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cookbook: Added recipe for handling pointer events on an actor

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Added a recipe about handling enter, leave, and motion events
on an actor.

Gives some pointers to data available from motion events,
explains a bit about stage-relative and actor-relative coords,
and covers how overlapping actors and reactivity of actors
can affect events occurring.

Examples include a simple scribble app showing how to integrate
pointer events into a more useful context.
This commit is contained in:
Elliot Smith 2010-08-20 13:54:04 +01:00
parent c480e5ec00
commit 81481cd803
3 changed files with 324 additions and 0 deletions
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1
doc/cookbook/Makefile.am
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@ -38,6 +38,7 @@ IMAGE_FILES = \
images/text-shadow.png \
images/textures-sub-texture.png \
images/layouts-stacking-diff-actor-sizes.png \
images/events-pointer-motion-stacking.png \
$(NULL)
VIDEO_FILES = \
videos/animations-fading-out.ogv \

323
doc/cookbook/events.xml
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@ -698,4 +698,327 @@ g_signal_connect (viewport,
</section>
</section>
<section id="events-pointer-motion">
<title>Detecting pointer movements on an actor</title>
<section id="events-pointer-motion-problem">
<title>Problem</title>
<para>You want to be able to tell when the pointer (e.g. associated
with a mouse or touches on a screen) enters, leaves, or moves over
an actor.</para>
<para>Example use cases include:</para>
<itemizedlist>
<listitem>
<para>Adding a tooltip or hover effect to an actor when
a pointer moves onto it.</para>
</listitem>
<listitem>
<para>Tracing the path of the pointer over an actor (e.g.
in a drawing application).</para>
</listitem>
</itemizedlist>
</section>
<section id="events-pointer-motion-solution">
<title>Solution</title>
<para>Connect to the pointer motion signals emitted by the actor.</para>
<section>
<title>Responding to crossing events</title>
<para>To detect the pointer crossing the boundary of an actor
(entering or leaving), connect to the <code>enter-event</code>
and/or <code>leave-event</code> signals. For example:</para>
<informalexample>
<programlisting>
ClutterActor *actor = clutter_texture_new ();
/* ...set size, color, image etc., depending on the actor... */
/* make the actor reactive: see <link linkend="events-pointer-motion-discussion">Discussion</link> for more details */
clutter_actor_set_reactive (actor, TRUE);
/* connect to the signals */
g_signal_connect (actor,
"enter-event",
G_CALLBACK (_pointer_enter_cb),
NULL);
g_signal_connect (actor,
"leave-event",
G_CALLBACK (_pointer_leave_cb),
NULL);
</programlisting>
</informalexample>
<para>The signature for callbacks connected to each of these
signals is:</para>
<informalexample>
<programlisting>
gboolean
_on_crossing (ClutterActor *actor,
ClutterEvent *event,
gpointer user_data)
</programlisting>
</informalexample>
<para>In the callback, you can examine the event to get the
coordinates where the pointer entered or left the actor. For
example, <function>_pointer_enter_cb()</function> could
follow this template:</para>
<informalexample id="events-pointer-motion-callback-example">
<programlisting>
<![CDATA[
/* the event passed to the callback is of type ClutterCrossingEvent */
static gboolean
_pointer_enter_cb (ClutterActor *actor,
ClutterEvent *event,
gpointer user_data)
{
/* get the coordinates where the pointer crossed into the actor */
gfloat stage_x, stage_y;
clutter_event_get_coords (event, &stage_x, &stage_y);
/*
* as the coordinates are relative to the stage, rather than
* the actor which emitted the signal, it can be useful to
* transform them to actor-relative coordinates
*/
gfloat actor_x, actor_y;
clutter_actor_transform_stage_point (actor,
stage_x, stage_y,
&actor_x, &actor_y);
g_debug ("pointer at stage x %.0f, y %.0f; actor x %.0f, y %.0f",
stage_x, stage_y,
actor_x, actor_y);
return TRUE;
}
]]>
</programlisting>
</informalexample>
<para>See <link linkend="events-pointer-motion-example-1">the
code example in the appendix</link> for an example of how
you can implement a hover effect on a "button" (rectangle
with text overlay) using this approach.</para>
</section>
<section>
<title>Responding to motion events</title>
<para>Motion events occur when a pointer moves over an actor;
the actor emits a <code>motion-event</code> signal when this
happens. To respond to motion events, connect to this signal:</para>
<informalexample>
<programlisting>
/* set up the actor, make reactive etc., as above */
/* connect to motion-event signal */
g_signal_connect (actor,
"motion-event",
G_CALLBACK (_pointer_motion_cb),
transitions);
</programlisting>
</informalexample>
<para>The signature of the callback is the same as for
the <code>enter-event/leave-event</code> signals, so you can use
<link linkend="events-pointer-motion-callback-example">code
similar to the above</link> to handle it. However, the
type of the event is a <type>ClutterMotionEvent</type>
(rather than a <type>ClutterCrossingEvent</type>).</para>
</section>
</section>
<section id="events-pointer-motion-discussion">
<title>Discussion</title>
<para>A few more useful things to know about pointer motion
events:</para>
<itemizedlist>
<listitem>
<para>Each crossing event is accompanied by a motion event at
the same coordinates.</para>
</listitem>
<listitem>
<para>Before an actor will emit signals for pointer events,
it needs to be made reactive with:</para>
<informalexample>
<programlisting>
clutter_actor_set_reactive (actor, TRUE);
</programlisting>
</informalexample>
</listitem>
<listitem>
<para>A pointer event structure includes other data. Some
examples:</para>
<informalexample>
<programlisting>
/* keys and mouse buttons pressed down when the pointer moved */
ClutterModifierType modifiers = clutter_event_get_state (event);
/* time (since the epoch) when the event occurred */
guint32 event_time = clutter_event_get_time (event);
/* actor where the event originated */
ClutterActor *actor = clutter_event_get_actor (event);
/* stage where the event originated */
ClutterStage *stage = clutter_event_get_stage (event);
</programlisting>
</informalexample>
<para>There's no need to cast the event to use these
functions: they will work on any <type>ClutterEvent</type>.</para>
</listitem>
<listitem>
<para>The coordinates of an event (as returned by
<function>clutter_event_get_coords()</function>) are relative
to the stage where they originated, rather than the actor. Unless
the actor is the same size as the stage, you'll typically want
the actor-relative coordinates instead. To get those, use
<function>clutter_actor_transform_stage_point()</function>.</para>
</listitem>
</itemizedlist>
<para>The <link linkend="events-pointer-motion-example-4">simple
scribble application</link> gives a more
thorough example of how to integrate pointer events into a
Clutter application (in this case, for drawing on a
<type>ClutterTexture</type>).</para>
<para>The effect of actor depth on pointer motion events is
worth slightly deeper discussion, and is covered next.</para>
<section>
<title>Pointer events on actors at different depths</title>
<para>If you have actors stacked on top of each other, the
reactive actor nearest the "top" is the one
which emits the signal (when the pointer crosses into or moves
over it). "Top" here means either at the top of
the depth ordering (if all actors are at the same depth)
or the closest to the view point (if actors have different
depths in the <code>z</code> axis).</para>
<para>Here's an example of three rectangles overlapping each
other:</para>
<screenshot>
<mediaobject>
<imageobject>
<imagedata format="PNG"
fileref="images/events-pointer-motion-stacking.png" />
</imageobject>
<alt>
<para>Pointer events in actors with different depth ordering</para>
</alt>
</mediaobject>
</screenshot>
<para>The rectangles are all at the same point on the
<code>z</code> axis but stacked (different positions in the depth
order). They have the following properties:</para>
<itemizedlist>
<listitem>
<para>The <emphasis>red</emphasis> rectangle is lowest down
the depth ordering and reactive. Pointer motion signals are
emitted by this actor when the pointer crosses or moves on the
area of the rectangle <emphasis>not</emphasis> overlapped by the
green rectangle.</para>
</listitem>
<listitem>
<para>The <emphasis>green</emphasis> rectangle is in the
middle of the depth ordering and reactive. This actor emits
events over its whole surface, even though it is overlapped
by the blue rectangle (as the blue rectangle is not
reactive).</para>
<para>Even if the blue rectangle were fully opaque, a pointer
crossing into or moving on the green rectangle's area (even if
obscured by the blue rectangle) would still cause a signal
to be emitted.</para>
</listitem>
<listitem>
<para>The <emphasis>blue</emphasis> rectangle is at the top
of the depth ordering and <emphasis>not</emphasis> reactive.
This actor doesn't emit any pointer motion signals and doesn't
block events from occurring on any other actor.</para>
</listitem>
</itemizedlist>
<para>See <link linkend="events-pointer-motion-example-3">the
sample code in the appendix</link> for more details.</para>
</section>
</section>
<section>
<title>Full examples</title>
<example id="events-pointer-motion-example-1">
<title>Simple button with a hover animation (change in opacity
as the pointer enters and leaves it)</title>
<programlisting>
<xi:include href="examples/events-pointer-motion-crossing.c" parse="text">
<xi:fallback>a code sample should be here... but isn't</xi:fallback>
</xi:include>
</programlisting>
</example>
<example id="events-pointer-motion-example-2">
<title>Detecting pointer motion on a <type>ClutterRectangle</type></title>
<programlisting>
<xi:include href="examples/events-pointer-motion.c" parse="text">
<xi:fallback>a code sample should be here... but isn't</xi:fallback>
</xi:include>
</programlisting>
</example>
<example id="events-pointer-motion-example-3">
<title>How actors influence pointer events on each other</title>
<programlisting>
<xi:include href="examples/events-pointer-motion-stacked.c" parse="text">
<xi:fallback>a code sample should be here... but isn't</xi:fallback>
</xi:include>
</programlisting>
</example>
<example id="events-pointer-motion-example-4">
<title>Scribbling on a <type>ClutterTexture</type> in response
to pointer events</title>
<programlisting>
<xi:include href="examples/events-pointer-motion-scribbler.c" parse="text">
<xi:fallback>a code sample should be here... but isn't</xi:fallback>
</xi:include>
</programlisting>
</example>
</section>
</section>
</chapter>

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