Application and contexts

Everything in GPUI starts from an Application. Its run callback receives &mut App, the root context: global state, entities, windows, and system services all hang off it.

Contexts are the argument named cx. There are four you will meet:

Type	Where you get it	Use it for
App	Application::run, event handlers on elements	Reading and updating entities, globals, opening windows
Context<T>	Inside Entity<T>::update and Render::render	Same as App, plus cx.notify(), cx.emit(), cx.spawn() bound to this entity
AsyncApp	cx.spawn(...)	Held across .await points on the foreground thread
AsyncWindowContext	cx.spawn_in(window, ...)	As above, with a window attached

Context<T> dereferences to App, so a function that takes &App also accepts &Context<T>.

When a function takes callbacks, they come after cx. When a function needs the window, the argument is window: &mut Window and it comes before cx.

Inside an update closure, always use the inner cx the closure gives you. Reaching for an outer context inside the closure is a double borrow and panics at runtime.

Zed’s own Contexts in depth chapter describes the full type hierarchy.