Annotating a closure as running on the main thread

Often times when you call a function with a closure as a parameter, you would like that closure to be run on the main thread for example:

func foo(_ completion: @escaping () -> Void) {
  completion()
}

if you need your completion block to be run on the main thread you could use a dispatch queue

foo {
  DispatchQueue.main.async {
    print("Ran on main thread")
  }
}

This is ok but then you’d have to remember to wrap it every time you use the foo function.

One way around this is to annotate that completion block with @MainActor

func bar(_ completion: @escaping @MainActor () -> Void) {
  Task { await completion() }
}

This annotation will guarantee all users that the completion closure will be run on the main thread.

bar {
  print("Will run on main thread.  I guarantee it")
}