ios – Why does this methodology name should be marked with await regardless that it isn’t an async methodology?

If I wanted to hear for a notification, I’ve all the time finished like so:

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

But when I transfer that line inside the physique of an async methodology, the compiler all of the sudden needs that line to be marked with await:

func setup() async {
    let importantStuff = await someTask()
    // retailer my vital stuff, now setup is full

    // This line will not compile with out the await key phrase.
    // However addObserver is just not even an async methodology 🤨
    await NotificationCenter.default.addObserver(self, selector: #selector(foo), identify: UIApplication.didEnterBackgroundNotification, object: nil)

    // However this works fantastic
    hear()
}


func hear() {
    NotificationCenter.default.addObserver(self, selector: #selector(foo), identify: UIApplication.didEnterBackgroundNotification, object: nil)
}

What is going on on right here?