Document contexts

crates/gpui2/docs/contexts.md

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+# Contexts
+
+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.
+
+## `ViewContext<V>`
+
+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.
+
+## `AsyncAppContext` and `AsyncWindowContext`
+
+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.