GStreamer无法获得WAV文件持续时间

Question

我正在Vala中编写一个使用GStreamer加载和播放WAV文件的类。

class Sample : Object
{
    private Gst.Pipeline gst_pipeline;
    private Gst.Bus      gst_pipeline_bus;

    private Gst.Element? reader;
    private Gst.Element? decoder;
    private Gst.Element? convertor;
    private Gst.Element? output;

    /* Read-only values */

    private bool    __playing = false;
    private string? __source  = null;
    private int64   __length  = -1;

    public bool playing {
        get { return __playing; }
    }

    public string? source {
        get { return __source; }
    }

    public int64 position {
        get {
            if (this.__source == null)
            {
                return -1;
            }
            else
            {
                int64 pos;
                this.gst_pipeline.query_position(Gst.Format.TIME, out pos);
                return pos;
            }
        }
    }

    public int64 length {
        get { return __length; }
    }

    /* Methods */

    public Sample()
    {
        this.gst_pipeline = new Gst.Pipeline(null);

        /* Create the elements */
        this.reader    = Gst.ElementFactory.make("filesrc", null);
        this.gst_pipeline.add(this.reader);

        this.decoder   = Gst.ElementFactory.make("wavparse", null);
        this.gst_pipeline.add(this.decoder);
        this.reader.link(this.decoder);

        this.convertor = Gst.ElementFactory.make("audioconvert", null);
        this.gst_pipeline.add(this.convertor);
        this.decoder.link(this.convertor);

        this.output    = Gst.ElementFactory.make("alsasink", null);
        this.gst_pipeline.add(this.output);
        this.convertor.link(this.output);

        /* Get the bus to send messages down */
        this.gst_pipeline_bus = this.gst_pipeline.get_bus();
        this.gst_pipeline_bus.add_signal_watch();
        this.gst_pipeline_bus.message.connect(this.on_gst_message);
    }

    ~Sample()
    {
        this.gst_pipeline.set_state(Gst.State.NULL);
    }

    public signal void finished();  // Called when the sample finished playing
    public signal void error(Error err);    // Called when GStreamer passes us an error message

    public void load(string path)
    {
        this.__source = path;

        /* Stop the playback */
        this.gst_pipeline.set_state(Gst.State.NULL);    // This stops the playback and resets the pipeline
        this.finished();

        /* Change the location */
        this.reader.set_property("location", path);

        /* Get the sample length */
        this.gst_pipeline.set_state(Gst.State.PAUSED);
        print("\t" + this.gst_pipeline.query_duration(Gst.Format.TIME, out this.__length).to_string() + "\n");
        this.gst_pipeline.set_state(Gst.State.NULL);
    }

    public void play()
    {
        this.__playing = true;
        this.gst_pipeline.set_state(Gst.State.PLAYING);
    }

    public void pause()
    {
        this.gst_pipeline.set_state(Gst.State.PAUSED);
        this.__playing = false;
    }

    private void on_gst_message(Gst.Message message)
    {
        /* Here we handle any messages that GStreamer may send us */

        switch (message.type)
        {
            case (Gst.MessageType.EOS): // End Of Stream
            {
                this.gst_pipeline.set_state(Gst.State.NULL);
                this.__playing = false;
                this.finished();

                break;
            }
            case (Gst.MessageType.ERROR):
            {
                /* Stop playing */
                this.gst_pipeline.set_state(Gst.State.NULL);
                this.__playing = false;

                /* Get the error */
                Error err;
                message.parse_error(out err, null);

                /* Display the error */
                this.error(err);

                break;
            }
        }
    }
}

我使用filesrc，wavparse，audioconvert和alsasink打击垫来播放文件。

在load方法中，我尝试通过调用我创建的管道上的Gst.Element.query_duration()来获取示例的持续时间， 但每当我尝试这样做时，它会返回false并将我引用的值设置为0。

我知道管道必须预先滚动以便能够获得其持续时间，尽管我已经尝试通过将其状态设置为Gst.State.PAUSED来预先推送我的管道，{{1不断失败。

我应该更改哪些内容才能成功获取已加载的WAV文件的持续时间？