Creating You Own Behaviours

Matthew Allum

<mallum@openedhand.com>

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.

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.

Example 11.

This example demonstrates a behaviour that produces a vertical 'wipe' like affect by modifying the actors clip region

static void
clutter_behaviour_foo_alpha_notify (ClutterBehaviour *behaviour,
                                    guint32           alpha_value)
{
  ClutterActor *actor
  gint          i, n;
  gdouble       factor;

  /* Normalise alpha value */
  factor = (gdouble) alpha_value / CLUTTER_ALPHA_MAX_ALPHA;

  n = clutter_behaviour_get_n_actors (behaviour);

  /* Change clip height of each applied actor. Note usually better to use
   * clutter_behaviour_actors_foreach () for performance reasons.
   */
  for (i = 0; i<n; i++)
    {
      int 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);
   }
}

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.