bevy/release-content/release-notes/event_split.md

title	authors	pull_requests
Event Split	@Jondolf	19647 20101

In past releases, all event types were defined by simply deriving the Event trait:

#[derive(Event)]
struct Speak {
    message: String,
}

You could then use the various event handling tools in Bevy to send and listen to the event. The common options include:

• Use trigger to trigger the event and react to it with a global Observer
• Use trigger_targets to trigger the event with specific entity target(s) and react to it with an entity Observer or global Observer
• Use EventWriter::write to write the event to an event buffer and EventReader::read to read it at a later time

The first two are observer APIs, while the third is a fully separate "buffered" API for pull-based event handling. All three patterns are fundamentally different in both the interface and usage. Despite the same event type being used everywhere, APIs are typically built to support only one of them.

This has led to a lot of confusion and frustration for users. A common footgun was using a "buffered event" with an observer, or an observer event with EventReader, leaving the user wondering why the event is not being detected.

Bevy 0.17 aims to solve this ambiguity by splitting the event traits into Event, EntityEvent, and BufferedEvent.

• Event: A shared trait for observer events.
• EntityEvent: An Event that additionally supports targeting specific entities and propagating the event from one entity to another.
• BufferedEvent: An event that supports usage with EventReader and EventWriter for pull-based event handling.

Using Events

A basic Event can be defined like before, by deriving the Event trait.

#[derive(Event)]
struct Speak {
    message: String,
}

You can then trigger the event, and use a global observer for reacting to it.

app.add_observer(|trigger: On<Speak>| {
    println!("{}", trigger.message);
});

// ...

commands.trigger(Speak {
    message: "Hello!".to_string(),
});

To allow an event to be targeted at entities and even propagated further, you can also derive EntityEvent. It supports optionally specifying some options for propagation using the event attribute:

// When the `Damage` event is triggered on an entity, bubble the event up to ancestors.
#[derive(Event, EntityEvent)]
#[entity_event(traversal = &'static ChildOf, auto_propagate)]
struct Damage {
    amount: f32,
}

Every EntityEvent is also an Event, so you can still use trigger to trigger them globally. However, entity events also support targeted observer APIs such as trigger_targets and observe:

// Spawn an enemy entity.
let enemy = commands.spawn((Enemy, Health(100.0))).id();

// Spawn some armor as a child of the enemy entity.
// When the armor takes damage, it will bubble the event up to the enemy,
// which can then handle the event with its own observer.
let armor_piece = commands
    .spawn((ArmorPiece, Health(25.0), ChildOf(enemy)))
    .observe(|trigger: On<Damage>, mut query: Query<&mut Health>| {
        // Note: `On::target` only exists because this is an `EntityEvent`.
        let mut health = query.get(trigger.target()).unwrap();
        health.0 -= trigger.amount();
    })
    .id();

// Trigger the `Damage` event on the armor piece.
commands.trigger_targets(Damage { amount: 10.0 }, armor_piece);

To allow an event to be used with the buffered API, you can instead derive BufferedEvent:

#[derive(BufferedEvent)]
struct Message(String);

The event can then be used with EventReader/EventWriter:

fn write_hello(mut writer: EventWriter<Message>) {
    writer.write(Message("I hope these examples are alright".to_string()));
}

fn read_messages(mut reader: EventReader<Message>) {
    // Process all buffered events of type `Message`.
    for Message(message) in reader.read() {
        println!("{message}");
    }
}

In summary:

• Need a basic event you can trigger and observe? Derive Event!
• Need the observer event to be targeted at an entity? Derive EntityEvent!
• Need the event to be buffered and support the EventReader/EventWriter API? Derive BufferedEvent!