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cookbook: Added a recipe for reusing a complex animation

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This recipe explains how to "reuse" the same animation
definition for different actors, by creating a new
instance of a "rig" (empty container) and animation
for the rig each time the animation is required.

An actor is then re-parented to the rig and animated
using it, rather than being animated directly.

JSON is used to define the rig + animator, to make
creating new instances of them simpler. The recipe
also discusses various caveats around using this
approach, rather than directly animating an actor.
This commit is contained in:
Elliot Smith 2010-09-13 14:29:37 +01:00
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doc/cookbook/Makefile.am
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@ -58,6 +58,7 @@ VIDEO_FILES = \
videos/events-mouse-scroll.ogv \ videos/events-mouse-scroll.ogv \
videos/textures-crossfade-two-textures.ogv \ videos/textures-crossfade-two-textures.ogv \
videos/animations-complex.ogv \ videos/animations-complex.ogv \
videos/animations-reuse.ogv \
$(NULL) $(NULL)
EXTRA_DIST = \ EXTRA_DIST = \

378
doc/cookbook/animations.xml
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@ -1438,4 +1438,382 @@ clutter_actor_set_z_rotation_from_gravity (actor,
</section> </section>
<section id="animations-reuse">
<title>Reusing a complex animation on different actors</title>
<section id="animations-reuse-problem">
<title>Problem</title>
<para>You want to apply the same complex animation to several
different actors.</para>
</section>
<section id="animations-reuse-solution">
<title>Solution</title>
<para>Instead of animating each actor separately, create a
<emphasis>rig</emphasis>: an empty container with an associated
animation, which will be animated in lieu of
animating the actor directly. Do this as follows:</para>
<orderedlist>
<listitem>
<para>Initialise the stage and actors, including those
to be animated.</para>
</listitem>
<listitem>
<para>Define a <type>ClutterContainer</type> and a
<type>ClutterAnimator</type> animation to animate it.</para>
</listitem>
<listitem>
<para>When you need to animate an actor:</para>
<orderedlist>
<listitem>
<para>Create an instance of the rig and its animator.</para>
</listitem>
<listitem>
<para>Reparent the actor to the rig.</para>
</listitem>
<listitem>
<para>Run the rig's animation.</para>
</listitem>
</orderedlist>
</listitem>
</orderedlist>
<para>For this solution, we're using
<ulink url="http://json.org/">JSON</ulink> to define the
animation and the user interface elements. For more
details about this approach, see
<link linkend="script-introduction">the chapter
on <type>ClutterScript</type></link>.</para>
<para>Here's an extract of the JSON definition for the stage and
one of five rectangles placed at its left edge (the full definition
is in <link linkend="animations-reuse-example-1">the
appendix</link>):</para>
<informalexample>
<programlisting>
<![CDATA[
[
{
"type" : "ClutterStage",
"id" : "stage",
... stage properties, signal handlers etc. ...
"children" : [
{
"type" : "ClutterRectangle",
"id" : "rect1",
"color" : "white",
"width" : 50,
"height" : 50,
"y" : 50,
"reactive" : true,
"signals" : [
{ "name" : "button-press-event", "handler" : "foo_button_pressed_cb" }
]
},
... more children defined here ...
]
}
]
]]>
</programlisting>
</informalexample>
<para>The key point to note is how a signal handler is defined
for the <code>button-press-event</code>, so that the
<function>foo_button_pressed_cb()</function> function will trigger
the animation when a (mouse) button is pressed on each rectangle.</para>
<para>The second JSON definition includes the rig
(an empty <type>ClutterGroup</type>) and a
<type>ClutterAnimator</type> to animate it. The animation moves the
container across the stage and scales it to twice its original
size. (This is the <link linkend="animations-reuse-example-2">same
code as in the appendix</link>):</para>
<informalexample>
<programlisting>
<xi:include href="examples/animations-reuse-animation.json" parse="text">
<xi:fallback>a code sample should be here... but isn't</xi:fallback>
</xi:include>
</programlisting>
</informalexample>
<para>The remaining parts of the application code load
the user interface definition, setting up the stage and rectangles;
and define the callback. The full code is
<link linkend="animations-reuse-example-3">in the appendix</link>,
but below is the most important part, the callback function:</para>
<informalexample>
<programlisting>
<![CDATA[
gboolean
foo_button_pressed_cb (ClutterActor *actor,
ClutterEvent *event,
gpointer user_data)
{
ClutterScript *ui = CLUTTER_SCRIPT (user_data);
ClutterStage *stage = CLUTTER_STAGE (clutter_script_get_object (ui, "stage"));
ClutterScript *script;
ClutterActor *rig;
ClutterAnimator *animator;
/* load the rig and its animator from a JSON file */
script = clutter_script_new ();
/* use a function defined statically in this source file to load the JSON */
load_script_from_file (script, ANIMATION_FILE);
clutter_script_get_objects (script,
"rig", &rig,
"animator", &animator,
NULL);
/* remove the button press handler from the rectangle */
g_signal_handlers_disconnect_matched (actor,
G_SIGNAL_MATCH_FUNC,
0,
0,
NULL,
foo_button_pressed_cb,
NULL);
/* add a callback to clean up the script when the rig is destroyed */
g_object_set_data_full (G_OBJECT (rig), "script", script, g_object_unref);
/* add the rig to the stage */
clutter_container_add_actor (CLUTTER_CONTAINER (stage), rig);
/* place the rig at the same coordinates on the stage as the rectangle */
clutter_actor_set_position (rig,
clutter_actor_get_x (actor),
clutter_actor_get_y (actor));
/* put the rectangle into the top-left corner of the rig */
clutter_actor_reparent (actor, rig);
clutter_actor_set_position (actor, 0, 0);
/* animate the rig */
clutter_animator_start (animator);
return TRUE;
}
]]>
</programlisting>
</informalexample>
<para>The code creates a new rig and associated animation
at the point when the rectangle is clicked. It then positions the
rig at the same coordinates as the rectangle, reparents
the rectangle to the rig, and starts the rig's animation.</para>
<note>
<para>The signal handler has to be declared non-static and
you must use <code>-export-dynamic</code> as an option to the
compiler, otherwise the function isn't visible to
<type>ClutterScript</type> (as outlined
<link linkend="script-signals">in this recipe</link>).</para>
</note>
<para>This is what the animation looks like:</para>
<inlinemediaobject>
<videoobject>
<videodata fileref="videos/animations-reuse.ogv"/>
</videoobject>
<alt>
<para>Video of a simple reusable animation</para>
</alt>
</inlinemediaobject>
</section>
<section id="animations-reuse-discussion">
<title>Discussion</title>
<para>The above solution reparents an actor to be animated
into a rig (an empty placeholder). The rig is a container
which acts as a temporary parent for the actor we
<emphasis>really</emphasis> want to animate. By animating the rig,
it appears as though the actor inside it is being animated (but
<link linkend="animations-reuse-discussion-rig-not-actor">see
these caveats</link>). This means the same animation can be
easily applied to different actors: create an
instance of the rig, reparent an actor to it, then
run the rig's animation. This is simpler than creating
a separate animation for each actor individually, or
reusing a single <type>ClutterAnimator</type> on different
actors (see
<link linkend="animations-reuse-discussion-one-or-many">this
section</link>).</para>
<para>Using JSON enhances the animation's reusability (it's even
potentially reusable in another application), makes the code
simpler (an animation can be loaded directly from the script),
and makes refactoring easier (the animation can be modified
without recompiling the application code). However, it also puts
some minor limitations on the animation's reusability; namely, you
can only set absolute property values in a JSON animation
definition. This makes JSON less useful in cases where
you need to animate properties relative to their starting
values: for example, "move 50 pixels along the x axis" or
"rotate by 10 degrees more on the z axis". (This type of animation
is probably less portable anyway.) In such cases, the programmable
API may be a better option: see the <type>ClutterAnimator</type>
documentation for examples.</para>
<section id="animations-reuse-discussion-one-or-many">
<title>One animation vs. many</title>
<para>In the sample code, a new instance of the rig and its
animation are created for each actor. One side effect of this
is that all of the actors can animate simultaneously with the
"same" animation. If you don't want this behaviour, but still
want to use a rig approach, you could create a single instance
of the rig and its animation. Then, you could reparent each actor
to it in turn.</para>
<para>To ensure that the rig only animates one actor (or group
of actors) at a time, you could track whether the rig is
currently animating (e.g. by examining the animation's
timeline with <function>clutter_animator_get_timeline()</function>).
Then, if the animation is running, prevent any other actor
from being reparented to the rig.</para>
<para>Note that you would also need to "reset" the rig each time the
animation completed (move it back to the right start values for
its properties), ready to animate the next actor.</para>
</section>
<section id="animations-reuse-discussion-rig-not-actor">
<title>Caveats about animating a rig instead of an actor</title>
<para>There are a few issues to be aware of in cases
where you animate a rig with contained actors, rather than
animating the actor directly:</para>
<itemizedlist>
<listitem>
<para>Animating a rig doesn't <emphasis>always</emphasis>
produce the same visual effect as animating an actor directly.
For example, compare the following cases:</para>
<itemizedlist>
<listitem>
<para>You rotate an actor by 180 degrees in the
<code>y</code> axis, then by 90 degrees in the
<code>z</code> axis. The actor appears to rotate in
a <emphasis>clockwise</emphasis> direction.</para>
</listitem>
<listitem>
<para>You rotate the parent container of an actor
by 180 degrees in the <code>y</code> axis; then rotate
the actor by 90 degrees in the <code>z</code> axis.
The actor appears to rotate in an
<emphasis>anti-clockwise</emphasis> direction. By
rotating the container, the "back" of the
actor faces the view point, so the actor's movement
appears reversed. See
<link linkend="animations-rotating-discussion-direction">this
recipe</link> for more details.</para>
</listitem>
</itemizedlist>
<para>There may be other situations where you get similar
discrepancies.</para>
</listitem>
<listitem>
<para>Animating a rig doesn't change an actor's properties,
but animating the actor does.</para>
<para>When you animate a container rather than the actor
directly, the reported properties of the actor may not
reflect its visual appearance. For example, if you apply
a scale animation to a container, the final scale of
actors inside it (as returned by
<function>clutter_actor_get_scale()</function>) will not
reflect the scaling applied to their container; whereas
directly animating the actors would cause their scale
properties to change.</para>
</listitem>
<listitem>
<para>Reparenting an actor to a rig can cause the actor
to "jump" to the rig's position, unless you align the
actor to the rig first.</para>
<para>Note that in the sample code, the position of the actor
(<code>x</code>, <code>y</code> coordinates) is copied to
the rig before the reparenting happens. The actor is then
reparented to the rig, and positioned in the rig's
top-left corner. So the actor appears to be in the same
position, but is now actually inside a rig at the actor's old
position.</para>
<para>Why bother to do this? Because the rig has a default
position of <code>0,0</code> (top-left of <emphasis>its</emphasis>
container, the stage). If you reparent the actor to the rig,
without first copying the actor's position to the rig, the
actor appears to "jump" to the rig's position.</para>
</listitem>
</itemizedlist>
</section>
</section>
<section id="animations-reuse-examples">
<title>Full example</title>
<note>
<para>The three separate code examples in this section
constitute a single application which implements the above
solution.</para>
</note>
<example id="animations-reuse-example-1">
<title><type>ClutterScript</type> JSON defining several
rectangles with signal handlers</title>
<programlisting>
<xi:include href="examples/animations-reuse-ui.json" parse="text">
<xi:fallback>a code sample should be here... but isn't</xi:fallback>
</xi:include>
</programlisting>
</example>
<example id="animations-reuse-example-2">
<title><type>ClutterScript</type> JSON describing a "rig"
and a <type>ClutterAnimator</type> animation</title>
<programlisting>
<xi:include href="examples/animations-reuse-animation.json" parse="text">
<xi:fallback>a code sample should be here... but isn't</xi:fallback>
</xi:include>
</programlisting>
</example>
<example id="animations-reuse-example-3">
<title>Loading <type>ClutterScript</type> from JSON files
in response to events in a user interface</title>
<programlisting>
<xi:include href="examples/animations-reuse.c" parse="text">
<xi:fallback>a code sample should be here... but isn't</xi:fallback>
</xi:include>
</programlisting>
</example>
</section>
</section>
</chapter> </chapter>

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