GPUI Entities

This skill provides comprehensive patterns for working with entities and contexts in GPUI applications.

Overview

Entities are GPUI's core state management primitive:

• Type-safe handles to state with automatic lifecycle management
• Read/write access with borrow checking at runtime
• Weak references to avoid memory leaks
• Subscriptions for observing state changes

Creating Entities

In Window Context

use gpui::*;

struct MyView {
    count: usize,
}

fn main() {
    let app = Application::new();
    
    app.run(move |cx| {
        cx.spawn(async move |cx| {
            cx.open_window(WindowOptions::default(), |window, cx| {
                // Create entity with cx.new()
                cx.new(|_cx| MyView { count: 0 })
            })?;
            
            Ok::<_, anyhow::Error>(())
        }).detach();
    });
}

In Entity Context

struct ParentView {
    child: Entity<ChildView>,
}

impl ParentView {
    fn new(cx: &mut Context<Self>) -> Self {
        let child = cx.new(|_cx| ChildView::new());
        Self { child }
    }
}

Reading Entities

Direct Read

fn display_count(entity: &Entity<MyView>, cx: &App) {
    let view = entity.read(cx);
    println!("Count: {}", view.count);
}

Read with Closure

fn get_count(entity: &Entity<MyView>, cx: &App) -> usize {
    entity.read_with(cx, |view, _cx| view.count)
}

Important: The closure return value is returned directly from .read_with().

Updating Entities

Basic Update

fn increment(entity: &Entity<MyView>, cx: &mut App) {
    entity.update(cx, |view, cx| {
        view.count += 1;
        cx.notify(); // Trigger re-render
    });
}

Update with Window

Use .update_in() when you need window access:

fn increment_and_dispatch(
    entity: &Entity<MyView>,
    cx: &mut AsyncWindowContext,
) -> anyhow::Result<()> {
    entity.update_in(cx, |view, window, cx| {
        view.count += 1;
        window.dispatch_action(CountChanged.boxed_clone(), cx);
        cx.notify();
    })
}

Return Values

// update returns the closure's return value
let new_count = entity.update(cx, |view, cx| {
    view.count += 1;
    cx.notify();
    view.count // Return new count
});

println!("New count: {}", new_count);

WeakEntity

Use WeakEntity<T> to avoid memory leaks when entities reference each other.

Creating Weak References

struct Parent {
    child: Entity<Child>,
}

struct Child {
    parent: WeakEntity<Parent>, // Weak to avoid cycle
}

impl Parent {
    fn new(cx: &mut Context<Self>) -> Self {
        let parent_weak = cx.entity().downgrade();
        
        let child = cx.new(|_cx| Child {
            parent: parent_weak,
        });
        
        Self { child }
    }
}

Using Weak References

impl Child {
    fn notify_parent(&self, cx: &mut Context<Self>) {
        if let Some(parent) = self.parent.upgrade() {
            parent.update(cx, |parent, cx| {
                parent.handle_child_event(cx);
            });
        }
    }
}

Async Context with WeakEntity

When using async contexts, methods return anyhow::Result:

fn do_async_work(&self, cx: &mut Context<Self>) {
    cx.spawn(async move |this, cx| {
        // this: WeakEntity<Self>
        
        // Async work
        tokio::time::sleep(Duration::from_secs(1)).await;
        
        // Update returns Result in async context
        this.update(&mut *cx, |view, cx| {
            view.count += 1;
            cx.notify();
        })?;
        
        Ok(())
    }).detach();
}

Entity Lifecycle

Entity ID

let id = entity.entity_id();
// id: EntityId - unique identifier that persists

Checking Entity Validity

// Strong entity is always valid
let entity: Entity<MyView> = cx.new(|_| MyView { count: 0 });
// entity is guaranteed to be valid

// Weak entity might be invalid
let weak: WeakEntity<MyView> = entity.downgrade();

if let Some(entity) = weak.upgrade() {
    // Entity still exists
    entity.update(cx, |view, cx| {
        // ...
    });
} else {
    // Entity was dropped
    println!("Entity no longer exists");
}

Subscriptions

Subscribe to entity events to observe state changes.

Defining Events

use gpui::*;

#[derive(Clone, Debug)]
enum MyEvent {
    CountChanged { new_value: usize },
    Reset,
}

// Declare that this entity can emit MyEvent
impl EventEmitter<MyEvent> for MyView {}

Emitting Events

impl MyView {
    fn increment(&mut self, cx: &mut Context<Self>) {
        self.count += 1;
        cx.emit(MyEvent::CountChanged { new_value: self.count });
        cx.notify();
    }
    
    fn reset(&mut self, cx: &mut Context<Self>) {
        self.count = 0;
        cx.emit(MyEvent::Reset);
        cx.notify();
    }
}

Subscribing to Events

struct ParentView {
    child: Entity<ChildView>,
    _subscription: Subscription,
}

impl ParentView {
    fn new(cx: &mut Context<Self>) -> Self {
        let child = cx.new(|_| ChildView::new());
        
        let subscription = cx.subscribe(&child, |this, child, event, cx| {
            // this: &mut ParentView
            // child: Entity<ChildView>
            // event: &MyEvent
            // cx: &mut Context<ParentView>
            
            match event {
                MyEvent::CountChanged { new_value } => {
                    println!("Child count changed: {}", new_value);
                }
                MyEvent::Reset => {
                    println!("Child was reset");
                }
            }
        });
        
        Self {
            child,
            _subscription: subscription, // Keeps subscription alive
        }
    }
}

Important: Store Subscription in a field to keep it active. When dropped, the subscription is automatically cancelled.

Observing Changes

Use cx.observe() to be notified when an entity calls cx.notify():

struct Dashboard {
    counter: Entity<Counter>,
    _observer: Subscription,
}

impl Dashboard {
    fn new(cx: &mut Context<Self>) -> Self {
        let counter = cx.new(|_| Counter { value: 0 });
        
        let observer = cx.observe(&counter, |this, observed, cx| {
            // Called whenever counter.cx.notify() is called
            // observed: Entity<Counter>
            println!("Counter changed!");
            cx.notify(); // Re-render dashboard
        });
        
        Self {
            counter,
            _observer: observer,
        }
    }
}

Context Methods

Getting Entity Reference

From within an entity's method:

impl MyView {
    fn get_self_reference(&self, cx: &Context<Self>) -> WeakEntity<Self> {
        cx.entity().downgrade()
    }
}

Notify

Trigger re-render for this entity:

impl MyView {
    fn update_state(&mut self, cx: &mut Context<Self>) {
        self.count += 1;
        cx.notify(); // Mark this entity as needing re-render
    }
}

Common Patterns

Parent-Child Communication

struct Parent {
    child: Entity<Child>,
}

impl Parent {
    fn tell_child_something(&self, cx: &mut Context<Self>) {
        self.child.update(cx, |child, cx| {
            child.handle_message_from_parent(cx);
        });
    }
}

struct Child {
    parent: WeakEntity<Parent>,
}

impl Child {
    fn tell_parent_something(&self, cx: &mut Context<Self>) {
        if let Some(parent) = self.parent.upgrade() {
            parent.update(cx, |parent, cx| {
                parent.handle_message_from_child(cx);
            });
        }
    }
}

Shared State Entity

#[derive(Clone)]
struct AppState {
    theme: String,
    user: Option<String>,
}

struct MyApp {
    state: Entity<AppState>,
}

impl MyApp {
    fn new(cx: &mut Context<Self>) -> Self {
        let state = cx.new(|_| AppState {
            theme: "dark".into(),
            user: None,
        });
        
        Self { state }
    }
    
    fn change_theme(&self, theme: String, cx: &mut Context<Self>) {
        self.state.update(cx, |state, cx| {
            state.theme = theme;
            cx.notify();
        });
    }
}

Entity as Global State

fn set_global_state(cx: &mut App, state: Entity<AppState>) {
    cx.set_global(state);
}

fn get_global_state(cx: &App) -> Entity<AppState> {
    cx.global::<Entity<AppState>>().clone()
}

// Usage
impl MyView {
    fn access_global(&self, cx: &Context<Self>) {
        let state = get_global_state(cx);
        state.read_with(cx, |state, _| {
            println!("Theme: {}", state.theme);
        });
    }
}

Common Mistakes

Mistake	Problem	Solution
Using outer cx in update	Borrow checker error	Use inner cx from closure
Nested updates	Runtime panic	Restructure to avoid updating while updating
Forgetting cx.notify()	UI doesn't update	Call cx.notify() after state changes
Strong circular references	Memory leak	Use WeakEntity for back-references
Dropping Subscription	Events stop working	Store Subscription in struct field
Not handling None from weak upgrade	Panic or undefined behavior	Always check upgrade() result

Best Practices

1. Use WeakEntity for back-references: Prevents memory leaks
2. Store subscriptions: Keep Subscription values in struct fields
3. Always call cx.notify(): After state changes that affect rendering
4. Handle weak reference failures: Always check if upgrade() returns Some
5. Use read_with for simple access: More ergonomic than .read()
6. Avoid long-lived strong references: Consider WeakEntity if entity won't be used frequently

Summary

• Create with cx.new()
• Read with .read() or .read_with()
• Update with .update() or .update_in()
• Use WeakEntity to avoid cycles
• Subscribe with cx.subscribe() for events
• Observe with cx.observe() for notify signals
• Always call cx.notify() after state changes

References

• GPUI Entities Documentation
• Zed GEMINI.md - Entities