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 ...]];

Using NSNotificationCenter - posting a notification

Objective C