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NSDistributedNotificationCenter


Inherits from:
NSNotificationCenter : NSObject
Conforms to:
NSObject
(NSObject)
Declared in:
Foundation/NSDistributedNotificationCenter.h



Class at a Glance


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

Principal Attributes


Creation


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

Commonly Used Methods



+ defaultCenter Accesses the default notification center.
- addObserver:selector:name:object:suspensionBehavior: Registers an object to receive a notification with a specified behavior when notification delivery is suspended.
- postNotificationName:object:userInfo:deliverImmediately: Creates and posts a notification.
- removeObserver:name:object: Specifies that an object no longer wants to receive certain notifications.


Class Description


An NSDistributedNotificationCenter object (or simply, distributed notification center) is a notification center that can distributed notifications to tasks other than the one in which the notification was posted.

The distributed notification center 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 distributed notification center. (See the NSNotification class specification for more on notifications.) The distributed 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 distributed notification center that you access with the defaultCenter class method. There may be different types of distributed notification centers. Right now there is a single type-NSLocalNotificationCenterType. This type of distributed notification center handles notifications that can be sent between tasks on a single machine. For communication between tasks on different machines, use Distributed Objects.


Registering to Receive Notifications

An object registers itself to receive a notification by sending the addObserver:selector:name:object:suspensionBehavior: method, specifying the message the notification should send, the name of the notification it wants to receive, the identifying string to match (the anObject argument), and the behavior to follow if notification delivery is suspended. Because the posting object and the observer may be in different tasks, notifications can't contain pointers to arbitrary objects. Therefore, NSDistributedNotificationCenter requires notifications to use an NSString as the anObject argument. Notification matching is done based on this string, rather than an object pointer. You should check the documentation of the object posting the notification to see what it uses as its identifying string.

There are four different types of suspension behavior, further described in the "Constants" section, each useful in different circumstances:


Suspending Notification Delivery

When a task is no longer interested in receiving notifications immediately, it may suspend notification delivery. This is often done when the application is hidden, or is put into the background. You suspend notifications by invoking setSuspended: with an argument of YES on the distributed notification center.

While notifications are suspended, the notification server handles notifications destined for the task that suspended notification delivery according to the suspension behavior specified by the observers when they registered to receive notifications. When the task unsuspends notification delivery, all queued notifications are delivered immediately. Note that a notification destined for an observer that registered with NSNotificationSuspensionBehaviorDeliverImmediately (described in "Constants" ), will automatically flush the queue as it is delivered, causing all queued notifications to be delivered at that time as well.


Posting Notifications for Immediate Delivery

If an object posting a notification wants to ensure that all observers receive the notification immediately (for example, if the notification is a warning that a server is about to shut down), it can invoke postNotificationName:object:userInfo:deliverImmediately: with a deliverImmediately argument of YES. The notification center will deliver the notification as if the observers had registered with NSNotificationSuspensionBehaviorDeliverImmediately (further described below, in "Constants" ).




Constants


There are four different types of suspension behavior, each useful in different circumstances.


Suspension Behavior Description
NSNotificationSuspensionBehaviorDrop The server does not queue any notifications with this name and object until setSuspended: with an argument of NO is called.
NSNotificationSuspensionBehaviorCoalesce The server only queues the last notification of the specified name and object; earlier notifications are dropped. In cover methods for which suspension behavior is not an explicit argument, NSNotificationSuspensionBehaviorCoalesce is the default.
NSNotificationSuspensionBehaviorHold The server holds all matching notifications until the queue has been filled (queue size determined by the server) at which point the server may flush queued notifications.
NSNotificationSuspensionBehaviorDeliverImmediately The server delivers notifications matching this registration irrespective of whether setSuspended: with an argument of YES has been called. When a notification with this suspension behavior is matched, it has the effect of first flushing any queued notifications. The effect is as if setSuspended: with an argument of NO were first called if the app is suspended, followed by the notification in question being delivered, followed by a transition back to the previous suspended or unsuspended state.



Method Types


Accessing distributed notification centers
+ defaultCenter
+ notificationCenterForType:
Adding and removing observers
- addObserver:selector:name:object:
- addObserver:selector:name:object:suspensionBehavior:
- removeObserver:name:object:
Posting notifications
- postNotificationName:object:
- postNotificationName:object:userInfo:
- postNotificationName:object:userInfo:deliverImmediately:
Suspending and enabling notification delivery
- setSuspended:
- suspended


Class Methods



defaultCenter

+ (NSNotificationCenter *)defaultCenter

Returns the default distributed notification center, the local notification center for the machine. Calls notificationCenterForType: with an argument of NSLocalNotificationCenterType.

notificationCenterForType:

+ (NSDistributedNotificationCenter *)notificationCenterForType:(NSString *)type

Returns the distributed notification center for the specified type. Currently only one type, NSLocalNotificationCenterType, is supported.


Instance Methods



addObserver:selector:name:object:

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

Registers anObserver to receive notifications with the name notificationName and/or the identifying string anObject.

This method calls addObserver:selector:name:object:suspensionBehavior: with a suspensionBehavior: argument of NSNotificationSuspensionBehaviorCoalesce (described in "Constants" ).



addObserver:selector:name:object:suspensionBehavior:

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

Registers anObserver to receive notifications with the name notificationName and/or the identifying string anObject.

When a notification of name notificationName with the identifying string 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 identifying string matching anObject. If anObject is nil, the notification center notifies the observer of all notifications with the name notificationName. The suspensionBehavior determines how the notification center handles notifications when notification delivery has been suspended. Here are the possible values (further described in "Constants" ):

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

See Also: - postNotificationName:object:userInfo:deliverImmediately:



postNotificationName:object:

- (void)postNotificationName:(NSString *)notificationName object:(NSString *)anObject

Creates a notification with the name notificationName, associates it with the string anObject, and posts it to the notification center. anObject may be nil.

This method invokes postNotificationName:object:userInfo:deliverImmediately: with a userInfo: argument of nil and a deliverImmediately: argument of NO.

See Also: - postNotificationName:object:userInfo:



postNotificationName:object:userInfo:

- (void)postNotificationName:(NSString *)notificationName object:(NSString *)anObject userInfo:(NSDictionary *)userInfo

Creates a notification with the name notificationName, associates it with the string anObject and dictionary userInfo, and posts it to the notification center. anObject and userInfo may be nil.

This method invokes postNotificationName:object:userInfo:deliverImmediately: with a deliverImmediately: argument of NO.

See Also: - postNotificationName:object:



postNotificationName:object:userInfo:deliverImmediately:

- (void)postNotificationName:(NSString *)notificationName object:(NSString *)anObject userInfo:(NSDictionary *)userInfo deliverImmediately:(BOOL)deliverImmediately

Creates a notification with the name notificationName, associates it with the string anObject and dictionary userInfo, and posts it to the notification center with delivery scheduled for deliverImmediately, as supplied by the invoker. This method is the preferred method for posting notifications.

The userInfo dictionary is serialized using the NSArchiver class, so it can be passed to another task. In the receiving task, it is deserialized using NSUnarchiver. This imposes some restrictions on the objects that can be placed in the userInfo dictionary. See the NSArchiver and NSUnarchiver documentation for details.

Posting with deliverImmediately set to NO allows the normal suspension behavior of the observers to take place. If deliverImmediately is set to YES, the notification is delivered immediately to all observers, regardless of their suspension behavior or suspension state.

See Also: - postNotificationName:object:userInfo:, - encodeRootObject: (NSArchiver), + unarchiveObjectWithData: (NSUnarchiver)



removeObserver:name:object:

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

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

If anObserver is nil, all objects are removed as observers of notificationName with identifying string anObject. If notificationName is nil, anObserver is removed as an observer of all notifications with identifying string anObject. If anObject is nil, anObserver is removed as an observer of notificationName with any identifying string.



setSuspended:

- (void)setSuspended:(BOOL)suspended

Suspends notification delivery when set to YES, and resumes immediate notification delivery when set to NO. Distributed notification centers enable or suspend notification delivery on a per-task basis. When a task suspends notification delivery, notifications are delivered according to the suspension behavior of the observer. When delivery is not suspended, notifications are always delivered immediately. See "Suspending Notification Delivery" in the class introduction for more information. NSApplication automatically suspends delivery when the application is not active.

See Also: - addObserver:selector:name:object:suspensionBehavior:, - postNotificationName:object:userInfo:deliverImmediately:, - suspended



suspended

- (BOOL)suspended

Returns YES if the notification center is delivering notifications for this application according to their suspension behavior, NO if it is delivering them immediately.

See Also: - setSuspended:




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