关于重复注册通知的问题

iOS中通过通知中心可以实现发送与接收通知的效果，其中若多次发送同一通知，对方就会多次响应，但如果多次注册通知，也会有多次响应的效果，这里容易比较隐形的bug。

多次发送：

//发送通知方

    //self 发送通知
    [[NSNotificationCenter defaultCenter] postNotificationName:@"justTest" object:nil];
    [[NSNotificationCenter defaultCenter] postNotificationName:@"justTest" object:nil];

    [[NSNotificationCenter defaultCenter] postNotificationName:@"justTest" object:nil];

//接收通知方

- (void) resignNotification{
    [[NSNotificationCenter defaultCenter] addObserverForName:@"justTest" object:nil queue:nil
                                                  usingBlock:^(NSNotification *note) {
                                                      NSLog(@"luozhiwei");

                                                  }];

}

结果：

2015-07-25 16:15:46.491 multipleRegistrationNotification[5233:171998] luozhiwei
2015-07-25 16:15:46.491 multipleRegistrationNotification[5233:171998] luozhiwei

2015-07-25 16:15:46.491 multipleRegistrationNotification[5233:171998] luozhiwei

多次接收：

//发送通知方

[[NSNotificationCenter defaultCenter] postNotificationName:@"justTest" object:nil];

//接收通知方

- (void) resignNotification{
    [[NSNotificationCenter defaultCenter] addObserverForName:@"justTest" object:nil queue:nil
                                                  usingBlock:^(NSNotification *note) {
                                                      NSLog(@"luozhiwei");

                                                  }];

    [[NSNotificationCenter defaultCenter] addObserverForName:@"justTest" object:nil queue:nil
                                                  usingBlock:^(NSNotification *note) {
                                                      NSLog(@"luozhiwei");

                                                  }];

    [[NSNotificationCenter defaultCenter] addObserverForName:@"justTest" object:nil queue:nil
                                                  usingBlock:^(NSNotification *note) {
                                                      NSLog(@"luozhiwei");

                                                  }];

}

结果：

2015-07-25 16:15:46.491 multipleRegistrationNotification[5233:171998] luozhiwei
2015-07-25 16:15:46.491 multipleRegistrationNotification[5233:171998] luozhiwei

2015-07-25 16:15:46.491 multipleRegistrationNotification[5233:171998] luozhiwei

结论：为了避免因此重复注册而造成错误，建议每次注册一个通知之前，可以先注销一次该通知。