Bevy Ecs

Bevy ECS

Structure a Bevy game in Rust around the Entity Component System: the App and plugins, components and resources, systems with queries, scheduling, and frame-rate-independent updates. New examples target Bevy 0.19. If the project already pins another release, keep that release and use its matching migration guide.

When to use

• Use when wiring a Bevy App, defining Component/Resource types, writing systems that query entities, ordering/filtering systems, or fixing borrow-conflict panics and frame-dependent movement.
• Use when Cargo.toml depends on bevy and code calls App::new(), add_systems, Query, or Commands.

*When not to use:* this is the ECS core. Deep rendering, custom shaders/ pipelines, UI layout, and audio are separate concerns. For engine-agnostic AI or procedural algorithms, pair with game-ai / procedural-gen.

Core workflow

1. Detect and pin the version. Read Cargo.toml and Cargo.lock first. For a new project use bevy = "0.19"; never silently migrate an existing project across a Bevy minor release. Treat the matching docs and migration guides as truth.
2. Build the `App`. App::new().add_plugins(DefaultPlugins) gives windowing, input, rendering, time, etc. Register systems into schedules: Startup (once) and Update (every frame).
3. Model data as components, globals as resources. #[derive(Component)] for per-entity data; #[derive(Resource)] for one-of-a-kind data (score, settings, the Time clock). In 0.19 Resource extends Component, so do not derive both.
4. Write systems as plain functions. Parameters declare data access: Query<...> for entities, Res<T>/ResMut<T> for resources, Commands for deferred spawn/despawn. Systems run in parallel when their accesses don't conflict.
5. Drive motion by `time.delta_secs()` so speed is frame-rate independent.
6. Order only what must be ordered with .chain() or explicit constraints; gate systems with run_if. Group related setup into Plugins. Build with cargo run and read the panics — Bevy reports conflicting queries at startup.

Patterns

1. Cargo.toml + minimal App

# Cargo.toml — pin the version; the API differs across minor releases.
[dependencies]
bevy = "0.19"

// main.rs
use bevy::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)            // window, input, render, time, ...
        .add_systems(Startup, setup)            // runs once at startup
        .add_systems(Update, move_players)      // runs every frame
        .run();
}

2. Components, resources, and spawning

#[derive(Component)]
struct Player;

#[derive(Component)]
struct Velocity(Vec2);

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

fn setup(mut commands: Commands) {
    commands.insert_resource(Score(0));

    // Camera2d is a component with required components (bundles removed in 0.16);
    // spawning it pulls in Transform, Camera, etc. automatically.
    commands.spawn(Camera2d);

    // Spawn an entity as a tuple of components.
    commands.spawn((
        Player,
        Velocity(Vec2::new(150.0, 0.0)),
        Transform::from_xyz(0.0, 0.0, 0.0),
    ));
}

3. A system with a query + the Time resource

// Iterate every entity that has BOTH Velocity and Transform; mutate Transform.
fn move_players(time: Res<Time>, mut query: Query<(&Velocity, &mut Transform)>) {
    for (velocity, mut transform) in &mut query {
        // delta_secs() is f32 seconds (renamed from delta_seconds() in 0.16).
        transform.translation += velocity.0.extend(0.0) * time.delta_secs();
    }
}

4. Query filters (With / Without / Changed)

// Only entities tagged Player (the Player component itself isn't read).
fn aim_player(mut q: Query<&mut Transform, With<Player>>) { /* ... */ }

// Disjoint two mutable Transform queries so they don't conflict at runtime.
fn separate(
    mut players: Query<&mut Transform, With<Player>>,
    mut enemies: Query<&mut Transform, Without<Player>>,
) { /* ... */ }

// React only when Health changed since last run (change detection).
fn on_health_change(q: Query<&Health, Changed<Health>>) {
    for health in &q { /* update the HUD, etc. */ }
}

5. Resources: read and write

fn add_points(mut score: ResMut<Score>) {
    score.0 += 10;                 // ResMut = write access
}

fn show_score(score: Res<Score>) {
    info!("score: {}", score.0);   // Res = read access
}

6. Ordering, run conditions, and plugins

fn main() {
    App::new()
        .add_plugins((DefaultPlugins, GameplayPlugin))
        // .chain() forces order: damage resolves before death is checked.
        .add_systems(Update, (apply_damage, check_deaths).chain())
        // run_if gates a system on a condition each frame.
        .add_systems(Update, spawn_wave.run_if(wave_timer_finished))
        .run();
}

struct GameplayPlugin;
impl Plugin for GameplayPlugin {
    fn build(&self, app: &mut App) {
        app.insert_resource(Score(0))
           .add_systems(Startup, setup)
           .add_systems(Update, (move_players, add_points));
    }
}

Pitfalls

• `delta_seconds()` not found → it was renamed to time.delta_secs() (and elapsed_secs()) in 0.16. Using the old name fails to compile.
• Movement speed scales with frame rate → multiply per-frame changes by time.delta_secs(). Never assume a fixed frame time.
• Panic: "conflicting accesses" / "&mut T and &mut T" → two Querys in one system both write the same component, or one reads while another writes overlapping entities. Make them disjoint with With/Without, or use ParamSet.
• `Camera2dBundle`/`SpriteBundle` not found → bundles were deprecated in 0.15 and removed in 0.16. Spawn the components directly (Camera2d, Sprite, Transform); required components fill in the rest.
• "trait `Component` is not implemented" → you forgot #[derive(Component)] (or #[derive(Resource)] for a resource).
• Spawned entity not visible to a later query in the same frame → Commands are deferred and applied at the next sync point. Read the entity in a subsequent system, not the one that spawned it.
• System order assumed but not enforced → systems run in parallel by default. If B must follow A, add (A, B).chain() or an explicit ordering constraint.
• Deriving both `Resource` and `Component` in 0.19 → Resource now extends Component; derive Resource alone to avoid conflicting implementations.
• Copy-pasting older Bevy snippets → APIs shift between minor versions. The buffered event system became the message system in recent releases. Verify against the docs and migration guide for your pinned version; don't mix versions.

References

• For schedules and SystemSet ordering, States/OnEnter/OnExit, change detection, Commands lifecycle and sync points, ParamSet for conflicting queries, and a version note on the events/observers API, read references/queries-and-scheduling.md.

Related skills

• game-ai — FSMs/behavior trees/steering as portable concepts to implement in ECS.
• procedural-gen — noise/RNG/generation algorithms to drive from systems.
• pygame-core / love2d-core — lighter-weight engines for smaller projects.