mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-12-15 21:06:32 +00:00
103 lines
4.4 KiB
XML
103 lines
4.4 KiB
XML
<chapter id="chapter-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>
|
|
|
|
<sect1 id="section-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>
|
|
</sect1>
|
|
</chapter>
|