Fix: Allow rewind of Mp3MediaStreamSource back to start

My comment at:

https://github.com/loarabia/ManagedMediaHelpers/issues/16

While trying to use Mp3MediaSource at ClipFlair Studio’s AudioRecorder control (http://clipflair.codeplex.com), I noticed that when I was doing Stop() at MediaElement and then Play() it kept on playing from where it was before at Mp3MediaStreamSource

So I did the following fix:

 

1) added these fields

    /// <summary>
    /// The first known frame (for rewind)
    /// </summary>     
    private MpegFrame firstFrame;

    /// <summary>
    /// The first audio stream position (for rewind)
    /// </summary>
    private long firstAudioStreamPosition;

 

2) added to the end of ReadPastId3v2TagsCallback:

this.firstFrame = mpegLayer3Frame; //keeping the 1st frame position for rewinding
this.firstAudioStreamPosition = audioStream.Position;

 

3) changed SeekAsync:

    /// <summary>
    /// <para>
    /// Only supporting seeking back to start of the stream 
    ///                              (e.g. when doing MediaElement.Stop()).
    /// </para>
    /// <para>
    /// In a fuller MediaStreamSource, the logic here would be to actually seek to
    /// the correct mpeg frame matching the seekToTime passed in.
    /// </para>
    /// </summary>
    /// <param name="seekToTime">
    /// The time to seek to (in 100-nanosecond units [hns])
    /// </param>
    protected override void SeekAsync(long seekToTime)
    {
        /*
        if (seekToTime > this.trackDuration.TotalMilliseconds * 10)
        {
          throw new InvalidOperationException(
                      "The seek position is beyond the length of the stream");
        }
        */
        if (seekToTime != 0) //only supporting rewinding back to start
            throw new InvalidOperationException(
                       "Only supporting seeking back to start of the stream");
        else
        {
            this.currentFrame = firstFrame;
            this.currentFrameStartPosition = MpegFrame.FrameHeaderSize;
            this.audioStream.Position = firstAudioStreamPosition;
        }

        this.ReportSeekCompleted(seekToTime);
    }

 

note that I changed the documentation for that method to say that time passed to it is in 100 ns units, not in ns units (according to WaveMediaStreamSource code that I found, unless that one has it wrong)