Bevy events and messaging

When to use

Load this when one system should notice that something happened without storing that fact on every entity. Pick the mechanism from the pages below. Do not invent a third channel.

ECS shape

• Messages (pull). Message is a buffered message for pull-based handling. MessageWriter writes. MessageReader reads and tracks what it already saw. Messages holds what occurred within the last two updates. Register the type with App::add_message. The engine updates the buffer; gameplay systems only write and read.
• Events (push). Derive Event, add an observer, trigger. Commands::trigger and World::trigger run observers immediately inside the call. On<E> is the system parameter. Event::Trigger defaults to GlobalTrigger for derived events.
• Observers. An Observer is a component that holds a system. EntityEvent targets one entity: the event carries the entity field, and On::event_target returns it.
• State transitions. StateTransitionEvent<S> fires when state S changes. That type lives with states. See bevy-state-machines.

Examples

[dependencies]
bevy = "0.20"

A hit is a fact other systems poll. Register the message, write it from the strike system, and read it later in the same schedule. add_message keeps the double-buffered queue.

#[derive(Message)]
struct Hit {
    amount: i32,
}

fn install(app: &mut App) {
    app.add_message::<Hit>()
        .add_systems(Update, (deal, apply).chain());
}

fn deal(mut hits: MessageWriter<Hit>) {
    hits.write(Hit { amount: 12 });
}

fn apply(mut hits: MessageReader<Hit>) {
    for hit in hits.read() {
        info!("hit {}", hit.amount);
    }
}

A shout should run listeners now. Derive Event, observe with On<Shout>, and trigger. The observer runs inside trigger before the system returns.

#[derive(Event)]
struct Shout {
    text: String,
}

fn listen(shout: On<Shout>) {
    info!("{}", shout.event().text);
}

fn setup(mut commands: Commands) {
    commands.spawn(Observer::new(listen));
    commands.trigger(Shout {
        text: "round".into(),
    });
}

An entity-targeted event names its target. event_target is that entity; the observer still runs immediately.

#[derive(EntityEvent)]
struct Stagger {
    entity: Entity,
}

fn on_stagger(hit: On<Stagger>) {
    info!("stagger {:?}", hit.event_target());
}

fn strike(mut commands: Commands, fighters: Query<Entity>) {
    for entity in &fighters {
        commands.trigger(Stagger { entity });
    }
}

Scope

• Register a message with App::add_message before any MessageWriter or MessageReader.
• Entity-targeted triggers use EntityEvent and Commands::trigger with the entity field set.
• Write and read a message in the same schedule with .chain() so the reader sees the write before the buffer updates.

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 message-registration or trigger methods. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

• No third-party crates. 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

• ECS messages - Systems, Query, and where a buffered notice fits beside components.
• Observer example - Event, EntityEvent, Observer, and trigger.
• Examples - Example tree on the latest branch.

Related Skills

Prerequisites

• bevy-master - ECS ownership before adding a notice channel.
• bevy-ecs-composition - Systems that write or read the notice.

Complements

• bevy-state-machines - StateTransitionEvent for app-wide state changes.

Downstream / consumers

• Gameplay skills cover combat, abilities, and similar loops.

Master

• bevy-master - Bevy router.