通知的使用

 //建立通知
    [[NSNotificationCenter defaultCenter] postNotificationName:kMusicPlaybackStateChangeNotification object:musicIdentifier];

//監聽通知
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(musicPlabackStateChange:) name:kMusicPlaybackStateChangeNotification object:nil];

//銷燬通知
- (void)dealloc {
    [[NSNotificationCenter defaultCenter] removeObserver:self];
}