gstreamer/docs/pwg/building-state.xml

<chapter id="cha-statemanage-states">
  <title>
    What are states?
  </title>
  <para>
    A state describes whether the element instance is initialized, whether it
    is ready to transfer data and whether it is currently handling data. There
    are four states defined in &GStreamer;: <classname>GST_STATE_NULL</classname>,
    <classname>GST_STATE_READY</classname>, <classname>GST_STATE_PAUSED</classname>
    and <classname>GST_STATE_PLAYING</classname>.
  </para>
  <para>
    <classname>GST_STATE_NULL</classname> (from now on referred to as
    <quote>NULL</quote>) is the default state of an element. In this state, it
    has not allocated any runtime resources, it has not loaded any runtime
    libraries and it can obviously not handle data.
  </para>
  <para>
    <classname>GST_STATE_READY</classname> (from now on referred to as
    <quote>READY</quote>) is the next state that an element can be in. In the
    READY state, an element has all default resources (runtime-libraries,
    runtime-memory) allocated. However, it has not yet allocated or defined
    anything that is stream-specific. When going from NULL to READY state
    (<classname>GST_STATE_NULL_TO_READY</classname>), an element should
    allocate any non-stream-specific resources and should load runtime-loadable
    libraries (if any). When going the other way around (from READY to NULL,
    <classname>GST_STATE_READY_TO_NULL</classname>), an element should unload
    these libraries and free all allocated resources. Examples of such
    resources are hardware devices. Note that files are generally streams,
    and these should thus be considered as stream-specific resources; therefore,
    they should <emphasis>not</emphasis> be allocated in this state.
  </para>
  <para>
    <classname>GST_STATE_PAUSED</classname> (from now on referred to as
    <quote>PAUSED</quote>) is a state in which an element is by all means able
    to handle data; the only 'but' here is that it doesn't actually handle
    any data. When going from the READY state into the PAUSED state
    (<classname>GST_STATE_READY_TO_PAUSED</classname>), the element will
    usually not do anything at all: all stream-specific info is generally
    handled in the <function>_link ()</function>, which is called during caps
    negotiation. Exceptions to this rule are, for example, files: these are
    considered stream-specific data (since one file is one stream), and should
    thus be opened in this state change. When going from the PAUSED back to
    READY (<classname>GST_STATE_PAUSED_TO_READY</classname>), all
    stream-specific data should be discarded.
  </para>
  <para>
    <classname>GST_STATE_PLAYING</classname> (from now on referred to as
    <quote>PLAYING</quote>) is the highest state that an element can be in. It
    is similar to PAUSED, except that now, data is actually passing over the
    pipeline. The transition from PAUSED to PLAYING
    (<classname>GST_STATE_PAUSED_TO_PLAYING</classname>) should be as small
    as possible and would ideally cause no delay at all. The same goes for the
    reverse transition (<classname>GST_STATE_PLAYING_TO_PAUSED</classname>).
  </para>
</chapter>
<chapter id="cha-statemanage-filters">
  <title>
    Mangaging filter state
  </title>
  <para>
    An element can be notified of state changes through a virtual function
    pointer. Inside this function, the element can initialize any sort of
    specific data needed by the element, and it can optionally fail to
    go from one state to another.
  </para>
  <programlisting>
static GstElementStateReturn
		gst_my_filter_change_state	(GstElement *element);

static void
gst_my_filter_class_init (GstMyFilterClass *klass)
{
  GstElementClass *element_class = GST_ELEMENT_CLASS (klass);

  element_class->change_state = gst_my_filter_change_state;
}

static GstElementStateReturn
gst_my_filter_change_state (GstElement *element)
{
  GstMyFilter *filter = GST_MY_FILTER (element);

  switch (GST_STATE_TRANSITION (element)) {
    case GST_STATE_NULL_TO_READY:
      if (!gst_my_filter_allocate_memory (filter))
        return GST_STATE_FAILURE;
      break;
    case GST_STATE_READY_TO_NULL:
      gst_my_filter_free_memory (filter);
      break;
    default:
      break;
  }

  if (GST_ELEMENT_CLASS (parent_class)->change_state)
    return GST_ELEMENT_CLASS (parent_class)->change_state (element);

  return GST_STATE_SUCCESS;
}
  </programlisting>
</chapter>