What is MainActor.assumeIsolated?

MainActor.assumeIsolated is a method in Swift that allows you to inform the Swift compiler that a particular piece of code is already running on the main actor. This method does not start a new task to asynchronously execute on the main actor; instead, it asserts that the current code execution context is already on the main actor. This is useful when you have a delegate protocol or callback that you know is always called on the main actor, and you want to avoid the overhead of starting a new task.

Using assumeIsolated can help prevent race conditions by ensuring that the code is executed in the expected context. If the function is called from a context that is not the main actor, it will trap, stopping the code execution, which is preferable to encountering a race condition that could corrupt user data.

For more details, you can refer to the session Migrate your app to Swift 6 (24:15).