Typed notifications with NSNotificationCenter

TypedNotifications.swift

public protocol BPFNotification: AnyObject {
    static var notificationName: String { get }
    static var payloadKey: String { get }
}

public extension BPFNotification {
    public static var notificationName: String {
        return NSStringFromClass(Self)
    }

    public static var payloadKey: String {
        return "\(self.notificationName)-payload"
    }
}

public extension NSNotificationCenter {

    private class BPFObserverWrapper {
        let notificationCenter: NSNotificationCenter
        let observer: AnyObject

        init(notificationCenter: NSNotificationCenter, observer: AnyObject) {
            self.notificationCenter = notificationCenter
            self.observer = observer
        }

        deinit {
            self.notificationCenter.removeObserver(self.observer)
        }
    }

    public func bpf_postNotification<Notification: BPFNotification>(sender sender: AnyObject? = nil, payload: Notification) {
        let name = Notification.notificationName
        let payloadKey = Notification.payloadKey
        let userInfo = [payloadKey : payload]
        self.postNotificationName(name, object: sender, userInfo: userInfo)
    }

    public func bpf_addObserver<Notification: BPFNotification>(forSender sender: AnyObject? = nil, queue: NSOperationQueue = NSOperationQueue.mainQueue(), usingBlock block: (Notification) -> Void) -> AnyObject {
        let name = Notification.notificationName
        let payloadKey = Notification.payloadKey
        let observer = self.addObserverForName(name, object: sender, queue: queue) { (notification) in
            if let payload = notification.userInfo?[payloadKey] as? Notification {
                block(payload)
            } else {
                assert(false, "Unexpected notification received: \(notification)")
            }
        }
        return BPFObserverWrapper(notificationCenter: self, observer: observer)
    }
}