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Overview

GPUI provides event system for component coordination:

Event Mechanisms:

• Custom Events: Define and emit type-safe events
• Observations: React to entity state changes
• Subscriptions: Listen to events from other entities
• Global Events: App-wide event handling

Quick Start

Define and Emit Events

#[derive(Clone)]
enum MyEvent {
    DataUpdated(String),
    ActionTriggered,
}

impl MyComponent {
    fn update_data(&mut self, data: String, cx: &mut Context<Self>) {
        self.data = data.clone();

        // Emit event
        cx.emit(MyEvent::DataUpdated(data));
        cx.notify();
    }
}

Subscribe to Events

impl Listener {
    fn new(source: Entity<MyComponent>, cx: &mut App) -> Entity<Self> {
        cx.new(|cx| {
            // Subscribe to events
            cx.subscribe(&source, |this, emitter, event: &MyEvent, cx| {
                match event {
                    MyEvent::DataUpdated(data) => {
                        this.handle_update(data.clone(), cx);
                    }
                    MyEvent::ActionTriggered => {
                        this.handle_action(cx);
                    }
                }
            }).detach();

            Self { source }
        })
    }
}

Observe Entity Changes

impl Observer {
    fn new(target: Entity<Target>, cx: &mut App) -> Entity<Self> {
        cx.new(|cx| {
            // Observe entity for any changes
            cx.observe(&target, |this, observed, cx| {
                // Called when observed.update() calls cx.notify()
                println!("Target changed");
                cx.notify();
            }).detach();

            Self { target }
        })
    }
}

Common Patterns

1. Parent-Child Communication

// Parent emits events
impl Parent {
    fn notify_children(&mut self, cx: &mut Context<Self>) {
        cx.emit(ParentEvent::Updated);
        cx.notify();
    }
}

// Children subscribe
impl Child {
    fn new(parent: Entity<Parent>, cx: &mut App) -> Entity<Self> {
        cx.new(|cx| {
            cx.subscribe(&parent, |this, parent, event, cx| {
                this.handle_parent_event(event, cx);
            }).detach();

            Self { parent }
        })
    }
}

2. Global Event Broadcasting

struct EventBus {
    listeners: Vec<WeakEntity<dyn Listener>>,
}

impl EventBus {
    fn broadcast(&mut self, event: GlobalEvent, cx: &mut Context<Self>) {
        self.listeners.retain(|weak| {
            weak.update(cx, |listener, cx| {
                listener.on_event(&event, cx);
            }).is_ok()
        });
    }
}

3. Observer Pattern

cx.observe(&entity, |this, observed, cx| {
    // React to any state change
    let state = observed.read(cx);
    this.sync_with_state(state, cx);
}).detach();

Best Practices

✅ Detach Subscriptions

// ✅ Detach to keep alive
cx.subscribe(&entity, |this, source, event, cx| {
    // Handle event
}).detach();

✅ Clean Event Types

#[derive(Clone)]
enum AppEvent {
    DataChanged { id: usize, value: String },
    ActionPerformed(ActionType),
    Error(String),
}

❌ Avoid Event Loops

// ❌ Don't create mutual subscriptions
entity1.subscribe(entity2) → emits event
entity2.subscribe(entity1) → emits event → infinite loop!

Reference Documentation

• API Reference: See api-reference.md

  • Event definition, emission, subscriptions
  • Observations, global events
  • Subscription lifecycle

• Patterns: See patterns.md

  • Event-driven architectures
  • Communication patterns
  • Best practices and pitfalls