NSNotificationCenter.
 
Mutliple observers can be notified of events.
 
notification sent by a specific object...
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(myFooHappened:) name:MyFooNotification object:foo];
 
set object to nil to receive notifications of that name sent by ANY object.
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(myFooHappened:) name:MyFooNotification object:nil];
 
Format for myFooHappened is..
-(void) myFooHappened:(NSNotification*)note {
    // [note name] - name of notification [note object] - sender
 
    // [note userInfo] is an NSDictionary of key-value pairs filled by the sender. 
}
 
Note: notifications are posted on the SAME THREAD as they were posted.
 
MUST unregister from all notifications in your dealloc
-(void) dealloc
    [[NSNotificationCenter defaultCenter] removeObserver:self];
}
 
Posting a notification..
Define a constant for the name of the notification..
in .h
extern NSString* const MyFooNotification;
 
in .m
NSString* const MyFooNotification = @"MyFooNotification";
 
-(void) postFoo  {
[[NSNotificationCenter defaultCenter] postNotificationName:MyFooNotification object:self userInfo:[NSDictionary ...]];