Why the Observer in NSNotification called twice....?

Question

If the class is a custom class, after posting the notification, the selector corresponding to the observer is called twice.. Is there any better solution so that the selector is called only once?

Answer 1

If the selector is called twice, you've probably registered for it twice.

Answer 2

If the observer class is registered for a notification by name, but not against a specific object, it will receive multiple messages, as it will be invoked every time the notification occurs, regardless of the originating object.

Likewise, if the observer is registered against a specific object, but not against a named notification, it will be messaged every time there is a notification concerning that object.

An alternative mechanism for being informed of changes in a distant state, is Key Value observing -http://developer.apple.com/documentation/Cocoa/Conceptual/KeyValueObserving/KeyValueObserving.html

Answer 3

What worked for me was a couple of things:

1-In the class where the addObserver registration call is made, was to add a specific object.

2-In the class where the postNotification call is made, as the same type of object.

I also do my postNotificaiton in a dispatchAsync call as well, but that's not related to your problem.