gstreamer/docs/manual/highlevel-components.xml

<chapter id="chapter-components">
  <title>Components</title>

  <para>
    &GStreamer; includes several higher-level components to simplify an
    application developer's life. All of the components discussed here (for now) are
    targetted at media playback. The idea of each of these components is
    to integrate as closely as possible with a &GStreamer; pipeline, but
    to hide the complexity of media type detection and several other
    rather complex topics that have been discussed in <xref
    linkend="part-advanced"/>.
  </para>

  <para>
    We currently recommend people to use either playbin (see <xref
    linkend="section-components-playbin"/>) or decodebin (see <xref
    linkend="section-components-decodebin"/>), depending on their needs.
    Playbin is the recommended solution for everything related to simple
    playback of media that should just work. Decodebin is a more flexible
    autoplugger that could be used to add more advanced features, such
    as playlist support, crossfading of audio tracks and so on. Its
    programming interface is more low-level than that of playbin, though.
  </para>

  <sect1 id="section-components-playbin">
    <title>Playbin</title>

    <para>
      Playbin is an element that can be created using the standard &GStreamer;
      API (e.g. <function>gst_element_factory_make ()</function>). The factory
      is conveniently called <quote>playbin</quote>. By being a
      <classname>GstPipeline</classname> (and thus a
      <classname>GstElement</classname>), playbin automatically supports all
      of the features of this class, including error handling, tag support,
      state handling, getting stream positions, seeking, and so on.
    </para>

    <para>
      Setting up a playbin pipeline is as simple as creating an instance of
      the playbin element, setting a file location using the
      <quote>uri</quote> property on playbin, and then setting the element
      to the <classname>GST_STATE_PLAYING</classname> state (the location has to be a valid
      URI, so <quote>&lt;protocol&gt;://&lt;location&gt;</quote>, e.g.
      file:///tmp/my.ogg or http://www.example.org/stream.ogg). Internally,
      playbin will set up a pipeline to playback the media location.
    </para>

    <programlisting><!-- example-begin playbin.c a -->
#include &lt;gst/gst.h&gt;
<!-- example-end playbin.c a -->
[.. my_bus_callback goes here ..]<!-- example-begin playbin.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:
      /* unhandled message */
      break;
  }

  /* remove message from the queue */
  return TRUE;
}
--><!-- example-end playbin.c b -->
<!-- example-begin playbin.c c -->
gint
main (gint   argc,
      gchar *argv[])
{
  GMainLoop *loop;
  GstElement *play;
  GstBus *bus;

  /* init GStreamer */
  gst_init (&amp;argc, &amp;argv);
  loop = g_main_loop_new (NULL, FALSE);

  /* make sure we have a URI */
  if (argc != 2) {
    g_print ("Usage: %s &lt;URI&gt;\n", argv[0]);
    return -1;
  }

  /* set up */
  play = gst_element_factory_make ("playbin", "play");
  g_object_set (G_OBJECT (play), "uri", argv[1], NULL);

  bus = gst_pipeline_get_bus (GST_PIPELINE (play));
  gst_bus_add_watch (bus, my_bus_callback, loop);
  gst_object_unref (bus);

  gst_element_set_state (play, GST_STATE_PLAYING);

  /* now run */
  g_main_loop_run (loop);

  /* also clean up */
  gst_element_set_state (play, GST_STATE_NULL);
  gst_object_unref (GST_OBJECT (play));

  return 0;
}
    <!-- example-end playbin.c c --></programlisting>

    <para>
      Playbin has several features that have been discussed previously:
    </para>
    <itemizedlist>
      <listitem>
        <para>
          Settable video and audio output (using the <quote>video-sink</quote>
          and <quote>audio-sink</quote> properties).
        </para>
      </listitem>
      <listitem>
        <para>
          Mostly controllable and trackable as a
          <classname>GstElement</classname>, including error handling, eos
          handling, tag handling, state handling (through the
          <classname>GstBus</classname>), media position handling and
          seeking.
        </para>
      </listitem>
      <listitem>
        <para>
          Buffers network-sources, with buffer fullness notifications being
          passed through the <classname>GstBus</classname>.
        </para>
      </listitem>
      <listitem>
        <para>
          Supports visualizations for audio-only media.
        </para>
      </listitem>
      <listitem>
        <para>
          Supports subtitles, both in the media as well as from separate
          files. For separate subtitle files, use the <quote>suburi</quote>
          property.
        </para>
      </listitem>
      <listitem>
        <para>
          Supports stream selection and disabling. If your media has
          multiple audio or subtitle tracks, you can dynamically choose
          which one to play back, or decide to turn it off altogether
          (which is especially useful to turn off subtitles). For each
          of those, use the <quote>current-text</quote> and other related
          properties.
        </para>
      </listitem>
    </itemizedlist>
    <para>
      For convenience, it is possible to test <quote>playbin</quote> on
      the commandline, using the command <quote>gst-launch-1.0 playbin
      uri=file:///path/to/file</quote>.
    </para>
  </sect1>

  <sect1 id="section-components-decodebin">
    <title>Decodebin</title>

    <para>
      Decodebin is the actual autoplugger backend of playbin, which was
      discussed in the previous section. Decodebin will, in short, accept
      input from a source that is linked to its sinkpad and will try to
      detect the media type contained in the stream, and set up decoder
      routines for each of those. It will automatically select decoders.
      For each decoded stream, it will emit the <quote>pad-added</quote>
      signal, to let the client know about the newly found decoded stream.
      For unknown streams (which might be the whole stream), it will emit
      the <quote>unknown-type</quote> signal. The application is then
      responsible for reporting the error to the user.
    </para>
    <programlisting><!-- example-begin decodebin.c a -->
#include &lt;gst/gst.h&gt;
<!-- example-end decodebin.c a -->
[.. my_bus_callback goes here ..]<!-- example-begin decodebin.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:
      /* unhandled message */
      break;
  }

  /* remove message from the queue */
  return TRUE;
}
--><!-- example-end decodebin.c b -->
<!-- example-begin decodebin.c c -->
GstElement *pipeline, *audio;

static void
cb_newpad (GstElement *decodebin,
	   GstPad     *pad,
	   gpointer    data)
{
  GstCaps *caps;
  GstStructure *str;
  GstPad *audiopad;

  /* only link once */
  audiopad = gst_element_get_static_pad (audio, "sink");
  if (GST_PAD_IS_LINKED (audiopad)) {
    g_object_unref (audiopad);
    return;
  }

  /* check media type */
  caps = gst_pad_query_caps (pad, NULL);
  str = gst_caps_get_structure (caps, 0);
  if (!g_strrstr (gst_structure_get_name (str), "audio")) {
    gst_caps_unref (caps);
    gst_object_unref (audiopad);
    return;
  }
  gst_caps_unref (caps);

  /* link'n'play */
  gst_pad_link (pad, audiopad);

  g_object_unref (audiopad);
}

gint
main (gint   argc,
      gchar *argv[])
{
  GMainLoop *loop;
  GstElement *src, *dec, *conv, *sink;
  GstPad *audiopad;
  GstBus *bus;

  /* init GStreamer */
  gst_init (&amp;argc, &amp;argv);
  loop = g_main_loop_new (NULL, FALSE);

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

  /* setup */
  pipeline = gst_pipeline_new ("pipeline");

  bus = gst_pipeline_get_bus (GST_PIPELINE (pipeline));
  gst_bus_add_watch (bus, my_bus_callback, loop);
  gst_object_unref (bus);

  src = gst_element_factory_make ("filesrc", "source");
  g_object_set (G_OBJECT (src), "location", argv[1], NULL);
  dec = gst_element_factory_make ("decodebin", "decoder");
  g_signal_connect (dec, "pad-added", G_CALLBACK (cb_newpad), NULL);
  gst_bin_add_many (GST_BIN (pipeline), src, dec, NULL);
  gst_element_link (src, dec);

  /* create audio output */
  audio = gst_bin_new ("audiobin");
  conv = gst_element_factory_make ("audioconvert", "aconv");
  audiopad = gst_element_get_static_pad (conv, "sink");
  sink = gst_element_factory_make ("alsasink", "sink");
  gst_bin_add_many (GST_BIN (audio), conv, sink, NULL);
  gst_element_link (conv, sink);
  gst_element_add_pad (audio,
      gst_ghost_pad_new ("sink", audiopad));
  gst_object_unref (audiopad);
  gst_bin_add (GST_BIN (pipeline), audio);

  /* run */
  gst_element_set_state (pipeline, GST_STATE_PLAYING);
  g_main_loop_run (loop);

  /* cleanup */
  gst_element_set_state (pipeline, GST_STATE_NULL);
  gst_object_unref (GST_OBJECT (pipeline));

  return 0;
}
    <!-- example-end decodebin.c c --></programlisting>

    <para>
      Decodebin, similar to playbin, supports the following features:
    </para>
    <itemizedlist>
      <listitem>
        <para>
          Can decode an unlimited number of contained streams to decoded
          output pads.
        </para>
      </listitem>
      <listitem>
        <para>
          Is handled as a <classname>GstElement</classname> in all ways,
          including tag or error forwarding and state handling.
        </para>
      </listitem>
    </itemizedlist>
    <para>
      Although decodebin is a good autoplugger, there's a whole lot of
      things that it does not do and is not intended to do:
    </para>
    <itemizedlist>
      <listitem>
        <para>
          Taking care of input streams with a known media type (e.g. a DVD,
          an audio-CD or such).
        </para>
      </listitem>
      <listitem>
        <para>
          Selection of streams (e.g. which audio track to play in case of
          multi-language media streams).
        </para>
      </listitem>
      <listitem>
        <para>
          Overlaying subtitles over a decoded video stream.
        </para>
      </listitem>
    </itemizedlist>
    <para>
      Decodebin can be easily tested on the commandline, e.g. by using the
      command <command>gst-launch-1.0 filesrc location=file.ogg ! decodebin
      ! audioconvert ! audioresample ! autoaudiosink</command>.
    </para>
    <para>
      The uridecodebin element is very similar to decodebin, only that it
      automatically plugs a source plugin based on the protocol of the URI
      given.
    </para>
  </sect1>

</chapter>