NSNotificationCenter - removing observer when observed objected is deallocated

Question

I use the following line to add an observer:

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(itemDidFinishPlaying) name:AVPlayerItemDidPlayToEndTimeNotification object:self.playerItem];

My observer (self) will never be deallocated.

But when the user starts playing a different item, the old self.playerItem will be deallocated---but will always be replaced with a new one, which I want to continue observing.

When that happens, what happens in regards to my observer's status as an observer? Do I need to do something to stop observing the deallocated object, as is required with KVO? Or will I continue observing the new object at self.playerItem? Or will my observer automatically be "unregistered?"

If I need to remove the observer, I wonder why there's no corresponding removeObserver method that enables one to specify a selector; it seems I can remove an observer only wholesale via removeObserver:(id)notificationObserver.

Answer 1

According to the NSNotificationCenter class reference:

Be sure to invoke removeObserver: or removeObserver:name:object: before notificationObserver or any object specified in addObserver:selector:name:object: is deallocated.

So: you should unregister your observer before self.playerItem deallocated.

But when the user starts playing a different item, the old self.playerItem will be deallocated---but will always be replaced with a new one, which I want to continue observing.

You may pass nil as the last parameter of addObserver:selector:name:object: method:

Adds an entry to the receiver’s dispatch table with an observer, a notification selector and optional criteria: notification name and sender. If you don't specify

If you pass nil, the notification center doesn’t use a notification’s sender to decide whether to deliver it to the observer.

So you will receive notification AVPlayerItemDidPlayToEndTimeNotification from any object that posts it.

Answer 2

Since iOS 9 it is no longer needed to remove an observer from an object:

In OS X 10.11 and iOS 9.0 NSNotificationCenter and NSDistributedNotificationCenter will no longer send notifications to registered observers that may be deallocated.

However, block based observers need to be un-registered as before:

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.

More information can be found here:

https://developer.apple.com/library/content/releasenotes/Foundation/RN-FoundationOlderNotes/index.html#10_11NotificationCenter