Notification Queue Basics

Using the NSNotificationCenter’s postNotification: method and its variants, you can post a notification to a notification center. However, the invocation of the method is synchronous: before the posting object can resume its thread of execution, it must wait until the notification center dispatches the notification to all observers and returns. A notification queue, on the other hand, maintains notifications (instances of NSNotification) generally in a First In First Out (FIFO) order. When a notification rises to the front of the queue, the queue posts it to the notification center, which in turn dispatches the notification to all objects registered as observers.

Every thread has a default notification queue, which is associated with the default notification center for the process. You can create your own notification queues and have multiple queues per center and thread.

Posting Notifications Asynchronously

With NSNotificationQueue’s enqueueNotification:postingStyle: and enqueueNotification:postingStyle:coalesceMask:forModes: methods, you can post a notification asynchronously to the current thread by putting it in a queue. These methods immediately return to the invoking object after putting the notification in the queue.

The notification queue is emptied and its notifications posted based on the posting style and run loop mode specified in the enqueuing method. The mode argument specifies the run loop mode in which the queue will be emptied. For example, if you specify NSModalPanelRunLoopMode, the notifications will be posted only when the run loop is in this mode. If the run loop is not currently in this mode, the notifications wait until the next time that mode is entered. See Run Loop Modes for more information.

Posting to a notification queue can occur in one of three different styles: NSPostASAP, NSPostWhenIdle, and NSPostNow. These styles are described in the following sections.

Coalescing Notifications

In some situations, you may want to post a notification if a given event occurs at least once, but you want to post no more than one notification even if the event occurs multiple times. For example, in an application that receives data in discrete packets, upon receipt of a packet you may wish to post a notification to signify that the data needs to be processed. If multiple packets arrive within a given time period, however, you do not want to post multiple notifications. Moreover, the object that posts these notifications may not have any way of knowing whether more packets are coming or not, whether the posting method is called in a loop or not.

In some situations it may be possible to simply set a Boolean flag (whether an instance variable of an object or a global variable) to denote that an event has occurred and to suppress posting of further notifications until the flag is cleared. If this is not possible, however, in this situation you cannot directly use NSNotificationCenter since its behavior is synchronous—notifications are posted before returning, thus there is no opportunity for "ignoring” duplicate notifications; moreover, an NSNotificationCenter instance has no way of knowing whether more notifications are coming or not.

Rather than posting a notification to a notification center, therefore, you can add the notification to an NSNotificationQueue instance specifying an appropriate option for coalescing. Coalescing is a process that removes from a queue notifications that are similar in some way to a notification that was queued earlier. You indicate the criteria for similarity by specifying one or more of the following constants in the third argument of the enqueueNotification:postingStyle:coalesceMask:forModes: method.

You can perform a bitwise-OR operation with the NSNotificationCoalescingOnName and NSNotificationCoalescingOnSender constants to specify coalescing using both the notification name and notification object. The following example illustrates how you might use a queue to ensure that, within a given event loop cycle, all notifications named MyNotificationName are coalesced into a single notification.

// MyNotificationName defined globally

NSString *MyNotificationName = @"MyNotification";

id object = <#The object associated with the notification#>;

NSNotification *myNotification =

        [NSNotification notificationWithName:MyNotificationName object:object]

[[NSNotificationQueue defaultQueue]

        enqueueNotification:myNotification

        postingStyle:NSPostWhenIdle

        coalesceMask:NSNotificationCoalescingOnName

        forModes:nil];