@interface NSNotificationCenter : NSObject {@packagevoid * __strong _impl;void * __strong _callback;void *_pad[11];
}//单例获得消息中心对象
+ (NSNotificationCenter *)defaultCenter;//增加消息监听 第一个参数是观察者为本身，第二个参数表示消息回调的方法，第三个消息通知的名字，第四个为nil表示表示接受所有发送者的消息
- (void)addObserver:(id)observer selector:(SEL)aSelector name:(nullable NSString *)aName object:(nullable id)anObject;//发送通知 三种方式
- (void)postNotification:(NSNotification *)notification;
- (void)postNotificationName:(NSString *)aName object:(nullable id)anObject;
- (void)postNotificationName:(NSString *)aName object:(nullable id)anObject userInfo:(nullable NSDictionary *)aUserInfo;//移除监听
- (void)removeObserver:(id)observer;
//移除监听特定消息
- (void)removeObserver:(id)observer name:(nullable NSString *)aName object:(nullable id)anObject;//增加监听 又提供了一个以block方式实现的添加观察者的方法
- (id <NSObject>)addObserverForName:(nullable NSString *)name object:(nullable id)obj queue:(nullable NSOperationQueue *)queue usingBlock:(void (^)(NSNotification *note))block NS_AVAILABLE(10_6, 4_0);@end

        NSNotification *notification =[NSNotification notificationWithName:@"qjwallet" object:nil userInfo:nil];//通过通知中心发送通知[[NSNotificationCenter defaultCenter] postNotification:notification];

  [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(qjwalletNotification:) name:@"qjwallet" object:nil];

- (void)qjwalletNotification:(NSNotification *)text{//NSLog(@"%@",text.userInfo[@"orderid"]);NSLog(@"－－－－－接收到通知------");
}

-(void)dealloc
{[[NSNotificationCenter defaultCenter]removeObserver:self name:@"qjwallet" object:nil];
}

方法1：[[NSNotificationCenter defaultCenter] postNotificationName:@"First" object:@"博客园"]; 方法2：
NSDictionary *dict=[[NSDictionary alloc]initWithObjects:@[@"keso"] forKeys:@[@"key"]];
[[NSNotificationCenter defaultCenter] postNotificationName:@"Second" object:@"http://www.cnblogs.com" userInfo:dict];

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(notificationFirst:) name:@"First" object:nil];[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(notificationSecond:) name:@"Second" object:nil];

-(void)notificationFirst:(NSNotification *)notification{NSString  *name=[notification name];NSString  *object=[notification object];NSLog(@"名称:%@----对象:%@",name,object);
}-(void)notificationSecond:(NSNotification *)notification{NSString  *name=[notification name];NSString  *object=[notification object];NSDictionary  *dict=[notification userInfo];NSLog(@"名称:%@----对象:%@",name,object);NSLog(@"获取的值:%@",[dict objectForKey:@"key"]);
}

-(void)dealloc{NSLog(@"观察者销毁了");[[NSNotificationCenter defaultCenter] removeObserver:self];
}也可以针对某一个进行移除：[[NSNotificationCenter defaultCenter] removeObserver:self name:@"First" object:nil];

@implementation ViewController- (void)viewDidLoad {[super viewDidLoad];[[NSNotificationCenter defaultCenter] addObserverForName:TEST_NOTIFICATION object:nil queue:[NSOperationQueue mainQueue] usingBlock:^(NSNotification *note) {NSLog(@"receive thread = %@", [NSThread currentThread]);}];dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{NSLog(@"post thread = %@", [NSThread currentThread]);[[NSNotificationCenter defaultCenter] postNotificationName:TEST_NOTIFICATION object:nil];});
}@end

post thread = <NSThread: 0x7ffe0351f5f0>{number = 2, name = (null)}
receive thread = <NSThread: 0x7ffe03508b30>{number = 1, name = main}

@interface NSNotification : NSObject <NSCopying, NSCoding>
//这个成员变量是这个消息对象的唯一标识，用于辨别消息对象
@property (readonly, copy) NSString *name;
// 这个成员变量定义一个对象，可以理解为针对某一个对象的消息，代表通知的发送者
@property (nullable, readonly, retain) id object;
//这个成员变量是一个字典，可以用其来进行传值
@property (nullable, readonly, copy) NSDictionary *userInfo;// 初始化方法
- (instancetype)initWithName:(NSString *)name object:(nullable id)object userInfo:(nullable NSDictionary *)userInfo NS_AVAILABLE(10_6, 4_0) NS_DESIGNATED_INITIALIZER;
- (nullable instancetype)initWithCoder:(NSCoder *)aDecoder NS_DESIGNATED_INITIALIZER;@end

@interface NSNotification (NSNotificationCreation)+ (instancetype)notificationWithName:(NSString *)aName object:(nullable id)anObject;
+ (instancetype)notificationWithName:(NSString *)aName object:(nullable id)anObject userInfo:(nullable NSDictionary *)aUserInfo;- (instancetype)init /*NS_UNAVAILABLE*/;    /* do not invoke; not a valid initializer for this class */@end