Quite a while back, I posted about uncovering selectors at runtime. Detecting selectors allows you to reverse engineer the way Apple uses delegation calls to communicate between objects and clients. Delegation is a key way that objects communicate. Another way is intra-application notification.

As with selectors, you can uncover notifications at runtime. Notification are electronic semaphores that pass between objects within a program.

Notifications allow objects to broadcast information. Observers, objects that subscribe to those broadcasts, can respond to those notifications as needed by checking for new data, queuing a song, or any other state update process you can imagine.

Apple's media playback classes offer a particularly good example of this. They broadcast when songs start playing, when they stop, when the track selection changes, and so forth.

Unlike inter-application notifications, which use Darwin Notification Centers, intra-application notifications use the standard NSNotificationCenter class. Be aware that the iPhone generally does not use or properly implement its child class, NSDistributedNotificationCenter.

Notification Centers can broadcast both a message and provide context for that message in a paired User Data -- typically a dictionary or array and an Object -- usually the object that broadcast the notification in the first place.

In addition to providing user data, in-app Notification Centers differ from BSD/Darwin Notifications by allowing you to subscribe with wildcards. You don't need to know the name in advance of the notification you want to listen to. So to subscribe an object, all you need to do is pass a call like this:

[[NSNotificationCenter defaultCenter] addObserver:self 
         selector:@selector(trackNotifications:) name:nil object:nil];

This registers your object (self) as an observer for all notifications that pass through your program. Here, the selector trackNotifications: is called with the notification whenever one is detected.

To eavesdrop on those notifications, add the following method. This method extracts the notification name, object and user information from the notification itself.

- (void) trackNotifications: (NSNotification *) notification
{
   id nname = [notification name];
   id nobj = [notification object];
   id ndict = [notification userInfo];

   // notification name
   printf("%s\n", [nname cStringUsingEncoding:1]);

   // output accompanying data dictionary
   int i;
   id keys = [ndict allKeys];
   for (i = 1; i < [ndict count]; i++)
   {
      id key = [keys objectAtIndex:i];
      id object = [ndict objectForKey:key];
      NSLog(@"  %@ : %@", key, object);
   }
}