iOS 5 Programming Cookbook

1.30. Listening for Notifications Sent From NSNotificationCenter

Problem

You want to listen for different system and custom notifications broadcast using NSNotificationCenter.

Solution

Add your observer object to the notification center using the addObserver:selector:name:object: instance method of NSNotificationCenter before a notification is broadcast. To stop observing a notification, use the removeObserver:name:object: instance method of NSNotificationCenter and pass your observer object, then the name of the notification that you want to stop observing and the object that you originally subscribed to (this will be explained in detail in the Discussion section of this recipe).

Discussion

Any object can broadcast a notification and any object within the same app can opt into listening for notifications with specific names. Two notifications with the same name can be broadcast, but they must come from two different objects. For instance, you can have a notification with the name of DOWNLOAD_COMPLETED that gets fired from two classes, one being a download manager that downloads images from the Internet and another being a download manager that downloads data from an accessory connected to the iOS device. An observer might be interested only in the notifications of this name coming from a specific object, for instance, the download manager that downloads data from the accessory. You can specify this source object (broadcaster) when you start listening for notifications, using the object ...