Bevy Game Loop Waves Agent Skill

Bevy wave loop

When to use

Load this when enemies arrive in waves and the wave index advances after a clear.

ECS shape

• Components: a wave index and a remaining-count you define.
• Resources: run-wide tables live in a resource (bevy-data-resources).
• Systems: mutate those components on Update or the fixed timestep. Use .chain() when damage must land before death.
• Messages: one-shot facts (hit, pickup, quest advance) go through bevy-events-messaging. Wave advance below is a resource, not a new message type.

Examples

[dependencies]
bevy = "0.20"

The wave index and the spawn budget are one resource. Each enemy is an entity. Spawning and clearing share WaveDirector, so those systems are chained.

#[derive(Resource)]
struct WaveDirector {
    index: u32,
    budget: u32,
    intermission: f32,
}

#[derive(Component)]
struct Enemy;

fn boot(mut commands: Commands) {
    commands.insert_resource(WaveDirector {
        index: 1,
        budget: 4,
        intermission: 0.0,
    });
}

fn spawn_budget(mut commands: Commands, mut wave: ResMut<WaveDirector>) {
    if wave.intermission > 0.0 || wave.budget == 0 {
        return;
    }
    for _ in 0..wave.budget {
        commands.spawn(Enemy);
    }
    wave.budget = 0;
}

fn advance_on_clear(
    enemies: Query<(), With<Enemy>>,
    mut wave: ResMut<WaveDirector>,
    time: Res<Time>,
) {
    if !enemies.is_empty() {
        wave.intermission = 0.0;
        return;
    }
    wave.intermission += time.delta_secs();
    if wave.intermission < 2.0 {
        return;
    }
    wave.index += 1;
    wave.budget = 3 + wave.index;
    wave.intermission = 0.0;
}

fn install(app: &mut App) {
    app.add_systems(Startup, boot)
        .add_systems(Update, (spawn_budget, advance_on_clear).chain());
}

A fixed step is the right place to tick remaining time on a live enemy. The clear system above stays on Update and only reads Enemy.

#[derive(Component)]
struct Lifetime(f32);

fn age_enemies(
    mut commands: Commands,
    mut enemies: Query<(Entity, &mut Lifetime)>,
    time: Res<Time<Fixed>>,
) {
    for (entity, mut life) in &mut enemies {
        life.0 -= time.delta_secs();
        if life.0 <= 0.0 {
            commands.entity(entity).despawn();
        }
    }
}

fn install_lifetime(app: &mut App) {
    app.add_systems(FixedUpdate, age_enemies);
}

Scope

• Wave, spawner, and encounter data are yours to build as components.
• Enemy motion routes to bevy-ai-navigation and bevy-character-movement.

Expert Knowledge Delta

Target: Bevy 0.20+.

Core ECS types: Component, Resource, Query, Commands, Startup, Update, Fixed / FixedMain (bevy::time), .chain(), Plugin, DefaultPlugins, MinimalPlugins. Messages and observers are an ownership slot. See bevy-events-messaging for their APIs. Systems run in parallel unless .chain() orders the ones that conflict.

Who owns state

| Piece | Owner | Who may mutate | Schedule |

| --- | --- | --- | --- |

| Data on one entity | Component | One system via Query | Update or FixedMain |

| One value for the whole app | Resource | One system | Startup to insert, then the schedule that reads it |

| Spawn | Commands | The spawning system | Startup for the initial world; later schedules for runtime spawn |

| Order that must not race | .chain() | The chained systems only | Same schedule |

| Plugin boundary | Plugin | build registers systems and resources | Before the app runs |

Borrow rule: two systems that both mutably query the same component must be ordered (.chain()) or split so only one writes. Unrelated systems stay parallel. That is the ECS page rule, not a guess at a system-set API.

Decision

| If you need | Do this | Do not |

| --- | --- | --- |

| Per-entity data | Component + Query | One mutable blob shared by every entity |

| App-wide config or clock | Resource | Copy the value onto every entity |

| Stable simulation step | FixedMain and the Fixed clock | Tie simulation to raw frame Update |

| Startup spawn | Commands in Startup | Spawn from a render system |

| Cross-system notice that is not shared state | Message or observer slot in bevy-events-messaging | A new event type invented in this file |

| An API outside the documented set | Read the official Bevy pages | A made-up crate or API |

Load references/ecs-ownership.md for the ownership table.

NEVER

• Do not pin /0.20.0/ in docs URLs. WHY: links stay on latest; the 0.20 target is stated once in prose.
• Do not mutably query the same component from two unordered systems. WHY: the schedule runs systems in parallel unless .chain() orders them.
• Do not invent a physics, navigation, networking, or XR crate here. WHY: this skill only names documented Bevy crates and modules.

• No invented APIs or unnamed third-party crates as the chosen solver. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

Reference

The library pin on bevy-master stays 0.20.0. Official pages stay on the unversioned site.

Official Documentation

• Assets - Official asset catalog. No third-party crate was selected.
• Examples - Example tree on the latest branch.
• ECS - Component, systems, Query, Startup, Update, and .chain().
• Plugins - Plugin, DefaultPlugins, MinimalPlugins, and add_plugins.

Related Skills

Prerequisites

• bevy-master - Hub and schedules.
• bevy-ecs-composition - Components and systems.
• bevy-ai-navigation - What spawned actors do.
• bevy-data-resources - Tables and definitions.
• bevy-events-messaging - One-shot facts.

Complements

• bevy-state-machines - Mode flags.

Downstream / consumers

• None yet.

Master

• bevy-master - Bevy router.