iOS 5 Programming Cookbook

1.29. Sending Notifications with NSNotificationCenter

Problem

You want to broadcast an event in your app and allow any object that is willing to listen to it to take action, depending on the notification that you are broadcasting.

Solution

Use the postNotificationName:object:userInfo: method of the default notification center of type NSNotificationCenter to post a notification that carries an object (usually the object that fires the notification) and a user-info dictionary that can carry extra information about the notification and/or the object that fires the notification.

Discussion

Notification centers are dispatch centrals for notification objects. For instance, when the keyboard pops up anywhere while the user is inside your app, iOS will send a notification to your app. Any object inside your app willing to listen to this notification can add itself to the default notification center as an observer for that particular notification. Once your object’s lifetime comes to an end, it must remove itself from the notification center’s dispatch table. As a result, a notification is a message that gets broadcasted to observers through a notification center. A notification center is an instance of NSNotificationCenter class. We retrieve the default notification center object using the defaultCenter class method of NSNotificationCenter.

Notifications are objects of type NSNotification. A notification object has a name (specified as NSString) and can carry two key pieces of information:

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