If I needed to listen for a notification, I've always done like so:

NotificationCenter.default.addObserver(self, selector: #selector(foo), name: UIApplication.didEnterBackgroundNotification, object: nil)

But if I move that line within the body of an async method, the compiler suddenly wants that line to be marked with await:

func setup() async {
    let importantStuff = await someTask()
    // store my important stuff, now setup is complete

    // This line won't compile without the await keyword.
    // But addObserver is not even an async method