class RYNotificationCenter {
    private init(){}
    static let `default` = RYNotificationCenter()
    private var observers: [RYNotificationObserver] = []
}

class RYNotificationObserver {
    var name: String
    var block: (Notification) -> Void
    init(name: String, block: @escaping (Notification) -> Void) {
        self.name = name
        self.block = block
    }
}

func addObserver(name: String, block: @escaping (Notification) -> Void) -> RYNotificationObserver {
    let observer = RYNotificationObserver(name: name, block: block)
    observers.append(observer)
    return observer
}

/// 发送通知的本质是利用了观察者模式
/// 让观察者数组执行闭包中的代码
func post(name: String, userInfo: [AnyHashable: Any]? = nil) {
    let notification = Notification(name: Notification.Name(name), userInfo: userInfo)
    observers
        .filter({ $0.name == name })
        .forEach { $0.block(notification) }
}

func removeObserver(_ observer: RYNotificationObserver) {
    if let index = observers.firstIndex(where: { $0 === observer }) {
        observers.remove(at: index)
    }
}

typedef struct Obs {
  id observer;     
  SEL selector;     
  struct Obs *next; 
  int retained;     
  struct NCTbl *link; 
} Observation;

- (void) postNotificationName: (NSString*)name
      object: (id)object
    userInfo: (NSDictionary*)info
{
  // 先封装好notification
  GSNotification *notification;
  notification = (id)NSAllocateObject(concrete, 0, NSDefaultMallocZone());
  notification->_name = [name copyWithZone: [self zone]];
  notification->_object = [object retain];
  notification->_info = [info retain];
  [self _postAndRelease: notification];
}
// 然后调用观察者的selector方法
- (void) _postAndRealse: (NSNotification*)notification {
......
[o->observer perfORMSelector: o->selector withObject: notification];
......
}

- (id) initWithCoder: (NSCoder*)aCoder
{
  NSString *name;
  id object;
  NSDictionary *info;
  id n;
  [aCoder decodeValueOfObjCType: @encode(id) at: &name];
  [aCoder decodeValueOfObjCType: @encode(id) at: &object];
  [aCoder decodeValueOfObjCType: @encode(id) at: &info];
  n = [NSNotification notificationWithName: name object: object userInfo: info];
  RELEASE(name);
  RELEASE(object);
  RELEASE(info);
  DESTROY(self);
  return RETAIN(n);
}

- (void) enqueueNotification: (NSNotification*)notification postingStyle:(NSPostingStyle)postingStyle coalesceMask: (NSUInteger)coalesceMask forModes: (NSArray*)modes
{
    if (modes == nil)
    {
      modes = defaultMode;
    }
    if (coalesceMask != NSNotificationNoCoalescing)
    {
      [self dequeueNotificationsMatching: notification coalesceMask: coalesceMask];
    }
    switch (postingStyle) {
        case NSPostNow: {
            NSString *mode;
            mode = [[NSRunLoop currentRunLoop] currentMode];
            if (mode == nil || [modes indexOfObject: mode] != NSNotFound)
            {
                [_center postNotification: notification];
            }
        }
        break;
      case NSPostASAP:
            add_to_queue(_asapQueue, notification, modes, _zone);
            break;
      case NSPostWhenIdle:
            add_to_queue(_idleQueue, notification, modes, _zone);
            break;
    }
}

void GSPrivateNotifyIdle(NSString *mode) {
    NotificationQueueList *item;
    for (item = currentList(); item; item = item->next)
    {
        if (item->queue) {
            notify(item->queue->_center,
                   item->queue->_idleQueue,
                   mode,
                   item->queue->_zone);
        }
    }
}

class ViewController: UIViewController {
    private weak var observer: NSObjectProtocol!
    func addObserver() {
        observer = NotificationCenter.default.addObserver(forName: NSNotification.Name("test"), object: nil, queue: OperationQueue.main) { _ in
            self.view.backgroundColor = UIColor.white
        }
    }
    deinit {
        NotificationCenter.default.removeObserver(observer!)
    }
}

func addObserver() {
    NotificationCenter.default.addObserver(self, selector: #selector(click), name: NSNotification.Name.init("test"), object: nil)
    DispatchQueue.global().async {
        NotificationCenter.default.post(name: NSNotification.Name.init("test"), object: nil)
        NSLog("curretThread1: \(Thread.current)")
    }
}
@objc func click() {
    NSLog("curretThread2: \(Thread.current)")
}
// curretThread2: <NSThread: 0x600001358240>{number = 6, name = (null)}
// curretThread1: <NSThread: 0x600001358240>{number = 6, name = (null)}