NSNotifications: a broadcasting mechanism

Notifications are a strong mechanism to broadcast messages to multiple recipients , and also to loosely couple objects.

Typical examples are :

- GUI elements that want to notify other GUI elements about an event. (for example : a window that closes can notify all controls on the window about the close).
- In a MVC application, the model can notify about changes in his internal state (internal variables that are changing).

To be able to use a notifications we need 2 things : a central broadcast component and a way to register an intrest for notifications (and a way to post notifications).

Apple in his Cocoa framework offers the following implementation :

1. The central broadcast component

[NSNotificationCenter defaultCenter]

The NSNotificationCenter is the class that implements the central broadcast component, and by sending the message defaultCenter you get a singleton instance of it.
Note that each application can only have one nsnotificationcenter.

2. Register for an interest in a notification

[[NSNotificationCenter defaultCenter]
addObserver:myInstance
selector:@selector(doSomethingWhenNotified:)
name:@"someName"
object:nil ] ;

With the above statement you register with the notification center the following :
when somebody (a class in your application) sends a notification with name "someName", then to the object myInstance the message doSomethingWhenNotified: will be sent.

Important note : the message doSomethingWhenNotified: must have the following signature :
- (void) doSomethingWhenNotified: (NSNotification *) notification

(= your notification message must accepts one argument of type NSNotification *).

Note 2 : This method is best called in the awakeFromNib method. Because then you are sure that your window is properly initialized.

3. Posting a notification

To post a notification you do the following :

[[NSNotificationCenter defaultCenter] postNotificationName:@"someName" object: nil ]

Note: this is for example used in a method which changes a variable.

General note : in both cases you see the "object: nil", instead of nil you can use any object, but then the notificationcenter will only post the notifications for that particular object.