Swifty_system_notifications.swift

// Following on from:
//    https://gist.github.com/azureblue75/b28dfb0b12dd0e3a8139a92b9a0243ec
// ...and building on the approaches discussed in:
//    https://talk.objc.io/episodes/S01E27-typed-notifications-part-1

// Let's define a protocol for receiving system notifications:

protocol ReceivableNotification {
    static var name: Notification.Name { get }
    init(notification: Notification)
}

// Now we can define a struct to describe the dictionary passed in a system notification,
// for example a PlaygroundPageNeedsIndefiniteExecutionDidChangeNotification :

struct PlaygroundPayload {
    let needsIndefiniteExecution: Bool
}

// And conform to our Receivable protocol:

extension PlaygroundPayload: ReceivableNotification {
    static let name = Notification.Name("PlaygroundPageNeedsIndefiniteExecutionDidChangeNotification")

    init(notification note: Notification) {
        needsIndefiniteExecution = note.userInfo!["PlaygroundPageNeedsIndefiniteExecution"] as! Bool
    }
}

// And finally, in a similar way as in our custom notifications approach, we'd like a way to register
// a handler for these notifications:

extension NotificationSource {
    func addObserver<N: ReceivableNotification>(using block: @escaping (N) -> ()) -> Token {
        return Token(token: addObserver(forName: N.name, object: nil, queue: nil, using: { note in
            block(N.init(notification: note))
        }), center: self)
    }
}

// Again, usage at the call site is then just:

token = NotificationCenter.default.addObserver { (note:PlaygroundPayload) in
    print("Indefinite now: \(note.needsIndefiniteExecution)")
}