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::() .add_message::() .init_state::() .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::), ) .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>) { for e in &query { commands.entity(e).despawn(); } }

fn read_input(mut writer: MessageWriter) { 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) { 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](references/system-params.md) |
| `#[derive(SystemSet)]` requirements, `.in_set`, `.configure_sets`, `.chain()` | [references/system-sets.md](references/system-sets.md) |
| Every built-in condition, `.and()`/`.or()`/`not()`, custom conditions, cost rule | [references/run-conditions.md](references/run-conditions.md) |
| `OnEnter(S)` / `OnExit(S)` / `OnTransition`, `NextState<S>`, `set_if_neq` | [references/state-schedules.md](references/state-schedules.md) |
| `.before`, `.after`, `.chain()`, `.ambiguous_with`, debugging ambiguity errors | [references/ordering.md](references/ordering.md) |
| `remove_systems_in_set`, cleanup policies, cross-schedule runtime removal, side-effect limits | [references/runtime-removal.md](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](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](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](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 `SystemParam`s; `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.

About this skill

When to use this skill

Canonical pattern