2022-03-07(通知：添加多次，执行多次)

对同一通知重复添加监听，监听方法会重复执行

- (void)addObserver{
  [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(notificationTestAction) name:@"kNotificationTest" object:nil];

  [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(notificationTestAction) name:@"kNotificationTest" object:nil];
}

- (void)notificationTestAction{
    NSLog(@"--> notificationTestAction");
}

/*
打印结果：    
    NSLog(@"--> notificationTestAction");
    NSLog(@"--> notificationTestAction");
*/

无论添加多少次，只要移除一次即可：

- (void)addObserver{
  [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(notificationTestAction) name:@"kNotificationTest" object:nil];

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

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

  [[NSNotificationCenter defaultCenter] removeObserver:self name:@"kNotificationTest" object:nil];

  [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(notificationTestAction) name:@"kNotificationTest" object:nil];
}

- (void)notificationTestAction{
    NSLog(@"--> notificationTestAction");
}

/*
打印结果：    
    NSLog(@"--> notificationTestAction");
*/