Extension of NSNotification to use a key enum as the notification name.

Swift NSNotification extension

public enum NotificationKey: String {
    case UserSignedIn = "UserSignedInNotification"
    case UserSignedOut = "UserSignedOutNotification"
    case SomeOtherEvent = "SomeOtherEventNotification"
}

extension NSNotificationCenter {
    
    func addObserver(observer: AnyObject, selector aSelector: Selector, key aKey: NotificationKey) {
        self.addObserver(observer, selector: aSelector, name: aKey.rawValue, object: nil)
    }

    func addObserver(observer: AnyObject, selector aSelector: Selector, key aKey: NotificationKey, object anObject: AnyObject?) {
        self.addObserver(observer, selector: aSelector, name: aKey.rawValue, object: anObject)
    }
    
    func removeObserver(observer: AnyObject, key aKey: NotificationKey, object anObject: AnyObject?) {
        self.removeObserver(observer, name: aKey.rawValue, object: anObject)
    }
    
    func postNotificationKey(key: NotificationKey, object anObject: AnyObject?) {
        self.postNotificationName(key.rawValue, object: anObject)
    }
    
    func postNotificationKey(key: NotificationKey, object anObject: AnyObject?, userInfo aUserInfo: [NSObject : AnyObject]?) {
        self.postNotificationName(key.rawValue, object: anObject, userInfo: aUserInfo)
    }
    
    func addObserverForKey(key: NotificationKey, object obj: AnyObject?, queue: NSOperationQueue?, usingBlock block: (NSNotification!) -> Void) -> NSObjectProtocol {
        return self.addObserverForName(key.rawValue, object: obj, queue: queue, usingBlock: block)
    }
}

But how about UIKeyboardWillHideNotification?