GPUI makes extensive use of _context parameters_, typically named `cx` and positioned at the end of the parameter list, unless they're before a final function parameter. A context reference provides access to application state and services.
There are multiple kinds of contexts, and contexts implement the `Deref` trait so that a function taking `&mut AppContext` could be passed a `&mut WindowContext` or `&mut ViewContext` instead.
```
AppContext
/ \
ModelContext WindowContext
/
ViewContext
```
- The `AppContext` forms the root of the hierarchy
-`ModelContext` and `WindowContext` both dereference to `AppContext`
-`ViewContext` dereferences to `WindowContext`
## `AppContext`
Provides access to the global application state. All other kinds of contexts ultimately deref to an `AppContext`. You can update a `Model<T>` by passing an `AppContext`, but you can't update a view. For that you need a `WindowContext`...
## `WindowContext`
Provides access to the state of an application window, and also derefs to an `AppContext`, so you can pass a window context reference to any method taking an app context. Obtain this context by calling `WindowHandle::update`.
## `ModelContext<T>`
Available when you create or update a `Model<T>`. It derefs to an `AppContext`, but also contains methods specific to the particular model, such as the ability to notify change observers or emit events.
Available when you create or update a `View<V>`. It derefs to a `WindowContext`, but also contains methods specific to the particular view, such as the ability to notify change observers or emit events.
Whereas the above contexts are always passed to your code as references, you can call `to_async` on the reference to create an async context, which has a static lifetime and can be held across `await` points in async code. When you interact with `Model`s or `View`s with an async context, the calls become fallible, because the context may outlive the window or even the app itself.
## `TestAppContext` and `TestVisualContext`
These are similar to the async contexts above, but they panic if you attempt to access a non-existent app or window, and they also contain other features specific to tests.
