Question

So far I have been using NSNotificationCenter with the method postNotification:aString object:anyObjectOfInterestForTheReceiver. But recently I read in the documentation that the object field should only be passed self. Is there any terrible side effect I am unaware of that should convince me to only pass self in the future, or is it OK to pass any object?
Thank you!

Answer 1

You can pass any object as the object of a notification, but the convention is that the object is the “thing that is doing the notifying” (and you put other relevant state in the userInfo dictionary).

The reason why it’s mostly self is because usually the object doing the notifying usually wants to reference itself. That way, for example, if you had many Foo objects, and one of them completed a task and sent a notification, anyone observing the notification could just look at object to see which Foo was the one in question. The observer can also choose to observe only notifications from a particular Foo when you follow this scheme.

It’s also reasonable (though less common) to use something besides “self” when posting a notification– let’s say you’re sending a notification “on behalf of” another object. For example, you could be a singleton controller object that completes a Bar task, and you could send the notification with a reference to the particular Bar as the object. That makes more sense than using the singleton as the object, since there’d be no interesting variance there.

Again, this is a (useful) convention only. When you make up your own notification, you get to define the “contract” of the notification, which is the name, what kind of object is used as the object, and what’s in the userInfo.