gstreamer/docs/manual/xml.xml

<chapter id="cha-xml">
  <title>XML in <application>GStreamer</application></title>
  <para> 
    <application>GStreamer</application> uses XML to store and load
    its pipeline definitions. XML is also used internally to manage the
    plugin registry. The plugin registry is a file that contains the definition
    of all the plugins <application>GStreamer</application> knows about to have 
    quick access to the specifics of the plugins.
  </para>

  <para>
    We will show you how you can save a pipeline to XML and how you can reload that
    XML file again for later use. 
  </para>
  
  <sect1 id="sec-xml-write">
    <title>Turning GstElements into XML</title>

    <para>
      We create a simple pipeline and write it to stdout with
      gst_xml_write_file (). The following code constructs an MP3 player
      pipeline with two threads and then writes out the XML both to stdout
      and to a file. Use this program with one argument: the MP3 file on disk.
    </para>

    <programlisting>
/* example-begin xml-mp3.c */
#include &lt;stdlib.h&gt;
#include &lt;gst/gst.h&gt;

gboolean playing;

int 
main (int argc, char *argv[]) 
{
  GstElement *filesrc, *osssink, *queue, *queue2, *decode;
  GstElement *bin;
  GstElement *thread, *thread2;

  gst_init (&amp;argc,&amp;argv);

  if (argc != 2) {
    g_print ("usage: %s &lt;mp3 filename&gt;\n", argv[0]);
    exit (-1);
  }

  /* create a new thread to hold the elements */
  thread = gst_element_factory_make ("thread", "thread");
  g_assert (thread != NULL);
  thread2 = gst_element_factory_make ("thread", "thread2");
  g_assert (thread2 != NULL);

  /* create a new bin to hold the elements */
  bin = gst_bin_new ("bin");
  g_assert (bin != NULL);

  /* create a disk reader */
  filesrc = gst_element_factory_make ("filesrc", "disk_source");
  g_assert (filesrc != NULL);
  g_object_set (G_OBJECT (filesrc), "location", argv[1], NULL);

  queue = gst_element_factory_make ("queue", "queue");
  queue2 = gst_element_factory_make ("queue", "queue2");

  /* and an audio sink */
  osssink = gst_element_factory_make ("osssink", "play_audio");
  g_assert (osssink != NULL);

  decode = gst_element_factory_make ("mad", "decode");
  g_assert (decode != NULL);

  /* add objects to the main bin */
  gst_bin_add_many (GST_BIN (bin), filesrc, queue, NULL);

  gst_bin_add_many (GST_BIN (thread), decode, queue2, NULL);

  gst_bin_add (GST_BIN (thread2), osssink);
  
  gst_element_link_many (filesrc, queue, decode, queue2, osssink, NULL);

  gst_bin_add_many (GST_BIN (bin), thread, thread2, NULL);

  /* write the bin to stdout */
  gst_xml_write_file (GST_ELEMENT (bin), stdout);

  /* write the bin to a file */
  gst_xml_write_file (GST_ELEMENT (bin), fopen ("xmlTest.gst", "w"));

  exit (0);
}
/* example-end xml-mp3.c */
    </programlisting>
    <para>
      The most important line is:
    </para>
    <programlisting>
  gst_xml_write_file (GST_ELEMENT (bin), stdout);
    </programlisting>
    <para>
      gst_xml_write_file () will turn the given element into an xmlDocPtr that 
      is then formatted and saved to a file. To save to disk, pass the result
      of a fopen(2) as the second argument.
    </para>
    <para>
      The complete element hierarchy will be saved along with the inter element
      pad links and the element parameters. Future <application>GStreamer</application>
      versions will also allow you to store the signals in the XML file.
    </para>
  </sect1>

  <sect1 id="sec-xml-load">
    <title>Loading a GstElement from an XML file</title>
    <para>
      Before an XML file can be loaded, you must create a GstXML object. 
      A saved XML file can then be loaded with the 
      gst_xml_parse_file (xml, filename, rootelement) method.
      The root element can optionally left NULL. The following code example loads
      the previously created XML file and runs it.
    </para>
    <programlisting>
#include &lt;stdlib.h&gt;
#include &lt;gst/gst.h&gt;

int 
main(int argc, char *argv[]) 
{
  GstXML *xml;
  GstElement *bin;
  gboolean ret;

  gst_init (&amp;argc, &amp;argv);

  xml = gst_xml_new ();

  ret = gst_xml_parse_file(xml, "xmlTest.gst", NULL);
  g_assert (ret == TRUE);

  bin = gst_xml_get_element (xml, "bin");
  g_assert (bin != NULL);
  
  gst_element_set_state (bin, GST_STATE_PLAYING);

  while (gst_bin_iterate(GST_BIN(bin)));

  gst_element_set_state (bin, GST_STATE_NULL);

  exit (0);
}
    </programlisting>
    <para>
      gst_xml_get_element (xml, "name") can be used to get a specific element 
      from the XML file. 
    </para>
    <para>
      gst_xml_get_topelements (xml) can be used to get a list of all toplevel elements
      in the XML file.
    </para>
    <para>
      In addition to loading a file, you can also load a from a xmlDocPtr and
      an in memory buffer using gst_xml_parse_doc and gst_xml_parse_memory
      respectively. Both of these methods return a gboolean indicating
      success or failure of the requested action.
    </para>
  </sect1>
  <sect1 id="sec-xml-custom">
    <title>Adding custom XML tags into the core XML data</title>

    <para>
      It is possible to add custom XML tags to the core XML created with
      gst_xml_write. This feature can be used by an application to add more
      information to the save plugins. The editor will for example insert
      the position of the elements on the screen using the custom XML tags.
    </para>
    <para>
      It is strongly suggested to save and load the custom XML tags using
      a namespace. This will solve the problem of having your XML tags 
      interfere with the core XML tags.
    </para>
    <para>
      To insert a hook into the element saving procedure you can link
      a signal to the GstElement using the following piece of code:
    </para>
    <programlisting>
xmlNsPtr ns;

  ...
  ns = xmlNewNs (NULL, "http://gstreamer.net/gst-test/1.0/", "test");
    ...
  thread = gst_element_factory_make ("thread", "thread");
  g_signal_connect (G_OBJECT (thread), "object_saved", 
  		     G_CALLBACK (object_saved), g_strdup ("decoder thread"));
    ...
    </programlisting>
    <para>
      When the thread is saved, the object_save method will be called. Our example
      will insert a comment tag:
    </para>
    <programlisting>
static void
object_saved (GstObject *object, xmlNodePtr parent, gpointer data)
{
  xmlNodePtr child;

  child = xmlNewChild (parent, ns, "comment", NULL);
  xmlNewChild (child, ns, "text", (gchar *)data);
}
    </programlisting>
    <para>
      Adding the custom tag code to the above example you will get an XML file
      with the custom tags in it. Here's an excerpt:
    </para>
    <programlisting>
          ...
        &lt;gst:element&gt;
          &lt;gst:name&gt;thread&lt;/gst:name&gt;
          &lt;gst:type&gt;thread&lt;/gst:type&gt;
          &lt;gst:version&gt;0.1.0&lt;/gst:version&gt;
	  ...
        &lt;/gst:children&gt;
        &lt;test:comment&gt;
          &lt;test:text&gt;decoder thread&lt;/test:text&gt;
        &lt;/test:comment&gt;
      &lt;/gst:element&gt;
          ...
    </programlisting>
    <para>
      To retrieve the custom XML again, you need to attach a signal to 
      the GstXML object used to load the XML data. You can then parse your
      custom XML from the XML tree whenever an object is loaded.
    </para>

    <para>
      We can extend our previous example with the following piece of
      code.
    </para>

    <programlisting>
  xml = gst_xml_new ();

  g_signal_connect (G_OBJECT (xml), "object_loaded", 
  		     G_CALLBACK (xml_loaded), xml);

  ret = gst_xml_parse_file (xml, "xmlTest.gst", NULL);
  g_assert (ret == TRUE);
    </programlisting>

    <para>
      Whenever a new object has been loaded, the xml_loaded function will
      be called. This function looks like:
    </para>
    <programlisting>
static void
xml_loaded (GstXML *xml, GstObject *object, xmlNodePtr self, gpointer data)
{
  xmlNodePtr children = self-&gt;xmlChildrenNode;

  while (children) {
    if (!strcmp (children-&gt;name, "comment")) {
      xmlNodePtr nodes = children-&gt;xmlChildrenNode;

      while (nodes) {
        if (!strcmp (nodes-&gt;name, "text")) {
          gchar *name = g_strdup (xmlNodeGetContent (nodes));
          g_print ("object %s loaded with comment '%s'\n",
                   gst_object_get_name (object), name);
        }
        nodes = nodes-&gt;next;
      }
    }
    children = children-&gt;next;
  }
}
    </programlisting>
    <para>
      As you can see, you'll get a handle to the GstXML object, the 
      newly loaded GstObject and the xmlNodePtr that was used to create
      this object. In the above example we look for our special tag inside
      the XML tree that was used to load the object and we print our
      comment to the console.
    </para>
  </sect1>

</chapter>