docs/manual/basics-bins.xml

<chapter id="chapter-bins">
  <title>Bins</title>
  <para> 
    A bin is a container element. You can add elements to a bin. Since a
    bin is an element itself, a bin can be handled in the same way as any
    other element. Therefore, the whole previous chapter (<xref
    linkend="chapter-elements"/>) applies to bins as well.
  </para>

  <sect1 id="section-bins">
    <title>What are bins</title>
    <para>
      Bins allow you to combine a group of linked elements into one
      logical element. You do not deal with the individual elements
      anymore but with just one element, the bin. We will see that
      this is extremely powerful when you are going to construct
      complex pipelines since it allows you to break up the pipeline
      in smaller chunks.
    </para>
    <para> 
      The bin will also manage the elements contained in it. It will
      figure out how the data will flow in the bin and generate an
      optimal plan for that data flow. Plan generation is one of the
      most complicated procedures in &GStreamer;. You will learn more
      about this process, called scheduling, in <xref
      linkend="section-threads-scheduling"/>.
    </para>

    <figure float="1" id="section-bin-img">
      <title>Visualisation of a bin with some elements in it</title>
        <mediaobject>  
          <imageobject>
            <imagedata scale="75" fileref="images/bin-element.&image;" format="&IMAGE;"/>
          </imageobject>
        </mediaobject>
    </figure>

    <para> 
      There is one specialized type of bin available to the
      &GStreamer; programmer:
    </para>
    <itemizedlist>
      <listitem>
        <para>
          A pipeline: a generic container that allows scheduling of the
          containing elements.  The toplevel bin has to be a pipeline,
          every application thus needs at least one of these. Pipelines will
          automatically run themselves in a background thread when started.
        </para>
      </listitem>
    </itemizedlist>
  </sect1>

  <sect1 id="section-bin-create">
    <title>Creating a bin</title>
    <para>
      Bins are created in the same way that other elements are created,
      i.e. using an element factory. There are also convenience functions
      available (<function>gst_bin_new ()</function> and 
      <function>gst_pipeline_new ()</function>).
      To add elements to a bin or remove elements from a
      bin, you can use <function>gst_bin_add ()</function> and
      <function>gst_bin_remove ()</function>. Note that the bin that you
      add an element to will take ownership of that element. If you
      destroy the bin, the element will be dereferenced with it. If you
      remove an element from a bin, it will be dereferenced automatically.
    </para>
    <programlisting><!-- example-begin bin.c a -->
#include &lt;gst/gst.h&gt;

int
main (int   argc,
      char *argv[])
{
  GstElement *bin, *pipeline, *source, *sink;

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

  /* create */
  pipeline = gst_pipeline_new ("my_pipeline");
  bin = gst_bin_new ("my_bin");
  source = gst_element_factory_make ("fakesrc", "source");
  sink = gst_element_factory_make ("fakesink", "sink");

  /* First add the elements to the bin */
  gst_bin_add_many (GST_BIN (bin), source, sink, NULL);
  /* add the bin to the pipeline */
  gst_bin_add (GST_BIN (pipeline), bin);

  /* link the elements */
  gst_element_link (source, sink);
<!-- example-end bin.c a -->
[..]<!-- example-begin bin.c b --><!--
  return 0;
--><!-- example-end bin.c b -->
<!-- example-begin bin.c c -->
}
    <!-- example-end bin.c c --></programlisting>
    <para>
      There are various functions to lookup elements in a bin. The most
      commonly used are <function>gst_bin_get_by_name ()</function> and
      <function>gst_bin_get_by_interface ()</function>. You can also
      iterate over all elements that a bin contains using the function
      <function>gst_bin_iterate_elements ()</function>. See the API references
      of <ulink type="http"
      url="&URLAPI;GstBin.html"><classname>GstBin</classname></ulink>
      for details.
    </para>
  </sect1>

  <sect1 id="section-bin-custom">
    <title>Custom bins</title>
    <para> 
      The application programmer can create custom bins packed with elements
      to perform a specific task. This allows you, for example, to write
      an Ogg/Vorbis decoder with just the following lines of code:
    </para>
    <programlisting>
int
main (int   argc,
      char *argv[])
{
  GstElement *player;

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

  /* create player */
  player = gst_element_factory_make ("oggvorbisplayer", "player");

  /* set the source audio file */
  g_object_set (player, "location", "helloworld.ogg", NULL);

  /* start playback */
  gst_element_set_state (GST_ELEMENT (player), GST_STATE_PLAYING);
[..]
}
    </programlisting>
    <para>
      (This is a silly example of course, there already exists a much more
      powerful and versatile custom bin like this: the playbin2 element.)
    </para>
    <para>
      Custom bins can be created with a plugin or an XML description. You
      will find more information about creating custom bin in the <ulink
      type="http"
      url="http://gstreamer.freedesktop.org/data/doc/gstreamer/head/pwg/html/index.html">Plugin
      Writers Guide</ulink>.
    </para>
    <para>
      Examples of such custom bins are the playbin2 and uridecodebin elements from<ulink
      type="http"
      url="http://gstreamer.freedesktop.org/data/doc/gstreamer/head/gst-plugins-base-plugins/html/index.html">
      gst-plugins-base</ulink>.
    </para>
  </sect1>
  <sect1 id="section-bin-state-change-handling">
    <title>Bins manage states of their children</title>
    <para>
      Bins manage the state of all elements contained in them. If you set
      a bin (or a pipeline, which is a special top-level type of bin) to
      a certain target state using <function>gst_element_set_state ()</function>,
      it will make sure all elements contained within it will also be set
      to this state. This means it's usually only necessary to set the state
      of the top-level pipeline to start up the pipeline or shut it down.
    </para>
    <para>
      Note, however, that if elements are added to a bin or pipeline that's
      already running, , e.g. from within a "pad-added" or "new-decoded-pad"
      signal callback, its state will not automatically be brought in line with
      the current state or target state of the bin or pipeline it was added to.
      Instead, you have to need to set it to the desired target state yourself
      using <function>gst_element_set_state ()</function> or
      <function>gst_element_sync_state_with_parent ()</function> when adding
      elements to an already-running pipeline.
    </para>
  </sect1>
</chapter>