c# - AudioPlayerAgent、计时器和网络服务

Question

我的应用程序读取了一个 shoutcast。

播放的音乐的元数据是从返回我 Json 的网络服务收集的(因此我不必解码流)。我使用计时器每 20 秒调用一次 Web 服务，这在我的应用程序中有效，但在 AudioPlayer.cs 中无效

    //Timer
    private DispatcherTimer timer;


    /// <remarks>
    /// AudioPlayer instances can share the same process. 
    /// Static fields can be used to share state between AudioPlayer instances
    /// or to communicate with the Audio Streaming agent.
    /// </remarks>
    public AudioPlayer()
    {
        if (!_classInitialized)
        {
            _classInitialized = true;
            // Subscribe to the managed exception handler
            Deployment.Current.Dispatcher.BeginInvoke(delegate
            {
                Application.Current.UnhandledException += AudioPlayer_UnhandledException;

            });
        }

        //I activate the timer
        timer = new DispatcherTimer
        {
            Interval = TimeSpan.FromSeconds(20) // <------- Error here UnauthorizedAccessException was unhandled. Invalid cross-thread access.
        };
        timer.Tick += timer_Tick;
        timer.Start();
    }


    private void timer_Tick(object sender, EventArgs e)
    {
        HttpWebRequest request = WebRequest.Create("http://127.0.0.81:8003/getmeta") as HttpWebRequest;
        request.BeginGetResponse(new AsyncCallback(AsyncBack), request);
    }

    private void AsyncBack(IAsyncResult ias)
    {
            HttpWebRequest req = (HttpWebRequest)ias.AsyncState;

            try
            {
                using (HttpWebResponse res = req.EndGetResponse(ias) as HttpWebResponse)
                {
                    StreamReader stream = new StreamReader(res.GetResponseStream());
                    String jsonToParse = stream.ReadToEnd();
                    JObject jObject = JObject.Parse(jsonToParse);

                    AudioTrack track = BackgroundAudioPlayer.Instance.Track;

                    track.BeginEdit();
                    track.Title = (string) jObject["title"];
                    track.Artist = (string) jObject["artist"];
                    track.Album = (string) jObject["album"];
                    track.EndEdit();


                    res.Close();
                }
            }
            catch (WebException e)
            {
                timer.Stop();
            }
    }

谢谢你的帮助

Answer 1

AudioPlayer 类非常独特。它只存在一小段时间。在使用 BackgroundAudioPlayer 的应用程序中，您对 AudioPlayer 类的实现只会保持事件状态以完成更改播放状态的任务。因此，当用户开始播放某些内容时，将创建 AudioPlayer 类的一个实例来完成开始播放的任务。一旦您在 OnUserAction 或 OnPlayStateChanged 中调用 NotifyComplete()，您的 AudioPlayer 实例就会消失。

与 AudioPlayer 相关联的后台线程仍将处于事件状态，您可以在该线程中让其他对象处于事件状态，但 AudioPlayer 将被终止。创建的默认 AudioPlayer 使用 _classInitialized 字段对此进行提示。这是静态的，因为 AudioPlayer 将被创建多次，但我们只想订阅该事件一次。

我会建议两件事之一。第一种是仅在需要前进到下一首歌曲时才获取 json 响应。在响应返回之前，您不会调用 NotifyComplete()。这是一些伪代码:

override OnUserAction(BackgroundAudioPlayer player, AudioTrack track, UserAction action, object param)
{
    GetJsonResponse();
}
private void GetJsonResponce()
{
    // async call to your service
    // When async completes:
    track.BeginEdit();
    track.Title = (string) jObject["title"];
    track.Artist = (string) jObject["artist"];
    track.Album = (string) jObject["album"];
    track.EndEdit();
    NotifyComplete();
}

第二个是有一个在后台执行此操作的类。该类将有一个静态属性来获取线程中存在的实例。然后您的 AudioPlayer 将从该对象中获取所需的信息

public class Songs
{
    static Songs _instance;
    public static Songs Instance
    { 
        get { return _instance ?? new Songs(); }
    }
    // Do you timer stuff here

    // Allow the ability to access the timer stuff also.
}
// in AudioPlayer
override OnUserAction(BackgroundAudioPlayer player, AudioTrack track, UserAction action, object param)
{
    Songs.Instance.GetStuff
    NotifyComplete();
}