How to stop the Observer in NSNotification to called twice?

Question

I have an observer of NSNotification which is called twice. I do not know what to do with it.

I googled it but no solution found.

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Answer 1

Try set a breakpoint on [[NSNotificationCenter defaultCenter] addObserver:self selector:aSelector name:aName object:nil]; and check if it is called more than once.

Answer 2

If your addObserver method is run multiple times, it will create multiple observers. My issue was that I somehow placed my observer in viewWillAppear which appeared multiple times before I posted the notification and it resulted in my observer being called multiple times.

While EmptyStack's 3rd solution works, there is a reason your observer is being called twice, so by finding it, you can prevent needless lines of code instead of removing and adding the same observer.

I would suggest putting your observer in viewDidLoad to avoid simple errors like the one I experienced.

Answer 3

Try to removeObserver in viewWillDisappear method :

-(void)viewWillDisappear:(BOOL)animated{

[[NSNotificationCenter defaultCenter] removeObserver:self name:@"startAnimating" object:nil]; }

Answer 4

I had two instances of the same class and it took me some time before I have realised that the notification is not running twice in one instance of that class but twice in two instances.

Answer 5

Solution 1: The first thing is to check if the notification itself is posted twice.

Solution 2: Even if the notification is posted only once, the action will be called as many times you've added the observer for the notification (no matter the notification is same or not). For example, the following two lines will register the observer(self) for the same notification(aSelector) twice.

[[NSNotificationCenter defaultCenter] addObserver:self selector:aSelector name:aName object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self selector:aSelector name:aName object:nil];

You have to find where you are adding observer for the second time, and remove it. And also make sure that the code where you are add the observer is not called twice.

Solution 3: If you are not sure whether you have already added the observer or not, you can simply do the following. This will make sure that the observer is added only once.

[[NSNotificationCenter defaultCenter] removeObserver:self name:aName object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self selector:aSelector name:aName object:nil];

Answer 6

I use notifications in a document-based app. Every document's observer class (a ViewController) caught the notification. Two documents open, function was called twice. Three documents open... you get the drift.

To limit who receives the notification, you can specify that you're interested in a particular object, but this has a twist: the object needs to be the same instance of a class object; you cannot simply compare a value; so UUID does not get matched, but an instance of

class DocumentID {
    var id = UUID()
}

works fine. Inject this from your Document to every class that sends or receives notifications, and you've limited notifications to your document.