NSNotificationCenter

NSNotificationCenter类
+ (id)defaultCenter;返回notification center [类方法,返回全局对象, 单件模式.cocoa的很多的全局对象都是通过类似方法实现]
    
- (void)addObserver:(id)observer selector:(SEL)aSelector name:(NSString *)aName object:(id)anObject;
 如果notificationName为nil. 那么notification center将anObject发送的所有notification转发给observer
. 如果anObject为nil.那么notification center将所有名字为notificationName的notification转发给observer


- (void)postNotification:(NSNotification *)notification;
- (void)postNotificationName:(NSString *)aName object:(id)anObject;
- (void)postNotificationName:(NSString *)aName object:(id)anObject userInfo:(NSDictionary *)aUserInfo;


- (void)removeObserver:(id)observer;
- (void)removeObserver:(id)observer name:(NSString *)aName object:(id)anObject;