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.