mutter/doc/reference/clutter/subclassing-ClutterActor.xml

<chapter id="clutter-subclassing-ClutterActor">
  <chapterinfo>
    <author>
      <firstname>Emmanuele</firstname>
      <surname>Bassi</surname>
      <affiliation>
        <address>
          <email>ebassi@linux.intel.com</email>
        </address>
      </affiliation>
    </author>
  </chapterinfo>

  <title>Implementing a new actor</title>

  <para>In order to implement a new #ClutterActor subclass the usual
  machinery for subclassing a #GObject should be used:</para>

  <informalexample>
    <programlisting>
#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)
{
}
    </programlisting>
  </informalexample>

  <para>The implementation of an actor roughly depends on what kind
  of actor the class should display.</para>

  <para>The implementation process can be broken down into sections:

    <variablelist>
      <varlistentry>
        <term>size requisition</term>
        <listitem><para>used by containers to know how much space
        an actor requires for itself and its eventual
        children.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term>size allocation</term>
        <listitem><para>used by containers to define how much space
        an actor should have for itself and its eventual
        children.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term>painting and picking</term>
        <listitem><para>the actual actor painting and the "picking"
        done to determine the actors that received events</para></listitem>
      </varlistentry>

      <varlistentry>
        <term>realization and visibility</term>
        <listitem><para>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.</para></listitem>
      </varlistentry>

    </variablelist>

  </para>

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

  <refsect1 id="actor-size-requisition">
    <title>Size requisition</title>

    <para>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.</para>

    <note><para>For instance, an actor that depends on the explicit
    sizing is the #ClutterRectangle actor.</para></note>

    <para>The size requisition is split into two different phases: width
    requisition and height requisition.</para>

    <para>The <classname>ClutterActor</classname>::<function>get_preferred_width</function> and
    <classname>ClutterActor</classname>::<function>get_preferred_height</function> 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 <function>get_preferred_width</function> and
    <function>get_preferred_height</function> virtual functions.</para>

    <para>The <function>get_preferred_width</function> and <function>get_preferred_height</function>
    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.</para>

    <note><para>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.</para></note>

    <para>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
    #ClutterActor:request-mode property.</para>

    <note><para>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
    <classname>ClutterActor</classname>::<function>allocate</function>
    implementation.</para></note>

    <para>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.</para>

    <note><para>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.</para></note>

    <note><para>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.</para></note>

    <example id="clutter-actor-get-width-request-example">
      <title>Width requisition implementation of a container</title>

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

      <para>The <function>get_preferred_height</function> implementation
      would be similar to the <function>get_preferred_width</function>
      implementation, so it is omitted.</para>

      <programlisting>
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-&gt;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,
                                         &amp;child_min,
                                         &amp;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 &lt; min_left)
            min_left = child_x;

          if (child_x &lt; natural_left)
            natural_left = child_x;

          if (child_x + child_min &gt; min_right)
            min_right = child_x + child_min;

          if (child_x + child_natural &gt; 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 &lt; 0)
    min_left = 0;

  if (natural_left &lt; 0)
    natural_left = 0;

  if (min_right &lt; 0)
    min_right = 0;

  if (natural_right &lt; 0)
    natural_right = 0;

  g_assert (min_right &gt;= min_left);
  g_assert (natural_right &gt;= natural_left);

  if (min_width_p)
    *min_width_p = min_right - min_left;

  if (natural_width_p)
    *natural_width_p = natural_right - min_left;
}
      </programlisting>
    </example>

  </refsect1> <!-- actor-size-requisition -->

  <refsect1 id="actor-size-allocation">
    <title>Size allocation</title>

    <para>The <classname>ClutterActor</classname>::<function>allocate</function>
    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 <function>allocate</function> virtual function and call
    clutter_actor_allocate() on each visible child.</para>

    <para>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.</para>

    <note><para>The <function>allocate</function> 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.</para></note>

    <example id="container-allocate-example">
      <title>Allocation of a container</title>
      <para>In this example, <classname>FooActor</classname> 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.</para>
      <programlisting language="C">
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)-&gt;allocate (actor, box, flags);

  clutter_actor_box_get_size (box,
                              &amp;available_width,
                              &amp;available_height);

  current_width = 0;
  current_height = 0;

  row_height = 0;

  for (l = foo_actor-&gt;children;
       l != NULL;
       l = l-&gt;next)
    {
      ClutterActor *child = l-&gt;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", &amp;mode, NULL);
      if (mode == CLUTTER_REQUEST_HEIGHT_FOR_WIDTH)
        {
          gfloat min, natural;

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

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

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

      /* overflow */
      if (current_width + child_width &gt; 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 &gt; 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,
                              &amp;child_box,
                              flags);

      row_height = MAX (row_height, child_height);
    }
}
      </programlisting>
    </example>

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

  </refsect1> <!-- actor-size-allocation -->

  <refsect1 id="actor-painting-and-picking">
    <title>Painting and picking</title>

    <para>The <classname>ClutterActor</classname>::<function>paint</function>
    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.</para>

    <example id="simple-actor-paint-example">
      <title>Paint implementation of a simple actor</title>
      <para>In this example, the <classname>FooActor</classname>
      implementation of the <function>paint</function> virtual function is
      drawing a rectangle with rounded corners with a custom color using the
      Cogl API.</para>
      <programlisting>
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-&gt;fgcol.red,
                            priv-&gt;fgcol.green,
                            priv-&gt;fgcol.blue,
                            clutter_actor_get_paint_opacity (actor));

  clutter_actor_get_allocation_box (actor, &amp;allocation);
  clutter_actor_box_get_size (&amp;allocation &amp;width, &amp;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-&gt;radius, 5);

  /* and fill it with the current color */
  cogl_path_fill ();
}
      </programlisting>
    </example>

    <note><para>When inside the <classname>ClutterActor</classname>::<function>paint</function>
    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.</para></note>

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

    <example id="container-actor-paint-example">
      <title>Paint implementation of a container</title>
      <para>In this example, <classname>FooActor</classname> is a simple
      container invoking clutter_actor_paint() on every visible child.</para>
      <programlisting>
static void
foo_actor_paint (ClutterActor *actor)
{
  FooActor *foo_actor = FOO_ACTOR (actor);
  GList *child;

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

      clutter_actor_paint (child_actor);
    }
}
      </programlisting>
    </example>

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

    <example id="simple-actor-pick-example">
      <title>Pick implementation of a simple actor</title>
      <para>In this example, <classname>FooActor</classname> overrides the
      <function>pick</function> 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.</para>
      <programlisting>
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, &amp;allocation);
  clutter_actor_box_get_size (&amp;allocation, &amp;width, &amp;height);

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

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

  /* and fill it with the current color */
  cogl_path_fill ();
}
      </programlisting>
    </example>

    <para>Containers should simply chain up to the parent class'
    <function>pick</function> implementation to get their silhouette painted
    and then paint their children:</para>

    <example id="container-actor-pick-example">
      <title>Pick implementation of a container</title>
      <para>In this example, <classname>FooActor</classname> allows the
      picking of each child it contains, as well as itself.</para>
      <programlisting>
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)-&gt;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-&gt;child);
}
      </programlisting>
    </example>

  </refsect1> <!-- actor-painting-and-picking -->

  <refsect1 id="implementing-clutter-container">
    <title>Implementing Containers</title>

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

    <para>
    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():
    </para>

    <example id="clutter-actor-set-parent-example">
      <title>Parenting an actor</title>
      <para>In this example, <classname>FooActor</classname> has an internal
      child of type <classname>BazActor</classname> which is assigned using a
      specific function called foo_actor_add_baz(). The
      <classname>FooActor</classname> instance takes ownership of the
      <classname>BazActor</classname> instance and sets the parent-child
      relationship using clutter_actor_set_parent().</para>
      <programlisting>
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-&gt;baz)
    clutter_actor_unparent (foo_actor-&gt;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");
}
      </programlisting>
    </example>

    <para>
    In order to implement the #ClutterContainer interface, these virtual
    functions must be defined:

      <variablelist>
        <varlistentry>
          <term>ClutterContainer::add</term>
          <listitem>
            <para>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
            #ClutterContainer::actor-added signal to notify
            handlers of the newly added actor.</para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term>ClutterContainer::remove</term>
          <listitem>
            <para>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
            #ClutterContainer::actor-removed signal and decrease the
            reference count.</para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term>ClutterContainer::foreach</term>
          <listitem>
            <para>The container should invoke the callback on every
            child it is holding.</para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term>ClutterContainer::foreach_with_internals</term>
          <listitem>
            <para>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.</para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term>ClutterContainer::raise</term>
          <listitem>
            <para>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.</para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term>ClutterContainer::lower</term>
          <listitem>
            <para>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.</para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term>ClutterContainer::sort_depth_order</term>
          <listitem>
            <para>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.</para>
          </listitem>
        </varlistentry>
      </variablelist>

    </para>

  </refsect1> <!-- implementing-clutter-container -->

</chapter>