Windows Phone 8.1 多媒体（3）：音乐
（1）前台
说是前台，其实音乐的播放都是利用系统后台的 BackgroundMediaPlayer：
BackgroundMediaPlayer.Current.SetUriSource(new Uri("ms-appx:///djlang59_-_Drops_of_H2O_(_The_Filtered_Water_Treatment_).mp3")); BackgroundMediaPlayer.Current.Play();
可以添加音乐播放状态改变事件：
BackgroundMediaPlayer.Current.CurrentStateChanged += MediaPlayerStateChanged; private async void MediaPlayerStateChanged(MediaPlayer sender, object args { await Dispatcher.RunAsync(CoreDispatcherPriority.Normal, () => { switch( BackgroundMediaPlayer.Current.CurrentState ) { case MediaPlayerState.Stopped: AppBarBtnPause.IsEnabled = false; AppBarBtnStop.IsEnabled = false; break; case MediaPlayerState.Playing: AppBarBtnPause.IsEnabled = true; AppBarBtnStop.IsEnabled = true; BackgroundMediaPlayer.SendMessageToBackground(new ValueSet { {"Title", "Drops of H2O"}, {"Artist", "J.Lang"}, }); break; case MediaPlayerState.Paused: AppBarBtnPause.IsEnabled = false; break; } }); }
其中 BackgroundMediaPlayer.SendMessageToBackground 方法可以向后台音乐播放程序发送消息，可在后台程序中添加处理事件：
BackgroundMediaPlayer.MessageReceivedFromForeground += BackgroundMediaPlayerOnMessageReceivedFromForeground; private void BackgroundMediaPlayerOnMessageReceivedFromForeground(object sender, MediaPlayerDataReceivedEventArgs e) { systemmediatransportcontrol.DisplayUpdater.Type = MediaPlaybackType.Music; systemmediatransportcontrol.DisplayUpdater.MusicProperties.Title = e.Data["Title"].ToString(); systemmediatransportcontrol.DisplayUpdater.MusicProperties.Artist = e.Data["Artist"].ToString(); systemmediatransportcontrol.DisplayUpdater.Update(); }
（2）后台
这里说的后台是指 SystemMediaTransportControls，也就是按下音量键后顶部弹出的控制音乐播放的控件。
1）新建 Windows Runtime Component 和继承 IBackgroundTask 的后台类
A：Run 方法中可设置 SystemMediaTransportControls 的各个按键属性以及事件的订阅（比如下一曲按钮可不可用，状态更改事件等）
public void Run(IBackgroundTaskInstance taskInstance) { systemmediatransportcontrol = SystemMediaTransportControls.GetForCurrentView(); systemmediatransportcontrol.ButtonPressed += SystemControlsButtonPressed; systemmediatransportcontrol.IsEnabled = true; systemmediatransportcontrol.IsPauseEnabled = true; systemmediatransportcontrol.IsStopEnabled = true; systemmediatransportcontrol.IsPlayEnabled = true; BackgroundMediaPlayer.Current.CurrentStateChanged -= BackgroundMediaPlayerCurrentStateChanged; BackgroundMediaPlayer.MessageReceivedFromForeground -= BackgroundMediaPlayerOnMessageReceivedFromForeground; BackgroundMediaPlayer.Current.CurrentStateChanged += BackgroundMediaPlayerCurrentStateChanged; BackgroundMediaPlayer.MessageReceivedFromForeground += BackgroundMediaPlayerOnMessageReceivedFromForeground; deferral = taskInstance.GetDeferral(); }
B：事件的处理
private void BackgroundMediaPlayerCurrentStateChanged(MediaPlayer sender, object args) { if (sender.CurrentState == MediaPlayerState.Playing) { systemmediatransportcontrol.PlaybackStatus = MediaPlaybackStatus.Playing; } else if (sender.CurrentState == MediaPlayerState.Paused) { systemmediatransportcontrol.PlaybackStatus = MediaPlaybackStatus.Paused; } } private static void SystemControlsButtonPressed(SystemMediaTransportControls sender, SystemMediaTransportControlsButtonPressedEventArgs args) { switch (args.Button) { case SystemMediaTransportControlsButton.Play: BackgroundMediaPlayer.Current.Play(); break; case SystemMediaTransportControlsButton.Pause: BackgroundMediaPlayer.Current.Pause(); break; case SystemMediaTransportControlsButton.Stop: BackgroundMediaPlayer.Current.Pause(); BackgroundMediaPlayer.Current.Position = TimeSpan.FromSeconds(0); break; } }
2）前台应用添加上一步新建的后台任务