## Implementing a new actor

### Emmanuele Bassi

`<ebassi`linux.intel.com`>`

In order to implement a new ClutterActor subclass the usual machinery for subclassing a GObject should be used:

    1
    2
    3
    4
    5
    6
    7
    8
    9
    10
    11
    12
    13
    14
    15
    16
    17
    18
    19
    20
    21
    22
    23
    24
    25
    26
    27
    28
    #define FOO_TYPE_ACTOR            (foo_actor_get_type ())
    #define FOO_ACTOR(obj)            (G_TYPE_CHECK_INSTANCE_CAST ((obj), FOO_TYPE_ACTOR, FooActor))
    #define FOO_IS_ACTOR(obj)         (G_TYPE_CHECK_INSTANCE_TYPE ((obj), FOO_TYPE_ACTOR))
    #define FOO_ACTOR_CLASS(klass)    (G_TYPE_CHECK_CLASS_CAST ((klass), FOO_TYPE_ACTOR, FooActorClass))
    #define FOO_IS_ACTOR_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE ((klass), FOO_TYPE_ACTOR))
    #define FOO_ACTOR_GET_CLASS(obj)  (G_TYPE_INSTANCE_GET_CLASS ((obj), FOO_TYPE_ACTOR, FooActorClass))

    typedef struct _FooActor
    {
      ClutterActor parent_instance;
    } FooActor;

    typedef struct _FooActorClass
    {
      ClutterActorClass parent_class;
    } FooActorClass;

    G_DEFINE_TYPE (FooActor, foo_actor, CLUTTER_TYPE_ACTOR);

    static void
    foo_actor_class_init (FooActorClass *klass)
    {
    }

    static void
    foo_actor_init (FooActor *actor)
    {
    }

The implementation of an actor roughly depends on what kind of actor the class should display.

The implementation process can be broken down into sections:

size requisition | used by containers to know how much space an actor requires for itself and its eventual children.
size allocation | used by containers to define how much space an actor should have for itself and its eventual children.
painting and picking | the actual actor painting and the "picking" done to determine the actors that received events
realization and visibility | used by containers and composite actors to determine whether their children should allocate (and deallocate) specific resources associated with being added to the ClutterStage, and whether their children should be painted or not.

Container actors should also implement the ClutterContainer interface to provide a consistent API for adding, removing and iterating over their children.

## Size requisition

Actors should usually implement the size requisition virtual functions unless they depend on explicit sizing by the developer, using the `clutter_actor_set_width()` and `clutter_actor_set_height()` functions and their wrappers.

### Note

For instance, an actor that depends on the explicit sizing is the ClutterRectangle actor.

The size requisition is split into two different phases: width requisition and height requisition.

The `ClutterActor`::`get_preferred_width` and `ClutterActor`::`get_preferred_height` methods of a ClutterActor are invoked when `clutter_actor_get_preferred_width()` and `clutter_actor_get_preferred_height()` are respectively called on an instance of that actor class. They are used to return the preferred size of the actor. Container actors, or composite actors with internal children, should call `clutter_actor_get_preferred_width()` and `clutter_actor_get_preferred_height()` on each visible child inside their implementation of the `get_preferred_width` and `get_preferred_height` virtual functions.

The `get_preferred_width` and `get_preferred_height` virtual functions return both the minimum size of the actor and its natural size. The minimum size is defined as the amount of space an actor must occupy to be useful; the natural size is defined as the amount of space an actor would occupy if nothing would constrain it.

### Note

The natural size must always be greater than, or equal to the minimum size. ClutterActor will just ignore a natural size lesser than a minimum size.

The height request may be computed for a specific width, which is passed to the implementation, thus allowing height-for-width geometry management. Similarly, the width request may be computed for a specific height, allowing width-for-height geometry management. By default, every ClutterActor uses the height-for-width geometry management, but the setting can be changed by using the "request-mode" property.

### Note

The `clutter_actor_get_preferred_size()` function will automatically check the geometry management preferred by the actor and return its preferred size depending on the value of the request-mode property and on the natural size of the actor. The `clutter_actor_get_preferred_size()` method, though, will ignore any notion of "available size" so it should not be used inside a `ClutterActor`::`allocate` implementation.

The size requisition starts from the ClutterStage and it is performed on every child of the stage following down the hierarchy of the scene graph.

### Note

The size requisition should not take into account the actor's scale, rotation or anchor point unless an actor is performing layout management depending on those properties.

### Note

All the sizes are expressed using pixels with subpixel precision. The sub-pixel precision is useful when animating actors but it can produce odd results on screen, so you might want to truncate the precision of the computed values.

Example 20. Width requisition implementation of a container

This example shows how an actor class should override the `get_preferred_width` virtual function of ClutterActor. In this case, the returned widths are the union of the extents of all the `FooActor` children.

The `get_preferred_height` implementation would be similar to the `get_preferred_width` implementation, so it is omitted.

    static void
    foo_actor_get_preferred_width (ClutterActor *actor,
                                   gfloat        for_height,
                                   gfloat       *min_width_p,
                                   gfloat       *natural_width_p)
    {
      GList *l;
      ClutterUnit min_left, min_right;
      ClutterUnit natural_left, natural_right;

      min_left = 0;
      min_right = 0;
      natural_left = 0;
      natural_right = 0;

      for (l = children; l != NULL; l = l->next)
        {
          ClutterActor *child = l->data;
          gfloat child_x, child_min, child_natural;

          child_x = clutter_actor_get_xu (child);

          clutter_actor_get_preferred_width (child, for_height,
                                             &child_min,
                                             &child_natural);

          if (l == children)
            {
              /* First child */
              min_left = child_x;
              natural_left = child_x;
              min_right = min_left + child_min;
              natural_right = natural_left + child_natural;
            }
          else
            {
              if (child_x < min_left)
                min_left = child_x;

              if (child_x < natural_left)
                natural_left = child_x;

              if (child_x + child_min > min_right)
                min_right = child_x + child_min;

              if (child_x + child_natural > natural_right)
                natural_right = child_x + child_natural;
            }
        }

      /* The request is defined as the width and height we want starting from
       * our origin, since our allocation will set the origin; so we now need
       * to remove any part of the request that is to the left of the origin.
       */
      if (min_left < 0)
        min_left = 0;

      if (natural_left < 0)
        natural_left = 0;

      if (min_right < 0)
        min_right = 0;

      if (natural_right < 0)
        natural_right = 0;

      g_assert (min_right >= min_left);
      g_assert (natural_right >= natural_left);

      if (min_width_p)
        *min_width_p = min_right - min_left;

      if (natural_width_p)
        *natural_width_p = natural_right - min_left;
    }

## Size allocation

The `ClutterActor`::`allocate` virtual function of a ClutterActor is invoked when `clutter_actor_allocate()` is called on an instance of that actor class. It is used by a parent actor to set the coordinates of the bounding box for its children actors. Hence, container actors, or composite actors with internal children, should override the `allocate` virtual function and call `clutter_actor_allocate()` on each visible child.

Each actor can know from their allocation box whether they have been moved with respect to their parent actor. Each child will also be able to tell whether their parent has been moved with respect to the stage.

### Note

The `allocate` virtual function implementation will be notified whether the actor has been moved, while `clutter_actor_allocate()` will usually be invoked with the `CLUTTER_ABSOLUTE_ORIGIN_CHANGED` flag, meaning that the parent has been moved.

Example 21. Allocation of a container

In this example, `FooActor` acts like a horizontal box with overflowing, like a toolbar which will display more children as it expands. The children falling outside of the allocated area will not be allocated.

    static void
    foo_actor_allocate (ClutterActor           *actor,
                        const ClutterActorBox  *box,
                        ClutterAllocationFlags  flags)
    {
      FooActor *foo_actor = FOO_ACTOR (actor);
      gfloat available_width, available_height;
      gfloat current_width, current_height;
      gfloat row_height;
      GList *l;

      /* chain up to store the allocation of the actor */
      CLUTTER_ACTOR_CLASS (foo_actor_parent_class)->allocate (actor, box, flags);

      clutter_actor_box_get_size (box,
                                  &available_width,
                                  &available_height);

      current_width = 0;
      current_height = 0;

      row_height = 0;

      for (l = foo_actor->children;
           l != NULL;
           l = l->next)
        {
          ClutterActor *child = l->data;
          ClutterActorBox child_box = { 0, };
          gfloat child_width, child_height;
          ClutterRequestMode mode;

          /* do not allocate invisible children */
          if (!CLUTTER_ACTOR_IS_VISIBLE (child))
            continue;

          g_object_get (G_OBJECT (child), "request-mode", &mode, NULL);
          if (mode == CLUTTER_REQUEST_HEIGHT_FOR_WIDTH)
            {
              gfloat min, natural;

              clutter_actor_get_preferred_width (child, available_height,
                                                 &min, &natural);
              child_width = MAX (min, MIN (natural, available_width));

              clutter_actor_get_preferred_height (child, child_width,
                                                  &min, &natural);
              child_height = MAX (min, MIN (natural, available_height));
            }
          else (mode == CLUTTER_REQUEST_WIDTH_FOR_HEIGHT)
            {
              gfloat min, natural;

              clutter_actor_get_preferred_height (child, available_width,
                                                  &min, &natural);
              child_height = MAX (min, MIN (natural, available_height));

              clutter_actor_get_preferred_width (child, child_height,
                                                 &min, &natural);
              child_width = MAX (min, MIN (natural, available_width));
            }
          else
            g_assert_not_reached ();

          /* overflow */
          if (current_width + child_width > available_width)
            {
              current_width = 0;
              current_height += row_height;
            }
          else
            current_width += child_width;

          /* stop allocating if we are overflowing the available height */
          if (current_height + child_height > available_height)
            break;

          child_box.x1 = current_width;
          child_box.y1 = current_height;
          child_box.x2 = child_box.x1 + child_width;
          child_box.y2 = child_box.y1 + child_height;

          /* update the allocation */
          clutter_actor_allocate (child,
                                  &child_box,
                                  flags);

          row_height = MAX (row_height, child_height);
        }
    }

The allocation is also the "paint area", that is the area where the paint operations should be performed.

## Painting and picking

The `ClutterActor`::`paint` virtual function should be overridden if the actor needs to control its drawing process, by painting other child actors or drawing with the Cogl 3D graphics API.

Example 22. Paint implementation of a simple actor

In this example, the `FooActor` implementation of the `paint` virtual function is drawing a rectangle with rounded corners with a custom color using the Cogl API.

    static void
    foo_actor_paint (ClutterActor *actor)
    {
      FooActor *foo_actor = FOO_ACTOR (actor);
      ClutterActorBox allocation = { 0, };
      gfloat width, height;

      /* FooActor has a specific background color
       *
       * the alpha component must take into account the absolute
       * opacity of the actor on the screen at this point in the
       * scenegraph; this value is obtained by calling
       * clutter_actor_get_paint_opacity().
       */
      cogl_set_source_color4ub (priv->fgcol.red,
                                priv->fgcol.green,
                                priv->fgcol.blue,
                                clutter_actor_get_paint_opacity (actor));

      clutter_actor_get_allocation_box (actor, &allocation);
      clutter_actor_box_get_size (&allocation &width, &height);

      /* paint a rounded rectangle using GL primitives; the area of
       * paint is (0, 0) - (width, height), which means the whole
       * allocation or, if the actor has a fixed size, the size that
       * has been set.
       */
      cogl_path_round_rectangle (0, 0, width, height, foo_actor->radius, 5);

      /* and fill it with the current color */
      cogl_path_fill ();
    }

### Note

When inside the `ClutterActor`::`paint` virtual function the actor is already positioned at the coordinates specified by its parent; all the paint operations should take place from the (0, 0) coordinates.

Container actors or composite actors with internal children should also override the paint method, and call `clutter_actor_paint()` on every visible child:

Example 23. Paint implementation of a container

In this example, `FooActor` is a simple container invoking `clutter_actor_paint()` on every visible child.

    static void
    foo_actor_paint (ClutterActor *actor)
    {
      FooActor *foo_actor = FOO_ACTOR (actor);
      GList *child;

      for (child = foo_actor->children;
           child != NULL;
           child = child->next)
        {
          ClutterActor *child_actor = child->data;

          clutter_actor_paint (child_actor);
        }
    }

If the actor has a non-rectangular shape, or it has internal children that need to be distinguished by the events delivery mechanism, the `ClutterActor`::`pick` virtual function should also be overridden. The `pick` virtual function works exactly like the `paint` virtual function, but the actor should paint just its shape with the passed colour:

Example 24. Pick implementation of a simple actor

In this example, `FooActor` overrides the `pick` virtual function default implementation to paint itself with a shaped silhouette, to allow events only on the actual shape of the actor instead of the whole paint area.

    static void
    foo_actor_pick (ClutterActor       *actor,
                    const ClutterColor *pick_color)
    {
      FooActor *foo_actor = FOO_ACTOR (actor);
      ClutterActorBox allocation = { 0, };
      gfloat width, height;

      /* it is possible to avoid a costly paint by checking whether the
       * actor should really be painted in pick mode
       */
      if (!clutter_actor_should_pick_paint (actor))
        return;

      clutter_actor_get_allocation_box (actor, &allocation);
      clutter_actor_box_get_size (&allocation, &width, &height);

      /* use the passed color to paint ourselves */
      cogl_set_source_color4ub (pick_color->red,
                                pick_color->green,
                                pick_color->blue,
                                pick_color->alpha);

      /* paint a round rectangle */
      cogl_path_round_rectangle (0, 0, width, height, foo_actor->radius, 5);

      /* and fill it with the current color */
      cogl_path_fill ();
    }

Containers should simply chain up to the parent class' `pick` implementation to get their silhouette painted and then paint their children:

Example 25. Pick implementation of a container

In this example, `FooActor` allows the picking of each child it contains, as well as itself.

    static void
    foo_actor_pick (ClutterActor       *actor,
                    const ClutterColor *pick_color)
    {
      FooActor *foo_actor = FOO_ACTOR (actor);

      /* this will paint a silhouette corresponding to the paint box */
      CLUTTER_ACTOR_CLASS (foo_actor_parent_class)->pick (actor, pick_color);

      /* clutter_actor_paint() is context-sensitive, and will perform
       * a pick paint if the scene graph is in pick mode
       */
      clutter_actor_paint (foo_actor->child);
    }

## Implementing Containers

The ClutterContainer interface should be implemented by subclasses of ClutterActor who wants to provide a general API for adding child actors.

If the ClutterActor subclass only handles internal children, or it's not suitable for having generic actors added to it, it should not implement the ClutterContainer interface, but simply use `clutter_actor_set_parent()`:

Example 26. Parenting an actor

In this example, `FooActor` has an internal child of type `BazActor` which is assigned using a specific function called `foo_actor_add_baz()`. The `FooActor` instance takes ownership of the `BazActor` instance and sets the parent-child relationship using `clutter_actor_set_parent()`.

    void
    foo_actor_add_baz (FooActor *foo_actor,
                       BazActor *baz_actor)
    {
      g_return_if_fail (FOO_IS_ACTOR (foo_actor));
      g_return_if_fail (BAZ_IS_ACTOR (baz_actor));

      /* unparent the previous BazActor; this will automatically call
       * g_object_unref() on the actor
       */
      if (foo_actor->baz)
        clutter_actor_unparent (foo_actor->baz);

      foo_actor->baz = baz_actor;

      /* this will cause the initial floating reference of ClutterActor to
       * disappear, and add a new reference on baz_actor. foo_actor has now
       * taken ownership of baz_actor, so that:
       *
       *   foo_actor_add_baz (foo_actor, baz_actor_new ());
       *
       * is a safe statement (no reference is leaked).
       */
      clutter_actor_set_parent (CLUTTER_ACTOR (baz_actor),
                                CLUTTER_ACTOR (foo_actor));

      /* a container should queue a change in the layout */
      clutter_actor_queue_relayout (CLUTTER_ACTOR (foo_actor));

      /* emit a signal and notification */
      g_signal_emit (foo_actor, foo_actor_signals[BAZ_CHANGED], 0, baz_actor);
      g_object_notify (G_OBJECT (foo_actor), "baz");
    }

In order to implement the ClutterContainer interface, these virtual functions must be defined:

ClutterContainer::add | The container actor should hold a pointer to the passed ClutterActor, call `clutter_actor_set_parent()` on it, queue a relayout on itself and then emit the "actor-added" signal to notify handlers of the newly added actor.
ClutterContainer::remove | The container actor should increase the reference count of the passed ClutterActor, remove the pointer held on the child and call `clutter_actor_unparent()` on it; then, emit the "actor-removed" signal and decrease the reference count.
ClutterContainer::foreach | The container should invoke the callback on every child it is holding.
ClutterContainer::foreach_with_internals | The container should invoke the callback on every child it is holding, including eventual private children that should not be handled by the ClutterContainer API. This method can be ignored if the container does not have internal children.
ClutterContainer::raise | The container should move the passed child on top of the given sibling, or on top of the paint stack in case the sibling is NULL. This method can be ignored if the container does not have overlapping children.
ClutterContainer::lower | The container should move the passed child below the given sibling, or on the bottom of the paint stack in case the sibling is NULL. This method can be ignored if the container does not have overlapping children.
ClutterContainer::sort_depth_order | The container should sort the paint stack depending on the relative depths of each child. This method can be ignored if the container does not have overlapping children.