mutter/doc/cookbook/actors.xml

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<chapter id="actors">
  <title>Actors</title>

  <epigraph>
    <attribution>Edmon Gween, actor, on his deathbed</attribution>
    <para>An actor's a guy who if you ain't talkin' about him, ain't
    listening.</para>
  </epigraph>

  <section id="actors-introduction">
    <title>Introduction</title>

    <para>When building a User Interface with Clutter, the visible part
    of the UI &mdash; that is, what is displayed on the screen &mdash; is
    commonly referred to as "the scene graph". Like every graph, a scene
    graph is composed by nodes.</para>

    <para>Every node on the Clutter scene graph is an
    <emphasis>actor</emphasis>. Every actor has a single relationship
    with the others: it can be the parent of another actor, or a child of
    another actor.</para>

    <note><para>The stage is an actor that can have children but cannot have
    any parent.</para></note>

    <para>Actors have different attributes: a position, a size, a
    scale factor, a rotation angle on each axis (relative to a specific
    center on the normal plane for that axis), an opacity factor.</para>

    <para>The scene graph is not fixed: it can be changed, not only
    by adding or removing actors, but also by changing the parent-child
    relationship: it is possible, for instance, to move an entire
    section of the scene graph from one parent actor to another.</para>

  </section>

  <section id="actors-allocation-notify">
    <title>Knowing when an actor's position or size changes</title>

    <section>
      <title>Problem</title>

      <para>You want to know when the position or the size, or
      both, of an actor change, for instance to update an unrelated
      actor or some internal state.</para>
    </section>

    <section>
      <title>Solution</title>

      <para>You can use the <emphasis>notify</emphasis> signal,
      detailed with the coordinate or the dimension you want
      to know has changed:</para>

      <informalexample>
        <programlisting>
g_signal_connect (actor, "notify::x",
                  G_CALLBACK (on_x_changed),
                  NULL);
g_signal_connect (actor, "notify::height",
                  G_CALLBACK (on_height_changed),
                  NULL);
g_signal_connect (actor, "notify::depth",
                  G_CALLBACK (on_depth_changed),
                  NULL);
        </programlisting>
      </informalexample>

      <para>If you want to know if any of the coordinates or dimensions of
      an actor have been changed, except for depth, you can use the
      <emphasis>allocation-changed</emphasis> signal:</para>

      <informalexample>
        <programlisting>
g_signal_connect (actor, "allocation-changed",
                  G_CALLBACK (on_allocation_changed),
                  NULL);
        </programlisting>
      </informalexample>

      <para>The signature for the handler of the "notify" signal is:</para>

      <informalexample>
        <programlisting>
void
on_notify (GObject    *gobject,
           GParamSpec *pspec,
           gpointer    user_data);
        </programlisting>
      </informalexample>

      <para>While the signature for the handler of the "allocation-changed"
      signal is:</para>

      <informalexample>
        <programlisting>
void
on_allocation_changed (ClutterActor           *actor,
                       const ClutterActorBox  *allocation,
                       ClutterAllocationFlags  flags,
                       gpointer                user_data);
        </programlisting>
      </informalexample>

    </section>

    <section>
      <title>Discussion</title>

      <para>Any change the position and size of an actor will cause a
      change in the allocation of the actor itself. This will update the
      values of the <property>x</property>, <property>y</property>,
      <property>width</property> and <property>height</property>
      properties as well.</para>

      <para>The first technique allows a greater deal of granularity,
      allowing you to know what exactly changed. Inside the callback
      for the signal you can query the value of the property:</para>

      <informalexample>
        <programlisting>
void
on_x_changed (GObject    *gobject,
              GParamSpec *pspec,
              gpointer    user_data)
{
  gint x_value = 0;

  /* Round the X coordinate to the nearest pixel */
  x_value = floorf (clutter_actor_get_x (CLUTTER_ACTOR (gobject))) + 0.5;

  g_print ("The new X coordinate is '%d' pixels\n", x_value);
}
        </programlisting>
      </informalexample>

      <para>The second technique is more indicated if you want to
      get notification that any of the positional or dimensional
      attributes changed, except for the depth:</para>

      <informalexample>
        <programlisting>
void
on_allocation_changed (ClutterActor           *actor,
                       const ClutterActorBox  *allocation,
                       ClutterAllocationFlags  flags,
                       gpointer                user_data)
{
  g_print ("The bounding box is now: (%.2f, %.2f) (%.2f x %.2f)\n",
           clutter_actor_box_get_x (allocation),
           clutter_actor_box_get_y (allocation),
           clutter_actor_box_get_width (allocation),
           clutter_actor_box_get_height (allocation));
}
        </programlisting>
      </informalexample>

      <para>All actors will update these properties when their size
      or position change.</para>

      <para>Note that the stage, on the other hand, will not notify on
      position changes, so it is not possible to use the
      <property>x</property> and <property>y</property>
      properties to know that the platform-specific window embedding the
      stage has been moved &mdash; if the platform supports a windowing
      system. In order to achieve that you will have to use backend-specific
      API to extract the surface used by the stage and then platform-specific
      API to retrieve its coordinates.</para>

    </section>

  </section>

  <section id="actors-paint-wrappers">
    <title>Overriding the paint sequence</title>

    <section>
      <title>Problem</title>

      <para>You want to override the way an actor paints itself
      without creating a subclass.</para>
    </section>

    <section>
      <title>Solution</title>

      <para>You can use the <emphasis>paint</emphasis> signal to
      invoke a callback that will be executed before the actor's
      paint implementation:</para>

      <informalexample>
        <programlisting>
g_signal_connect (actor, "paint", G_CALLBACK (on_paint), NULL);
        </programlisting>
      </informalexample>

      <para>You can paint something after the actor's paint implementation
      by using the <function>g_signal_connect_after()</function> function
      instead of <function>g_signal_connect()</function>:</para>

      <informalexample>
        <programlisting>
g_signal_connect_after (actor, "paint", G_CALLBACK (on_paint_after), NULL);
        </programlisting>
      </informalexample>

      <para>The signature for the handler of the "paint" signal is:</para>

      <informalexample>
        <programlisting>
void on_paint (ClutterActor *actor, gpointer user_data);
        </programlisting>
      </informalexample>

    </section>

    <section>
      <title>Discussion</title>

      <para>The paint cycle in Clutter works its way recursively from the
      stage through every child.</para>

      <para>Whenever an Actor is going to be painted it will be positioned in
      a new frame of reference according to the list of transformations
      (scaling, rotation and additional translations). After that, the "paint"
      signal will be emitted.</para>

      <para>The "paint" signal is defined as <emphasis>run-last</emphasis>,
      that is the signal handlers connected to it using
      <function>g_signal_connetc()</function> will be called first; then the
      default handler defined by the Actor's sub-class will be called;
      finally, all the signal handlers connected to the signal using
      <function>g_signal_connect_after()</function> will be called.</para>

      <para>This allows pre- and post-default paint handlers, and it also
      allows completely overriding the way an Actor draws itself by default;
      for instance:</para>

      <informalexample>
        <programlisting>
void
on_paint (ClutterActor *actor)
{
  do_my_paint (actor);

  g_signal_stop_emission_by_name (actor, "paint");
}
        </programlisting>
      </informalexample>

      <para>The code above will prevent the default paint implementation of
      the actor from running.</para>
    </section>

  </section>

</chapter>