bevy-core-concepts

Bevy 0.19 — Core concepts (App, Plugin, Schedule, World)

When to use this skill

Setting up a new Bevy app or library plugin.
Deciding which schedule (Startup, Update, FixedUpdate, PostUpdate, ...) a system belongs in.
Writing an exclusive system that needs &mut World.
Hitting schedule cycles, elevated ambiguity diagnostics, or deferred-command order bugs.
Asking "where should X run?"

Canonical pattern

use bevy::prelude::*;

fn main() { App::new() .add_plugins(DefaultPlugins) .add_plugins(GamePlugin) .run(); }

pub struct GamePlugin;

impl Plugin for GamePlugin { fn build(&self, app: &mut App) { app.insert_resource(Score(0)) .add_systems(Startup, spawn_world) .add_systems(Update, (read_input, apply_gravity).chain()) .add_systems(FixedUpdate, simulate_physics) .add_systems(PostUpdate, sync_transforms); } }

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

fn spawn_world(mut commands: Commands) { commands.spawn((Camera3d::default(), Transform::from_xyz(0.0, 4.0, 8.0))); }

fn read_input(_keys: Res<ButtonInput>) {} fn apply_gravity(_t: Res) {} fn simulate_physics(_t: Res<Time>) {} fn sync_transforms(_q: Query<&mut Transform>) {}

// Exclusive system — full mutable World access, runs alone. fn rebuild_index(world: &mut World) { let count = world.entities().len(); world.insert_resource(EntityCount(count)); }

#[derive(Resource)] struct EntityCount(u32);


## Schedule cheat sheet

| Schedule | Use for |
|---|---|
| `Startup` | One-shot setup. Runs once before `Update`. |
| `PreUpdate` | Reading input, networking ingress, frame-start bookkeeping. |
| `Update` | Per-frame game logic. Runs at render rate. **Default for game systems.** |
| `PostUpdate` | Reactive bookkeeping after `Update` (transform propagation lives here). |
| `Last` | Anything that must run after `PostUpdate` (rare). |
| `FixedFirst` / `FixedPreUpdate` / `FixedUpdate` / `FixedPostUpdate` / `FixedLast` | Deterministic, tick-rate work: physics, networking simulation, gameplay state. Runs zero or more times per render frame to catch up to `Time<Fixed>`. |

**Rule of thumb:** if a system must produce the same result for the same inputs regardless of frame rate, put it in `FixedUpdate`. Otherwise `Update`.

## Bevy 0.19 gotchas

- **`SimpleExecutor` was removed.** It used to apply deferred commands after every
  system. The remaining executors use dependency information to place
  `ApplyDeferred`; add `.before`, `.after`, or `.chain()` when downstream systems must
  see commands in the same schedule. Shared mutable access is serialized, but does
  not by itself define which system goes first.
- **`ScheduleBuildError` variants were renamed.** If you `match` on them, update: `HierarchyLoop` → `HierarchySort(DiGraphToposortError::Loop(...))`, `DependencyCycle` → `DependencySort(DiGraphToposortError::Cycle(...))`.
- **Executor configuration takes an executor value in 0.19.** Replace
  `set_executor_kind(ExecutorKind::SingleThreaded)` with
  `set_executor(SingleThreadedExecutor::new())` (or
  `MultiThreadedExecutor::new()` / `default_executor()`).
- **Resources implement `Component` in 0.19.** Generic bounds that used `Resource` only to exclude components no longer do so; audit broad component queries and generic APIs.
- **`State::set()` now always triggers a transition** — even if the requested state equals the current one. Use `next_state.set_if_neq(...)` for the old "no-op when equal" behavior.
- Don't put render-world passes in `Update`. Main-world components may prepare
  render data there, but GPU passes belong in render schedules. See `bevy-rendering`.
- Exclusive systems block everything in their schedule — use them only when you really need `&mut World` (scene loading, batch reflection, schema migrations).

## See also

- [`bevy-ecs-systems`](../bevy-ecs-systems/SKILL.md) — params, sets, conditions,
  ordering, and ambiguity diagnostics.
- [`bevy-ecs-components`](../bevy-ecs-components/SKILL.md) — entity data.
- [`bevy-rendering`](../bevy-rendering/SKILL.md) — render schedules and systems.
- [`bevy-physics`](../bevy-physics/SKILL.md) — Rapier schedule placement and
  `PhysicsSet` ordering.
- [`bevy-input-actions`](../bevy-input-actions/SKILL.md) — frame input transfer into fixed ticks.
- [`bevy-testing`](../bevy-testing/SKILL.md) — controlled `App` stepping and manual time.
- [`bevy-migration-0-18-to-0-19`](../bevy-migration-0-18-to-0-19/SKILL.md) — executor
  instances and resources-as-components.

About this skill

When to use this skill

Canonical pattern