NSNotificationCenter

Inherits From:
NSObject

Conforms To:
NSObject (NSObject)

Declared In:
Foundation/NSNotification.h

Class At A Glance

Purpose

NSNotificationCenter provides a way for objects that don't know about each other to communicate. It receives NSNotification objects and broadcasts them to all interested objects.

Principal Attributes

A table of objects that want to receive notifications, the notifications they want to receive, and about which objects.

Creation

Each task has a default notification center. You typically don't create your own.

Commonly Used Methods + defaultCenter Accesses the default notification center. - addObserver:selector:name:object: Registers an object to receive a notification. - postNotificationName:object: Creates and posts a notification. - removeObserver: Specifies that an object no longer wants to receive notifications.

+ defaultCenter	Accesses the default notification center.
- addObserver:selector:name:object:	Registers an object to receive a notification.
- postNotificationName:object:	Creates and posts a notification.
- removeObserver:	Specifies that an object no longer wants to receive notifications.

Class Description

An NSNotificationCenter object (or simply, notification center) is essentially a notification dispatch table. It notifies all observers of notifications meeting specific criteria. This information is encapsulated in NSNotification objects, also known as notifications. Client objects register themselves as observers of specific notifications posted by other objects. When an event occurs, an object posts an appropriate notification to the notification center. (See the NSNotification class specification for more on notifications.) The notification center dispatches a message to each registered observer, passing the notification as the sole argument. It is possible for the posting object and the observing object to be the same.

Each task has a default notification center that you access with the defaultCenter class method.

Registering to Receive Notifications

An object registers itself to receive a notification by sending the addObserver:selector:name:object: method, specifying the message the notification should send, the name of the notification it wants to receive, and about which object. However, the observer need not specify both the name and the object. If it specifies only the object, it will receive all notifications containing that object. If the object specifies only a notification name, it will receive that notification every time it's posted, regardless of the object associated with it.

It is possible for an observer to register to receive more than one message for the same notification. In such a case, the observer will receive all messages it is registered to receive for the notification, but the order in which it receives them cannot be determined.

Creating and Posting Notifications

Normally, you create an instance of NSNotification and post it using postNotification:. The methods postNotificationName:object: and postNotificationName:object:userInfo: are convenient ways to post notifications without having to create an NSNotification first.

Example

As an example of using the notification center, suppose your program can perform a number of conversions on text (for instance, RTF to ASCII). You have defined a class of objects that perform those conversions, Converter. Converter objects might be added or removed during program execution. Your program has a client object that wants to be notified when converters are added or removed, allowing the application to reflect the available services in a pop-up list. The client object would register itself as an observer by sending the following messages to the notification center:

[[NSNotificationCenter defaultCenter] addObserver:self
    selector:@selector(objectAddedToConverterList:)
    name:@"ConverterAdded" object:nil];

[[NSNotificationCenter defaultCenter] addObserver:self
    selector:@selector(objectRemovedFromConverterList:)
    name:@"ConverterRemoved" object:nil];

When a user installs or removes a Converter, the Converter sends one of the following messages to the notification center:

[[NSNotificationCenter defaultCenter]
    postNotificationName:@"ConverterAdded" object:self];

[[NSNotificationCenter defaultCenter]
    postNotificationName:@"ConverterRemoved" object:self];

The notification center identifies all observers who are interested in the "ConverterAdded" or "ConverterRemoved" notifications by invoking the method they specified in the selector argument of addObserver:selector:name:object:. In the case of our example observer, the selectors are objectAddedToConverterList: and objectRemovedFromConverterList:. Assume the Converter class has an instance method converterName that returns the name of the Converter object. Then the objectAddedToConverterList: method might have the following implementation:

- (void)objectAddedToConverterList:(NSNotification *)notification
{
    Converter *addedConverter = [notification object];

    // Add this to our popup (it will only be added if not there)...
    [myPopUpButton addItem:[addedConverter converterName]];
}

The Converters don't need to know anything about the pop-up list or any other aspect of the user interface to your program.

If there are other objects of interest to the observer, place them in the notification's optional dictionary or use postNotificationName:object:userInfo:.

Method Types

Accessing the default center

+ defaultCenter

Adding and removing observers

- addObserver:selector:name:object:
- removeObserver:
- removeObserver:name:object:

Posting notifications

- postNotification:
- postNotificationName:object:
- postNotificationName:object:userInfo:

Class Methods

defaultCenter

+ (NSNotificationCenter *)defaultCenter

Returns the current task's notification center, which is used for system notifications.

Instance Methods

addObserver:selector:name:object:

- (void)addObserver:(id)anObserver
selector:(SEL)aSelector
name:(NSString *)notificationName
object:(id)anObject

Registers anObserver to receive notifications with the name notificationName and/or containing anObject. When a notification of name notificationName containing the object anObject is posted, anObserver receives an aSelector message with this notification as the argument. The method for the selector specified in aSelector must have one and only one argument. If notificationName is nil, the notification center notifies the observer of all notifications with an object matching anObject. If anObject is nil, the notification center notifies the object of all notifications with the name notificationName.

The notification center does not retain anObserver or anObject. Therefore, you should always send removeObserver: or removeObserver:name:object: to the notification center before releasing these objects.

postNotification:

- (void)postNotification:(NSNotification *)notification

Posts notification to the notification center. You can create notification with the NSNotification class method notificationWithName:object: or notificationWithName:object:userInfo:. An exception is raised if notification is nil.

postNotificationName:object:

- (void)postNotificationName:(NSString *)notificationName

 
object:

(id)anObject

Creates a notification with the name notificationName, associates it with the object anObject, and posts it to the notification center. anObject is typically the object that is posting the notification. It may be nil.

This method invokes postNotificationName:object:userInfo: with a userInfo: argument of nil.

See also: - postNotification:

postNotificationName:object:userInfo:

- (void)postNotificationName:(NSString *)notificationName

 
object:

(id)anObject
userInfo:(NSDictionary *)userInfo

Creates a notification with the name notificationName, associates it with the object anObject and dictionary userInfo, and posts it to the notification center. This method is the preferred method for posting notifications. anObject is typically the object that is posting the notification. It may be nil. userInfo also may be nil.

See also: - postNotificationName:object:

removeObserver:

- (void)removeObserver:(id)anObserver

Removes anObserver from all notification associations in the notification center. Be sure to invoke this method (or removeObserver:name:object:) before releasing anObserver or any object specified in addObserver:selector:name:object:.

removeObserver:name:object:

- (void)removeObserver:(id)anObserver
name:(NSString *)notificationName
object:(id)anObject

Removes anObserver as the observer of notifications with the name notificationName and object anObject from the notification center. Be sure to invoke this method (or removeObserver:) before deallocating the observer object or any object specified in addObserver:selector:name:object:.

If anObserver is nil, all objects are removed as observers of notificationName containing anObject. (Recall that the object that a notification contains is usually the object that posted the notification.) If notificationName is nil, anObserver is removed as an observer of all notifications containing anObject. If anObject is nil, anObserver is removed as an observer of notificationName containing any object. For example, if you wanted all objects to stop observing notifications containing the object aWindow, you would sent this message:

[[NSNotificationCenter defaultCenter] removeObserver:nil name:nil object:aWindow];