gstreamer/docs/manual/advanced-position.xml

<chapter id="chapter-queryevents">
  <title>Position tracking and seeking</title>

  <para>
    So far, we've looked at how to create a pipeline to do media processing
    and how to make it run. Most application developers will be interested
    in providing feedback to the user on media progress. Media players, for
    example, will want to show a slider showing the progress in the song,
    and usually also a label indicating stream length. Transcoding
    applications will want to show a progress bar on how much percent of
    the task is done. &GStreamer; has built-in support for doing all this
    using a concept known as <emphasis>querying</emphasis>. Since seeking
    is very similar, it will be discussed here as well. Seeking is done
    using the concept of <emphasis>events</emphasis>.
  </para>

  <sect1 id="section-querying">
    <title>Querying: getting the position or length of a stream</title>

    <para>
      Querying is defined as requesting a specific stream-property related
      to progress tracking. This includes getting the length of a stream (if
      available) or getting the current position. Those stream properties
      can be retrieved in various formats such as time, audio samples, video
      frames or bytes. The function most commonly used for this is
      <function>gst_element_query ()</function>, although some convenience
      wrappers are provided as well (such as
      <function>gst_element_query_position ()</function>). You can generally
      query the pipeline directly, it'll figure out the internal details
      for you, like which element to query.
    </para>

    <para>
      Internally, queries will be sent to the sinks, and
      <quote>dispatched</quote> backwards until one element can handle it;
      that result will be sent back to the function caller. Usually, that
      is the demuxer, although with live sources (from a webcam), it is the
      source itself.
    </para>

    <programlisting><!-- example-begin query.c a -->
#include &lt;gst/gst.h&gt;
<!-- example-end query.c a -->
<!-- example-begin query.c b --><!--
static gboolean
my_bus_callback (GstBus     *bus,
                 GstMessage *message,
                 gpointer    data)
{
  GMainLoop *loop = data;

  switch (GST_MESSAGE_TYPE (message)) {
    case GST_MESSAGE_ERROR: {
      GError *err;
      gchar *debug;

      gst_message_parse_error (message, &amp;err, &amp;debug);
      g_print ("Error: %s\n", err-&gt;message);
      g_error_free (err);
      g_free (debug);

      g_main_loop_quit (loop);
      break;
    }
    case GST_MESSAGE_EOS:
      /* end-of-stream */
      g_main_loop_quit (loop);
      break;
    default:
      break;
  }

  /* remove from queue */
  return TRUE;
}
--><!-- example-end query.c b -->
<!-- example-begin query.c c -->
static gboolean
cb_print_position (GstElement *pipeline)
{
  GstFormat fmt = GST_FORMAT_TIME;
  gint64 pos, len;

  if (gst_element_query_position (pipeline, &amp;fmt, &amp;pos, &amp;len)) {
    g_print ("Time: %" GST_TIME_FORMAT " / %" GST_TIME_FORMAT "\r",
	     GST_TIME_ARGS (pos), GST_TIME_ARGS (len));
  }

  /* call me again */
  return TRUE;
}

gint
main (gint   argc,
      gchar *argv[])
{
  GstElement *pipeline;
<!-- example-end query.c c -->
[..]<!-- example-begin query.c d --><!--
  GMainLoop *loop;
  gchar *l;

  /* init */
  gst_init (&amp;argc, &amp;argv);

  /* args */
  if (argc != 2) {
    g_print ("Usage: %s &lt;filename&gt;\n", argv[0]);
    return -1;
  }

  /* build pipeline, the easy way */
  l = g_strdup_printf ("filesrc location=\"%s\" ! oggdemux ! vorbisdec ! "
		       "audioconvert ! audioscale ! alsasink",
		       argv[1]);
  pipeline = gst_parse_launch (l, NULL);
  g_free (l);
  gst_bus_add_watch (gst_pipeline_get_bus (GST_PIPELINE (pipeline)),
		     my_bus_callback, NULL);

  /* play */
  gst_element_set_state (pipeline, GST_STATE_PLAYING);
  loop = g_main_loop_new (NULL, FALSE);
--><!-- example-end query.c d -->
<!-- example-begin query.c e -->
  /* run pipeline */
  g_timeout_add (200, (GSourceFunc) cb_print_position, pipeline);
  g_main_loop_run (loop);
<!-- example-end query.c e -->
[..]<!-- example-begin query.c f --><!--
  /* clean up */
  gst_element_set_state (pipeline, GST_STATE_NULL);
  gst_object_unref (GST_OBJECT (pipeline));

  return 0;
--><!-- example-end query.c f -->
<!-- example-begin query.c g -->
}
    <!-- example-end query.c g --></programlisting>
  </sect1>

  <sect1 id="section-eventsseek">
    <title>Events: seeking (and more)</title>

    <para>
      Events work in a very similar way as queries. Dispatching, for
      example, works exactly the same for events (and also has the same
      limitations), and they can similarly be sent to the toplevel pipeline
      and it will figure out everything for you. Although there are more
      ways in which applications and elements can interact using events,
      we will only focus on seeking here. This is done using the seek-event.
      A seek-event contains a seeking offset, a seek method (which indicates
      relative to what the offset was given), a seek format (which is the
      unit of the offset, e.g. time, audio samples, video frames or bytes)
      and optionally a set of seeking-related flags (e.g. whether internal
      buffers should be flushed). The behaviour of a seek is also wrapped
      in the function <function>gst_element_seek ()</function>.
    </para>

    <programlisting>
static void
seek_to_time (GstElement *pipeline,
	      gint64      time_nanoseconds)
{
  gst_element_seek (pipeline,
		    GST_SEEK_METHOD_SET | GST_FORMAT_TIME |
		    GST_SEEK_FLAG_FLUSH, time_nanoseconds);
}
    </programlisting>
    <para>
      It is possible to do multiple seeks in short time-intervals, such as
      a direct response to slider movement. After a seek, internally, the
      pipeline will be paused (if it was playing), the position will be
      re-set internally, the demuxers and decoders will decode from the new
      position onwards and this will continue until all sinks have data
      again. If it was playing originally, it will be set to playing again,
      too. Since the new position is immediately available in a video output,
      you will see the new frame, even if your pipeline is not in the playing
      state.
    </para>
  </sect1>
</chapter>