-sets.md
Every built-in condition, .and()/.or()/not(), custom conditions, cost rule	references/run-conditions.md
OnEnter(S) / OnExit(S) / OnTransition, NextState<S>, set_if_neq	references/state-schedules.md
.before, .after, .chain(), .ambiguous_with, debugging ambiguity errors	references/ordering.md
remove_systems_in_set, cleanup policies, cross-schedule runtime removal, side-effect limits	references/runtime-removal.md

Bevy 0.19 gotchas

• Shared access does not define order. Conflicting mutable systems are serialized, but their relative order is unspecified. Enable ambiguity diagnostics and add .before, .after, or .chain() when behavior depends on order. See references/ordering.md.
• MessageReader / MessageWriter, not EventReader / EventWriter. Renamed in 0.17. Trait derive is #[derive(Message)]; registrar is app.add_message::<M>().
• next_state.set(S) always fires OnExit/OnEnter in 0.18. Use set_if_neq for the old behaviour. See references/state-schedules.md.
• Do not remove the currently running schedule through Schedules. It is temporarily absent from that resource. Remove Update systems from a later/different schedule such as Last. See references/runtime-removal.md.
• Resource: Component in 0.19. Generic system parameters still use Res<T>/ResMut<T>, but a T: Resource bound no longer proves that T cannot appear in a component query.

See also

• bevy-core-concepts — which schedule (Startup, Update, FixedUpdate) to add a system to.
• bevy-ecs-queries — query is one of many SystemParams; Changed<T> / Added<T> filters.
• bevy-ecs-components — defining the Component types your systems act on.
• bevy-migration-0-17-to-0-18 — full Message/Event rename, MaterialPlugin changes.
• bevy-migration-0-18-to-0-19 — resource bounds and executor configuration.