问Xamarin.iOS音频流无法工作，事件未被触发

EN

class AudioStreamer : IAudioStreamer // this is my dependency service interface
{
    bool outputStarted;

    AudioFileStream afs;
    OutputAudioQueue oaq;

    public void StartStreaming(string url)
    {
        afs = new AudioFileStream(AudioFileType.MP3);

        // event handlers, these are never triggered
        afs.PacketDecoded += OnPacketDecoded;
        afs.PropertyFound += OnPropertyFound;

        GetAudio(url);
    }

    void GetAudio(string url)
    {
        // HTTP
        NSUrlSession session = NSUrlSession.FromConfiguration(
            NSUrlSessionConfiguration.DefaultSessionConfiguration, 
            new SessionDelegate(afs), 
            NSOperationQueue.MainQueue);

        var dataTask = session.CreateDataTask(new NSUrl(url));
        dataTask.Resume();
    }

    // event handler - never executed
    void OnPropertyFound(object sender, PropertyFoundEventArgs e)
    {
        if(e.Property == AudioFileStreamProperty.ReadyToProducePackets)
        {
            oaq = new OutputAudioQueue(afs.StreamBasicDescription);
            oaq.BufferCompleted += OnBufferCompleted;
        }
    }

    // another event handler never executed
    void OnPacketDecoded(object sender, PacketReceivedEventArgs e)
    {
        IntPtr outBuffer;
        oaq.AllocateBuffer(e.Bytes, out outBuffer);
        AudioQueue.FillAudioData(outBuffer, 0, e.InputData, 0, e.Bytes);
        oaq.EnqueueBuffer(outBuffer, e.Bytes, e.PacketDescriptions);

        // start playing if not already
        if(!outputStarted)
        {
            var status = oaq.Start();
            if (status != AudioQueueStatus.Ok)
                System.Diagnostics.Debug.WriteLine("Could not start audio queue");
            outputStarted = true;
        }
    }

    void OnBufferCompleted(object sender, BufferCompletedEventArgs e)
    {
        oaq.FreeBuffer(e.IntPtrBuffer);
    }
}

// instantiated in GetAudio()
class SessionDelegate : NSUrlSessionDataDelegate
{
    readonly AudioFileStream afs;

    public SessionDelegate(AudioFileStream afs)
    {
        this.afs = afs;
    }

    // this is, too, never executed
    public override void DidReceiveData(NSUrlSession session, NSUrlSessionDataTask dataTask, NSData data)
    {
        afs.ParseBytes((int)data.Length, data.Bytes, false);
    }
}