gstreamer/docs/manual/elements-api.xml

<chapter id="cha-elements-api">
  <title>Elements</title>
  <sect1 id="sec-elements-create">
    <title>Creating a GstElement</title>
    <para> 
      A <classname>GstElement</classname> object is created from
      a factory.  To create an element, you have to get access to a
      <classname>GstElementFactory</classname> object using a unique
      factory name.
    </para> 
    <para> 
      The following code example is used to get a factory that can be used 
      to create the 'mad' element, an mp3 decoder.
    </para> 
    <programlisting>
 GstElementFactory *factory;

 factory = gst_element_factory_find ("mad");
    </programlisting>
    <para> 
      Once you have the handle to the element factory, you can create a 
      real element with the following code fragment:
    </para> 
    <programlisting>
 GstElement *element;

 element = gst_element_factory_create (factory, "decoder");
    </programlisting>
    <para>
      <function>gst_element_factory_create</function> will use the element
      factory to create an element with the given name. The name of the
      element is something you can use later on to look up the element in
      a bin, for example. You can pass <symbol>NULL</symbol> as the name
      argument to get a unique, default name.
    </para>
    <para>
      A simple shortcut exists for creating an element from a factory. The
      following example creates an element named "decoder" from the element
      factory named "mad". This convenience function is most widely used to
      create an element.
    </para>
    <programlisting>
 GstElement *element;

 element = gst_element_factory_make ("mad", "decoder");
    </programlisting>
    <para> 
      When you don't need the element anymore, you need to unref it, as shown in the following
      example. 
    </para> 
    <programlisting>
 GstElement *element;

  ...
 gst_element_unref (element);
    </programlisting>
  </sect1>
  <sect1 id="sec-elements-properties">
    <title>GstElement properties</title>
    <para> 
      A <classname>GstElement</classname> can have several properties
      which are implemented using standard <classname>GObject</classname>
      properties. The usual <classname>GObject</classname> methods to query,
      set and get property values and <classname>GParamSpecs</classname>
      are therefore supported.
    </para> 
    <para> 
      Every <classname>GstElement</classname> inherits at least
      one property of its parent <classname>GstObject</classname>:
      the "name" property. This is the name you provide to the
      functions <function>gst_element_factory_make</function> or
      <function>gst_element_factory_create</function>. You can get and set 
      this property using the functions 
      <function>gst_object_set_name</function>
      and <function>gst_object_get_name</function> or use the
      <classname>GObject</classname> property mechanism as shown below.
    </para> 
    <programlisting>
 GstElement *element;
 GValue value = { 0, }; /* initialize the GValue for g_object_get() */

 element = gst_element_factory_make ("mad", "decoder");
 g_object_set (G_OBJECT (element), "name", "mydecoder", NULL);
 ...

 g_value_init (&amp;value, G_TYPE_STRING);
 g_object_get_property (G_OBJECT (element), "name", &amp;value);
 ...
    </programlisting>
    <para> 
      Most plugins provide additional properties to provide more information
      about their configuration or to configure the element. 
      <command>gst-inspect</command> is a useful tool to query the properties
      of a particular element, it will also use property introspection to give
      a short explanation about the function of the property and about the
      parameter types and ranges it supports.
    </para> 
    <para> 
      For more information about <classname>GObject</classname>
      properties we recommend you read the <ulink
      url="http://developer.gnome.org/doc/API/2.0/gobject/index.html"
      type="http">GObject manual</ulink> and an introduction to <ulink
      url="http://le-hacker.org/papers/gobject/index.html" type="http">
      The Glib Object system</ulink>.
    </para> 
  </sect1>
  
  <sect1 id="sec-elements-signals">
    <title>GstElement signals</title>
    <para> 
      A <classname>GstElement</classname> also provides various 
      <classname>GObject</classname> signals that can be used as a flexible
      callback mechanism.
    </para> 
  </sect1>
  
  <sect1 id="sec-elements-factories">
    <title>More about GstElementFactory</title>
    <para> 
      We talk some more about the GstElementFactory object.
    </para> 
    
    <sect2 id="sec-elements-factories-details">
      <title>Getting information about an element using the factory details</title>
      <para> 
      </para> 
    </sect2>
    
    <sect2 id="sec-elements-factories-padtemplates">
      <title>Finding out what pads an element can contain</title>
      <para> 
      </para> 
    </sect2>
    
    <sect2 id="sec-elements-factories-query">
      <title>Different ways of querying the factories</title>
      <para> 
      </para> 
    </sect2>
  </sect1>
</chapter>