mutter/doc/reference/clutter/creating-behaviours.xml

<chapter id="creating-your-own-behaviours">
  <chapterinfo>
    <author>
      <firstname>Matthew</firstname>
      <surname>Allum</surname>
      <affiliation>
        <address>
          <email>mallum@openedhand.com</email>
        </address>
      </affiliation>
    </author>
  </chapterinfo>

  <title>Creating You Own Behaviours</title>
  <para>Clutter comes with a number of fairly generic prebuilt behaviour
  classes which provide a basis for transitions, animations and other
  visual effects. However even with the ability to combine a number of
  these behaviours sometimes they are not enough and a custom behaviour
  is needed to create a spcific animation.</para>

  <para>In order to implement a new #ClutterBehaviour subclass the usual
  machinery for subclassing a GObject should be used. The new subclass
  then just overides the #ClutterBehaviour::alpha_notify() method. This
  method is passed an alpha value which is then used to compute
  modifications to any actors the behaviour is applied to.</para>

  <example id="clutter-behaviour-alpha-notify-example">
    <title>Implementing the alpha-notify virtual function</title>
    <para>This example demonstrates a behaviour that produces a vertical
    'wipe' like affect by modifying the actors clip region</para>

    <programlisting>
static void
clutter_behaviour_foo_alpha_notify (ClutterBehaviour *behaviour,
                                    gdouble           factor)
{
  ClutterActor *actor
  gint i, n;

  n = clutter_behaviour_get_n_actors (behaviour);

  /* Change clip height of each applied actor. Note that it is
   * usually better to use clutter_behaviour_actors_foreach()
   * to avoid iterating multiple times
   */
  for (i = 0; i &lt; n; i++)
    {
      gfloat clip_height;

      actor = clutter_behaviour_get_nth_actor (behaviour, i);

      clip_height = clutter_actor_get_height (actor)
                  - (clutter_actor_get_height (actor) * factor);

      clutter_actor_set_clip (actor,
                              0,
                              0,
                              clutter_actor_get_width (actor),
                              clip_height);
   }
}
    </programlisting>
  </example>

  <para>If the new behaviour is meant to set an initial state on the
  actors to which its applied to, then the ClutterBehaviour::applied
  signal class handler should be overridden.</para>
</chapter>