## 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.