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<KeyCode>>) {}
fn apply_gravity(_t: Res<Time>) {}
fn simulate_physics(_t: Res<Time<Fixed>>) {}
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 — params, sets, conditions, ordering, and ambiguity diagnostics.
• bevy-ecs-components — entity data.
• bevy-rendering — render schedules and systems.
• bevy-physics — Rapier schedule placement and PhysicsSet ordering.
• bevy-input-actions — frame input transfer into fixed ticks.
• bevy-testing — controlled App stepping and manual time.
• bevy-migration-0-18-to-0-19 — executor instances and resources-as-components.