如何在Swift iOS中注销NSNotification

Question

I have two controllers

class CtrlA: UIViewController {
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)

        NotificationCenter.default.addObserver(CtrlB.self, selector: #selector(CtrlB.badge(notification:)), name: NSNotification.Name(rawValue: "badge"), object: nil)
    }

    override func viewWillDisappear(_ animated: Bool) {
        super.viewWillDisappear(animated)

        NotificationCenter.default.removeObserver(CtrlB.self, name: NSNotification.Name(rawValue: "badge"), object: nil)
    }
}

class CtrlB: UIViewController {
    static func badge (notification: NSNotification) {
        // blah blah
    }
}

Whats the correct way to unregister the notification listener above? I'm not certain this is correct: NotificationCenter.default.removeObserver(CtrlB.self, name: NSNotification.Name(rawValue: "badge"), object: nil)

I don't think I can use self either, since it was registered on CtrlB.self

Answer 1

I am not sure why you are registering/unregistering to notifications with a class and not an instance. 'CtrlB.self' - will not give you an instance of the CtrlB class, in fact it will return a class itself. Instead you should use something like this:

class CtrlA {

    let ctrlBInstance = CtrlB()

    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)

        NotificationCenter.default.addObserver(ctrlBInstance, selector: #selector(CtrlB.badge(notification:)), name: NSNotification.Name(rawValue: "badge"), object: nil)
    }

    override func viewWillDisappear(_ animated: Bool) {
        super.viewWillDisappear(animated)

        NotificationCenter.default.removeObserver(ctrlBInstance, name: NSNotification.Name(rawValue: "badge"), object: nil)
    }
}

And your ClassB should look like this in this case:

class CtrlB {
   func badge (notification: NSNotification) {
        // blah blah
    }
}

Answer 2

So the best way to implement the notification in your project is create one class called NotificationManager inside that declare one dictionary in which you can always update the observers

class NotificationManager {
var observers = [String: AnyObject]()
}

Create addObserver method, post notification method and remove observer method inside the same class.

func postNotification(_ name: String, userInfo: [AnyHashable: Any]? = nil) {
    NotificationCenter.default.post(name: name, object: nil, userInfo: userInfo)
}

func addObserver(_ name: String, block: @escaping (Notification) -> Void) {
 //if observer is already in place for this name, remove it
  removeObserverForName(name)
  let observer = NotificationCenter.default.addObserver(forName: name), object: nil, queue: OperationQueue.main, using: block)
  self.observers[name] = observer
}

func removeObserver(_ name: name) {
 guard let observer = self.observers[name] else { return }
 NotificationCenter.default.removeObserver(observer)
 self.observers.removeValue(forKey: name)
}

 //Removes all observers
func removeAllObservers() {
for observer in self.observers.values {
 NotificationCenter.default.removeObserver(observer)
 }self.observers = [:]
}

So access the above method in any of your class wherever its required and it will take care of everything. This will also prevent crash in your code. If try to remove the same observer more than one time.

Answer 3

You need to get the instance of the observer,which you haven't declared...

for instance you need to set class variable secondA...

class CtrlA: UIViewController {

var secondController: CtrlB?


override func viewDidLoad()
{
super.viewDidLoad()

    if let unwrappedController = storyboard.instantiateViewController(withIdentifier: "someViewController") as? CtrlB
    {
    secondController = unwrappedController        
    }

}

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)
    if let secondController = secondController
    {
    NotificationCenter.default.addObserver(CtrlB.self, selector: #selector(CtrlB.badge(notification:)), name: NSNotification.Name(rawValue: "badge"), object: nil)
    }
}

override func viewWillDisappear(_ animated: Bool) {
    super.viewWillDisappear(animated)
    if let secondController = secondController
    {
    NotificationCenter.default.removeObserver(CtrlB.self, name: NSNotification.Name(rawValue: "badge"), object: nil)
    }
}

//Also don't forget to remove listening on deinit

deinit
{
    if let secondController = secondController
    {
    NotificationCenter.default.removeObserver(secondController, name: NSNotification.Name(rawValue: "badge"), object: nil)
    }
  }
}

    class CtrlB: UIViewController {
        //Here you go with notification...
        static func badge (notification: NSNotification) {
            // blah blah
        }
    }