Programming iOS 6, 3rd Edition

Notifications

Notifications (Chapter 11) can be a way to communicate between objects that are conceptually distant from one another without bothering to provide any way for one to see the other. All they really need to have in common is a knowledge of the name of the notification. Using a notification in this way may seem lazy, an evasion of your responsibility to architect your objects sensibly. But sometimes one object doesn’t need to know, and indeed shouldn’t know, what object (or objects) it is sending a message to.

For example, in one of my apps, the app delegate may detect a need to tear down the interface and built it back up again from scratch. If this is to happen without causing memory leaks (and all sorts of other havoc), every view controller that is currently running a repeating NSTimer needs to invalidate that timer (Chapter 12). Rather than my having to work out what view controllers those might be, and endowing every view controller with a method that can be called, I simply have the app delegate shout “Everybody stop timers!” — by posting a notification. My view controllers that run timers have all registered for this notification, and they know what to do when they receive it.