Bevy Core Concepts

Bevy 0.18 — 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 ScheduleBuildError panics at startup ("ambiguity" or "cycle").
• 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>) {}
fn apply_gravity(_t: Res) {}
fn simulate_physics(_t: Res>) {}
fn sync_transforms(_q: Query) {}

// 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. |

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.

Gotchas (0.18)

• SimpleExecutor was removed. If two systems in the same schedule both touch the same data, the schedule no longer guesses an order — it panics. Make order explicit with .before(other), .after(other), .chain(), or .in_set(MySet).
• ScheduleBuildError variants were renamed. If you match on them, update: HierarchyLoop → HierarchySort(DiGraphToposortError::Loop(...)), DependencyCycle → DependencySort(DiGraphToposortError::Cycle(...)).
• FunctionSystem grew an In generic. Type aliases like type Boxed = FunctionSystem need FunctionSystem in 0.18.
• 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 rendering systems in Update. The render world is extracted automatically; reaching into it from Update is almost always a mistake. See bevy-rendering-core (future skill).
• 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 — system params, sets, run conditions, ordering.
• bevy-ecs-components — what entities are made of.
• bevy-migration-0-17-to-0-18 — full list of executor and schedule renames.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: chrisgliddon
• Source: chrisgliddon/bevy-skills
• License: MIT

Install and usage instructions live in the source repository linked above.