mutter/doc/cookbook/actors.xml

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<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<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 position or size change</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",
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G_CALLBACK (on_x_changed),
NULL);
g_signal_connect (actor, "notify::height",
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G_CALLBACK (on_height_changed),
NULL);
g_signal_connect (actor, "notify::depth",
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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",
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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,
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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,
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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 :x, :y, :width and :height 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,
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GParamSpec *pspec,
gpointer user_data)
{
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gint x_value = 0;
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/* Round the X coordinate to the nearest pixel */
x_value = floorf (clutter_actor_get_x (CLUTTER_ACTOR (gobject))) + 0.5;
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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,
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const ClutterActorBox *allocation,
ClutterAllocationFlags flags,
gpointer user_data)
{
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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 :x and :y
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 traslations). 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)
{
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do_my_paint (actor);
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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>