Notifications

Cocoa中，通知中心实现的观察者设计模式。通过下面两个类实现通知的发送和接收。

/****************    Notifications    ****************/

@interface NSNotification : NSObject <NSCopying, NSCoding>

- (NSString *)name;
- (id)object; // 发送该通知的对象
- (NSDictionary *)userInfo; // 额外的可选信息

@end

@interface NSNotification (NSNotificationCreation)

+ (id)notificationWithName:(NSString *)aName object:(id)anObject;
+ (id)notificationWithName:(NSString *)aName object:(id)anObject userInfo:(NSDictionary *)aUserInfo;

@end

/****************    Notification Center    ****************/

@interface NSNotificationCenter : NSObject {
    @package
    void * __strong _impl;
    void * __strong _callback;
    void *_pad[11];
}

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

// 发送通知，anObject：发送通知的对象，Observer通过- object方法可以访问该对象
- (void)postNotification:(NSNotification *)notification;
- (void)postNotificationName:(NSString *)aName object:(id)anObject;
- (void)postNotificationName:(NSString *)aName object:(id)anObject userInfo:(NSDictionary *)aUserInfo;

// 注册观察者
- (void)removeObserver:(id)observer;
- (void)removeObserver:(id)observer name:(NSString *)aName object:(id)anObject;

#if NS_BLOCKS_AVAILABLE
- (id)addObserverForName:(NSString *)name object:(id)obj queue:(NSOperationQueue *)queue usingBlock:(void (^)(NSNotification *note))block NS_AVAILABLE(10_6, 4_0);
    // The return value is retained by the system, and should be held onto by the caller in
    // order to remove the observer with removeObserver: later, to stop observation.
#endif

@end