Where to remove observer for NSNotification in Swift?

Question

Where should I remove the observer for NSNotification in Swift, since viewDidUnload and dealloc() are unavailable?

Answer 1

It is also good if you add your observer in viewWillAppear() and remove them in viewWillDisappear()

Answer 2

Swift 5

I have a chat application, so whenever I go from my ChatLogViewController to some other viewController and then comeback, I have 1 extra Observer of my keyboard notification. To remove that I remove all the observers when I change my viewController or disappear from my chatLogViewController.

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

    NotificationCenter.default.removeObserver(self)
}

Answer 3

As of iOS 9 (and OS X 10.11), you don't need to remove observers yourself, if you're not using block based observers though. The system will do it for you, since it uses zeroing-weak references for observers, where it can.

And if you are using block based observers, make sure you capture self weakly using [weak self] in the closure's capture list, and remove observer in deinit method. If you don't use weak reference to self, deinit method (and thus removal of that observer) will never be called since Notification Center will hold a strong reference to it indefinitely.

More info can be found at Foundation Release Notes for OS X v10.11 and iOS 9.

If the observer is able to be stored as a zeroing-weak reference the underlying storage will store the observer as a zeroing weak reference, alternatively if the object cannot be stored weakly (i.e. it has a custom retain/release mechanism that would prevent the runtime from being able to store the object weakly) it will store the object as a non-weak zeroing reference. This means that observers are not required to un-register in their deallocation method.

Block based observers via the -[NSNotificationCenter addObserverForName: object: queue: usingBlock] method still need to be un-registered when no longer in use since the system still holds a strong reference to these observers.

Answer 4

deinit {
    NotificationCenter.default.removeObserver(self)
}

Answer 5

I also want to point out that you should use this method:

func addObserver(_ observer: Any, selector aSelector: Selector, name aName: NSNotification.Name?, object anObject: Any?)

Instead of

func addObserver(forName name: NSNotification.Name?, object obj: Any?, queue: OperationQueue?, using block: @escaping (Notification) -> Void) -> NSObjectProtocol

The latter will not remove the observer (Ran into this problem recently). The former will remove the observer if you are using iOS9.

Answer 6

You can use three methods:

1. after popViewController, back navigationController or dismissViewControllerAnimated:

   deinit {
       print("Remove NotificationCenter Deinit")
       NSNotificationCenter.defaultCenter().removeObserver(self)
   }
   

2. viewDidDisappear, remove after it is already the next view controller:

   override func viewDidDisappear(animated: Bool) {
       NSNotificationCenter.defaultCenter().removeObserver(self)
   }
   

3. viewWillDisappear - before opening the next view:

   override func viewWillDisappear(animated: Bool) {
       NSNotificationCenter.defaultCenter().removeObserver(self)
   }
   

Swift 3.0 syntax:

NotificationCenter.default.removeObserver(self)