How NSNotification works

Question

I understand what in Notification, poster and observer.

But I am quite unable to understand how our app or OS understands and sends the flag/notification to the observer-class?

What is the mechanism behind this?

Your answer and help will be appreciated a lot.

Thanks

ID.

Answer 1

Imagine the Notification Center as a dictionary which has keys of the notification names and values of the list of observers (and their specified action methods). When a notification is posted, the list of observers for that notification name is obtained and iterated. Each observer has its action method called with the notification information.

Also, during the iteration there is a check to determine if the notification object is of interest to the observer (based on the parameters supplied when the observer was added).

The notification process is carried out on the thread from which the notification was posted.

Don't think about trying to rely on any implied order related to how and when the observers were added.

Answer 2

Basically the NotificationCenter keeps a reference to any object that is registered as an observer. With that reference, it also keeps track of what kind of notifications that object wants. When an object posts a notification, the center delivers it to each registered observer by sending the observer a message with that selector.

The default center is normally a global singleton. But you could create your own, perhaps if you want to ensure your notifications are private to your app.