gstreamer/docs/pwg/advanced-interfaces.xml

<chapter id="cha-advanced-interfaces">
  <title>Interfaces</title>
  <para>
    Previously, in the chapter <xref linkend="cha-building-args"/>, we have
    introduced the concept of GObject properties of controlling an element's
    behaviour. This is a very powerful, but has two big disadvantage: firstly,
    it is too generic, and secondly, it isn't dynamic.
  </para>
  <para>
    The first disadvantage has to do with customizability of the end-user
    interface that will be built to control the element. Some properties are
    more important than others. Some integer properties are better shown in a
    spin-button widget, whereas others would be better represented by a slider
    widget. Such things are not possible because the UI has no actual meaning
    in the application. A UI widget that stands for a bitrate property is the
    same as an UI widget that stands for the size of a video, as long as both
    are of the same <classname>GParamSpec</classname> type. Another problem,
    related to the one about parameter important, is that things like parameter
    grouping, function grouping or anything to make parameters coherent, is not
    really possible.
  </para>
  <para>
    The second argument against parameters are that they are not dynamic. In
    many cases, the allowed values for a property are not fixed, but depend
    on things that can only be detected at run-time. The names of inputs for
    a TV card in a video4linux source element, for example, can only be
    retrieved from the kernel driver when we've opened the device; this only
    happens when the element goes into the READY state. This means that we
    cannot create an enum property type to show this to the user.
  </para>
  <para>
    The solution to those problems is to create very specialized types of
    controls for certain often-used controls. We use the concept of interfaces
    to achieve this. The basis of this all is the glib
    <classname>GTypeInterface</classname> type. For each case where we think
    it's useful, we've created interfaces which can be implemented by elements
    at their own will. We've also created a small extension to
    <classname>GTypeInterface</classname> (which is static itself, too) which
    allows us to query for interface availability based on runtime properties.
    This extension is called <classname>GstImplementsInterface</classname>.
  </para>
  <para>
    One important note: interfaces do <emphasis>not</emphasis> replace
    properties. Rather, interfaces should be built <emphasis>next to</emphasis>
    properties. There are two important reasons for this. Firstly, properties
    can be saved in XML files. Secondly, properties can be specified on the
    commandline (<filename>gst-launch</filename>).
  </para>

  <sect1 id="sect1-iface-general" xreflabel="How to Implement Interfaces">
    <title>How to Implement Interfaces</title>
    <para>
      Implementing interfaces is intiated in the <function>_get_type ()</function>
      of your element. You can register one or more interfaces after having
      registered the type itself. Some interfaces have dependencies on other
      interfaces or can only be registered by certain types of elements. You
      will be notified of doing that wrongly when using the element: it will
      quit with failed assertions, which will explain what went wrong. In the
      case of GStreamer, the only dependency that <emphasis>some</emphasis>
      interfaces have is <classname>GstImplementsInterface</classname>. Per
      interface, we will indicate clearly when it depends on this extension.
      If it does, you need to register support for <emphasis>that</emphasis>
      interface before registering support for the interface that you're
      wanting to support. The example below explains how to add support for a
      simple interface with no further dependencies. For a small explanation
      on <classname>GstImplementsInterface</classname>, see the next section
      about the mixer interface: <xref linkend="sect1-iface-mixer"/>.
    </para>
    <programlisting>
static void	gst_my_filter_some_interface_init	(GstSomeInterface *iface);

GType
gst_my_filter_get_type (void)
{
  static GType my_filter_type = 0;
                                                                                
  if (!my_filter_type) {
    static const GTypeInfo my_filter_info = {
      sizeof (GstMyFilterClass),
      (GBaseInitFunc) gst_my_filter_base_init,
      NULL,
      (GClassInitFunc) gst_my_filter_class_init,
      NULL,
      NULL,
      sizeof (GstMyFilter),
      0,
      (GInstanceInitFunc) gst_my_filter_init
    };
    static const GInterfaceInfo some_interface_info = {
      (GInterfaceInitFunc) gst_my_filter_some_interface_init,
      NULL,
      NULL
    };

    my_filter_type =
	g_type_register_static (GST_TYPE_MY_FILTER,
				"GstMyFilter",
				&amp;my_filter_info, 0);
    g_type_add_interface_static (my_filter_type,
				 GST_TYPE_SOME_INTERFACE,
                                 &amp;some_interface_info);
  }

  return my_filter_type;
}

static void
gst_my_filter_some_interface_init (GstSomeInterface *iface)
{
  /* here, you would set virtual function pointers in the interface */
}
    </programlisting>
  </sect1>

  <sect1 id="sect1-iface-mixer" xreflabel="Mixer Interface">
    <title>Mixer Interface</title>
    <para>
      The goal of the mixer interface is to provide a simple yet powerful API
      to applications for audio hardware mixer/volume control. Most soundcards
      have hardware mixers, where volume can be changed, they can be muted,
      inputs can be modified to mix their content into what will be read from
      the device by applications (in our case: audio source plugins). The
      mixer interface is the way to control those. The mixer interface can
      also be used for volume control in software (e.g. the <quote>volume</quote>
      element). The end goal of this interface is to allow development of
      hardware volume control applications and for the control of audio volume
      and input/output settings.
    </para>
    <para>
      The mixer interface requires the <classname>GstImplementsInterface</classname>
      interface to be implemented by the element. The example below will
      feature both, so it serves as an example for the
      <classname>GstImplementsInterface</classname>, too. In the
      <classname>GstImplementsInterface</classname>, it is required to set a
      function pointer for the <function>supported ()</function> function. If
      you don't, this function will always return FALSE (default
      implementation) and the mixer interface implementation will not work. For
      the mixer interface, the only required function is
      <function>list_tracks ()</function>. All other function pointers in the
      mixer interface are optional, although it is strongly recommended to set
      function pointers for at least the <function>get_volume ()</function> and
      <function>set_volume ()</function> functions. The API reference for this
      interface documents the goal of each function, so we will limit ourselves
      to the implementation here.
    </para>
    <programlisting>
#include &lt;gst/mixer/mixer.h&gt;

typedef struct _GstMyFilter {
[..]
  gint volume;
  GList *tracks;
} GstMyFilter;

static void	gst_my_filter_implements_interface_init	(GstImplementsInterfaceClass *iface);
static void	gst_my_filter_mixer_interface_init	(GstMixerClass *iface);

GType
gst_my_filter_get_type (void)
{
[..]
    static const GInterfaceInfo implements_interface_info = {
      (GInterfaceInitFunc) gst_my_filter_implements_interface_init,
      NULL,
      NULL
    };
    static const GInterfaceInfo mixer_interface_info = {
      (GInterfaceInitFunc) gst_my_filter_mixer_interface_init,
      NULL,
      NULL
    };
[..]
    g_type_add_interface_static (my_filter_type,
				 GST_TYPE_IMPLEMENTS_INTERFACE,
				 &amp;implements_interface_info);
    g_type_add_interface_static (my_filter_type,
				 GST_TYPE_MIXER,
				 &amp;mixer_interface_info);
[..]
}

static void
gst_my_filter_init (GstMyFilter *filter)
{
  GstMixerTrack *track = NULL;
[..]
  filter->volume = 100;
  filter->tracks = NULL;
  track = g_object_new (GST_TYPE_MIXER_TRACK, NULL);
  track->label = g_strdup ("MyTrack");
  track->num_channels = 1;
  track->min_volume = 0;
  track->max_volume = 100;
  track->flags = GST_MIXER_TRACK_SOFTWARE;
  filter->tracks = g_list_append (filter->tracks, track);
}

static gboolean
gst_my_filter_interface_supported (GstImplementsInterface *iface,
				   GType                   iface_type)
{
  g_return_val_if_fail (iface_type == GST_TYPE_MIXER, FALSE);

  /* for the sake of this example, we'll always support it. However, normally,
   * you would check whether the device you've opened supports mixers. */
  return TRUE;
}

static void
gst_my_filter_implements_interface_init (GstImplementsInterfaceClass *iface)
{
  iface->supported = gst_my_filter_interface_supported;
}

/*
 * This function returns the list of support tracks (inputs, outputs)
 * on this element instance. Elements usually build this list during
 * _init () or when going from NULL to READY.
 */

static const GList *
gst_my_filter_mixer_list_tracks (GstMixer *mixer)
{
  GstMyFilter *filter = GST_MY_FILTER (mixer);

  return filter->tracks;
}

/*
 * Set volume. volumes is an array of size track->num_channels, and
 * each value in the array gives the wanted volume for one channel
 * on the track.
 */

static void
gst_my_filter_mixer_set_volume (GstMixer      *mixer,
				GstMixerTrack *track,
				gint          *volumes)
{
  GstMyFilter *filter = GST_MY_FILTER (mixer);

  filter->volume = volumes[0];

  g_print ("Volume set to %d\n", filter->volume);
}

static void
gst_my_filter_mixer_get_volume (GstMixer      *mixer,
				GstMixerTrack *track,
				gint          *volumes)
{
  GstMyFilter *filter = GST_MY_FILTER (mixer);

  volumes[0] = filter->volume;
}

static void
gst_my_filter_mixer_interface_init (GstMixerClass *iface)
{
  /* the mixer interface requires a definition of the mixer type:
   * hardware or software? */
  GST_MIXER_TYPE (iface) = GST_MIXER_SOFTWARE;

  /* virtual function pointers */
  iface->list_tracks = gst_my_filter_mixer_list_tracks;
  iface->set_volume  = gst_my_filter_mixer_set_volume;
  iface->get_volume  = gst_my_filter_mixer_get_volume;
}
    </programlisting>
  </sect1>

  <sect1 id="sect1-iface-tuner" xreflabel="Tuner Interface">
    <title>Tuner Interface</title>
    <para>
      WRITEME
    </para>
  </sect1>

  <sect1 id="sect1-iface-colorbalance" xreflabel="Color Balance Interface">
    <title>Color Balance Interface</title>
    <para>
      WRITEME
    </para>
  </sect1>

  <sect1 id="sect1-iface-propprobe" xreflabel="Property Probe Interface">
    <title>Property Probe Interface</title>
    <para>
      Property probing is a generic solution to the problem that properties'
      value lists in an enumeration are static. We've shown enumerations in
      <xref linkend="cha-building-args"/>. Property probing tries to accomplish
      a goal similar to enumeration lists: to have a limited, explicit list of
      allowed values for a property. There are two differences between
      enumeration lists and probing. Firstly, enumerations only allow strings
      as values; property probing works for any value type. Secondly, the
      contents of a probed list of allowed values may change during the life
      of an element. The contents of a enumeraiton list are static. Crrently,
      property probing is being used for detection of devices (e.g. for OSS
      elements, Video4linux elements, etc.). It could - in theory - be used
      for any property, though.
    </para>
    <para>
      Property probing stores the list of allowed (or recommended) values in a
      <classname>GValueArray</classname> and returns that to the user.
      <classname>NULL</classname> is a valid return value, too. The process of
      property probing is separated over two virtual functions: one for probing
      the property to create a <classname>GValueArray</classname>, and one to
      retrieve the current <classname>GValueArray</classname>. Those two are
      separated because probing might take a long time (several seconds). Also,
      this simpliies interface implementation in elements. For the application,
      there are functions that wrap those two. For more information on this,
      have a look at the API reference for the
      <classname>GstPropertyProbe</classname> interface.
    </para>
    <para>
      Below is a example of property probing for the audio filter element; it
      will probe for allowed values for the <quote>silent</quote> property.
      Indeed, this value is a <classname>gboolean</classname> so it doesn't
      make much sense. Then again, it's only an example.
    </para>
    <programlisting>
#include &lt;gst/propertyprobe/propertyprobe.h&gt;

static void	gst_my_filter_probe_interface_init	(GstPropertyProbeInterface *iface);

GType
gst_my_filter_get_type (void)
{
[..]
    static const GInterfaceInfo probe_interface_info = {
      (GInterfaceInitFunc) gst_my_filter_probe_interface_init,
      NULL,
      NULL
    };
[..]
    g_type_add_interface_static (my_filter_type,
				 GST_TYPE_PROPERTY_PROBE,
				 &amp;probe_interface_info);
[..]
}

static const GList *
gst_my_filter_probe_get_properties (GstPropertyProbe *probe)
{
  GObjectClass *klass = G_OBJECT_GET_CLASS (probe);
  static GList *props = NULL;

  if (!props) {
    GParamSpec *pspec;

    pspec = g_object_class_find_property (klass, "silent");
    props = g_list_append (props, pspec);
  }

  return props;
}

static gboolean
gst_my_filter_probe_needs_probe (GstPropertyProbe *probe,
				 guint             prop_id,
				 const GParamSpec *pspec)
{
  gboolean res = FALSE;

  switch (prop_id) {
    case ARG_SILENT:
      res = FALSE;
      break;
    default:
      G_OBJECT_WARN_INVALID_PROPERTY_ID (probe, prop_id, pspec);
      break;
  }

  return res;
}

static void
gst_my_filter_probe_probe_property (GstPropertyProbe *probe,
				    guint             prop_id,
				    const GParamSpec *pspec)
{
  switch (prop_id) {
    case ARG_SILENT:
      /* don't need to do much here... */
      break;
    default:
      G_OBJECT_WARN_INVALID_PROPERTY_ID (probe, prop_id, pspec);
      break;
  }
}

static GValueArray *
gst_my_filter_get_silent_values (GstMyFilter *filter)
{
  GValueArray *array = g_value_array_new (2);
  GValue value = { 0 };

  g_value_init (&amp;value, G_TYPE_BOOLEAN);

  /* add TRUE */
  g_value_set_boolean (&amp;value, TRUE);
  g_value_array_append (array, &amp;value);

  /* add FALSE */
  g_value_set_boolean (&amp;value, FALSE);
  g_value_array_append (array, &amp;value);

  g_value_unset (&amp;value);

  return array;
}

static GValueArray *
gst_my_filter_probe_get_values (GstPropertyProbe *probe,
				guint             prop_id,
				const GParamSpec *pspec)
{
  GstMyFilter *filter = GST_MY_FILTER (probe);
  GValueArray *array = NULL;

  switch (prop_id) {
    case ARG_SILENT:
      array = gst_my_filter_get_silent_values (filter);
      break;
    default:
      G_OBJECT_WARN_INVALID_PROPERTY_ID (probe, prop_id, pspec);
      break;
  }

  return array;
}

static void
gst_my_filter_probe_interface_init (GstPropertyProbeInterface *iface)
{
  iface->get_properties = gst_my_filter_probe_get_properties;
  iface->needs_probe    = gst_my_filter_probe_needs_probe;
  iface->probe_property = gst_my_filter_probe_probe_property;
  iface->get_values     = gst_my_filter_probe_get_values;
}
    </programlisting>
    <para>
      You don't need to support any functions for getting or setting values.
      All that is handled via the standard <classname>GObject</classname>
      <function>_set_property ()</function> and <function>_get_property ()</function>
      functions.
    </para>
  </sect1>

  <sect1 id="sect1-iface-profile" xreflabel="Profile Interface">
    <title>Profile Interface</title>
    <para>
      WRITEME
    </para>
  </sect1>
                                                                                
  <sect1 id="sect1-iface-xoverlay" xreflabel="X Overlay Interface">
    <title>X Overlay Interface</title>
    <para>
      WRITEME
    </para>
  </sect1>

  <sect1 id="sect1-iface-navigation" xreflabel="Navigation Interface">
    <title>Navigation Interface</title>
    <para>
      WRITEME
    </para>
  </sect1>
</chapter>