获取手机左边音量＋ －按键的事件方法或私有api

MPMusicPlayerController *ipodMusicPlayer = [MPMusicPlayerController iPodMusicPlayer];
????[ipodMusicPlayer beginGeneratingPlaybackNotifications];
????[[NSNotificationCenter defaultCenter] addObserver:self?
???????????????????????????????????????????? selector:@selector(trackTheDeviceVolume:)?
???????????????????????????????????????????????? name:MPMusicPlayerControllerVolumeDidChangeNotification?
?????????????????????????????????????????????? object:nil];