NSNotificationCenter

- (void)addObserver:(id)observer selector:(SEL)aSelector :(NSString *)aName object:(id)anObject;

register to receive notification

parameter:

1. observer: the object to receive notification

2. aName: name of notification, if it is nil, we receive all notifications sent out by anObject

3. anObject: specify object we want to observe. If it is nil, we receive all notification with aName

we can register many objects to receive same notification.

- (void)postNotificationName:(NSString *)aName object:(id)anObject;

send notfication

parameter:

anObject: sender