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cookbook: Add recipe about handling button events
Recipe covers adding handlers for button-press-event and button-release-event signals on actors, and how to examine the content of a ClutterButtonEvent via API functions. The discussion section explains about click count (the criteria for how clicks get counted, including distance and time settings); how button numbers are reported; and how to use ClutterClickAction as an alternative for press + release in certain scenarios.
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Elliot Smith
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@ -1021,4 +1021,392 @@ ClutterStage *stage = clutter_event_get_stage (event);
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</section>
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</section>
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<section id="events-buttons">
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<title>Making an actor respond to button events</title>
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<section>
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<title>Problem</title>
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<para>You want an actor to respond to button events. These might
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be buttons on an input device like a mouse;
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or input events caused by other means, like touches on a screen.</para>
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<para>Some examples of where this is useful:</para>
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<itemizedlist>
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<listitem>
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<para>For implementing button widgets which respond to
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button clicks.</para>
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</listitem>
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<listitem>
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<para>To make actor selections by mouse click (e.g.
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as part of a drawing application).</para>
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</listitem>
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<listitem>
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<para>To recognise a button press followed by pointer
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motion and button release (e.g. to implement drag
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and drop or kinetic animations).</para>
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</listitem>
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</itemizedlist>
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</section>
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<section>
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<title>Solution</title>
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<para>Connect a handler to the <emphasis>button-press-event</emphasis>
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and/or <emphasis>button-release-event</emphasis> signals of an
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actor.</para>
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<note>
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<para>The <emphasis>button-press-event</emphasis> is emitted
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when a button is pressed (not necessarily released) on a reactive
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actor; the <emphasis>button-release-event</emphasis> when a
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button is released on a reactive actor (even if the button was
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pressed down somewhere else).</para>
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</note>
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<para>First, ensure the actor is reactive:</para>
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<informalexample>
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<programlisting>
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clutter_actor_set_reactive (actor, TRUE);
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</programlisting>
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</informalexample>
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<para>Next, create a function to handle the signal(s) you are
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interested in. The function signature is the same for both the
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press and release signals:</para>
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<informalexample>
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<programlisting>
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gboolean
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callback_function (ClutterActor *actor,
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ClutterEvent *event,
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gpointer user_data);
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</programlisting>
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</informalexample>
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<para>You can use a single function as the
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callback for both signals (or write a different one for each signal).
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Here's an example function which can be used as a callback
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for both press and release signals, as it simply pulls data
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out of the event and displays it:</para>
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<informalexample>
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<programlisting>
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/* <varname>event</varname> is a <type>ClutterButtonEvent</type>
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* for both the press and the release signal; it contains
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* data about where the event occurred
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*/
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static gboolean
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button_event_cb (ClutterActor *actor,
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ClutterEvent *event,
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gpointer user_data)
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{
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gfloat x, y;
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gchar *event_type;
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guint button_pressed;
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ClutterModifierType state;
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gchar *ctrl_pressed;
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guint32 click_count;
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/* where the pointer was (relative to the stage)
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* when the button event occurred; use
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* <function>clutter_actor_transform_stage_point()</function>
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* to transform to actor-relative coordinates
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*/
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clutter_event_get_coords (event, &x, &y);
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/* check whether it was a press or release event */
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event_type = "released";
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if (clutter_event_type (event) == CLUTTER_BUTTON_PRESS)
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event_type = "pressed";
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/* which button triggered the event */
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button_pressed = clutter_event_get_button (event);
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/* keys down when the event occurred;
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* this is a bit mask composed of the bits for each key held down
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* when the button was pressed or released; see the
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* <type>ClutterModifierType</type> enum in the Clutter API docs
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* for a list of the available modifiers
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*/
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state = clutter_event_get_state (event);
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ctrl_pressed = "ctrl not pressed";
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if (state & CLUTTER_CONTROL_MASK)
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ctrl_pressed = "ctrl pressed";
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/* click count */
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click_count = clutter_event_get_click_count (event);
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g_debug ("button %d was %s at %.0f,%.0f; %s; click count %d",
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button_pressed,
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event_type,
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x,
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y,
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ctrl_pressed,
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click_count);
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return TRUE;
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}
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</programlisting>
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</informalexample>
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<para>Finally, connect the signals to the function(s):</para>
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<informalexample>
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<programlisting>
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/* connect the press event */
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g_signal_connect (actor,
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"button-press-event",
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G_CALLBACK (button_event_cb),
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NULL);
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/* connect the release event */
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g_signal_connect (actor,
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"button-release-event",
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G_CALLBACK (button_event_cb),
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NULL);
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</programlisting>
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</informalexample>
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<para>Pressing or releasing a button on the actor will now
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trigger a call to the <function>button_event_cb()</function>
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function. See <link linkend="events-buttons-example-1">the full
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example</link> for more details.</para>
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</section>
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<section>
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<title>Discussion</title>
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<para>Properties of the <type>ClutterButtonEvent</type>
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emitted by both signals should be examined using the
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<function>clutter_event_*</function> functions (rather than struct
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members directly), as in the example above. While most of these
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functions are self-explanatory, a couple require more explanation:
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see the sections below.</para>
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<para>Also covered below is an alternative approach to handling a
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button press followed by a release on a single actor: by adding
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a <type>ClutterClickAction</type> to an actor. See
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<link linkend="events-buttons-clutterclickaction">this section</link>
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for details.</para>
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<para>Finally, a <link linkend="events-buttons-example-3">longer
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example</link> is included, showing how to make use of button press,
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button release and pointer events in a simple drawing application.</para>
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<section>
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<title>Click count</title>
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<para>The click count records the number of times a press/release
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pair occurred in sequence. You can retrieve it via the
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<function>clutter_event_get_click_count()</function> function.</para>
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<note>
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<para>A press/release pair is effectively a click, so this term
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will be used from now on throughout this section,
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to make the explanation simpler. However, the click count has
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nothing to do with <type>ClutterClickActions</type>, described
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<link linkend="events-buttons-clutterclickaction">later</link>.
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</para>
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</note>
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<para>For clicks to be considered part of the same sequence (for
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the purposes of counting), all the clicks after the first one
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must occur within the global <varname>double_click_distance</varname>
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(pixels) of the first click; and the time between click
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<code>n</code> and click <code>n+1</code> must be <code><=</code>
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the global <varname>double_click_time</varname> (milliseconds).</para>
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<para>The clicks <emphasis>do not</emphasis> have to occur on
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the same actor: providing they occur within the double click
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distance and time, they are counted as part of the same click
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sequence. Also note that the clicks don't even have to happen
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on a reactive actor: providing they happen somewhere on the
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stage, they will still increment the click count.</para>
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<para>The default double click time and distance are
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stored in the <type>ClutterSettings</type> associated
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with an application. You can get/set their values like this:</para>
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<informalexample>
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<programlisting>
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gint double_click_distance;
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gint double_click_time;
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ClutterSettings *settings = clutter_settings_get_default ();
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/* get double click settings */
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g_object_get (settings,
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"double-click-distance", &double_click_distance,
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"double-click-time", &double_click_time,
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NULL);
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/* set */
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g_object_set (settings,
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"double-click-distance", 50,
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"double-click-time", 1000,
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NULL);
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</programlisting>
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</informalexample>
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</section>
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<section>
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<title>Button numbering</title>
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<para><function>clutter_event_get_button()</function> returns
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an integer representing the pressed or released button.</para>
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<para>In the case of a standard scroll mouse, the numbers
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returned are reliable across different hardware models:</para>
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<itemizedlist>
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<listitem>
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<para>1 = left mouse button</para>
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</listitem>
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<listitem>
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<para>2 = scroll wheel</para>
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</listitem>
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<listitem>
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<para>3 = right mouse button</para>
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</listitem>
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</itemizedlist>
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<para>For mice with more buttons, or other types of
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input devices, the mappings may not be so
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straightforward: you may have to experiment to see
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which button returns which value.</para>
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</section>
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<section id="events-buttons-clutterclickaction">
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<title><type>ClutterClickAction</type></title>
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<para><type>ClutterActions</type> add flexible event handling
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to <type>ClutterActors</type>. They recognise and abstract
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common sequences of low-level events into a single, more easily
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managed high-level event. In the case of a
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<type>ClutterClickAction</type>, the abstraction is over
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a press followed by a release on a single actor. This is
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achieved by "synthesising" the press and release signals on
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the actor: in other words, the action captures those
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two signals when emitted by a single actor; and, once captured, the
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action emits a single <emphasis>clicked</emphasis> signal
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<emphasis>instead</emphasis> of the two signals being
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emitted by the actor.</para>
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<para>The pointer can move off the actor between the press and
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release, but the press and release must both occur on the same
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actor, with no intervening presses or releases on other
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actors. In addition, there are no maximum distance or time
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constraints on the press and release.</para>
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<para>If a press occurs and you want to force it to be released
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(e.g. to break a pointer grab after a certain length of
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time has elapsed), use
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<function>clutter_click_action_release()</function>.</para>
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<para>On the down side, the <emphasis>clicked</emphasis> signal
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doesn't present the same detailed <type>ClutterButtonEvent</type>
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to the handler. So, for example, you can't get a click count from a
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<type>ClutterClickAction</type> (though you could count
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the clicks yourself, of course); and you don't have access
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to the coordinates where the press or release occurred.</para>
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<para>To add a click action to a <type>ClutterActor</type>:</para>
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<informalexample>
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<programlisting>
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ClutterAction *action = clutter_click_action_new ();
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clutter_actor_add_action (actor, action);
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</programlisting>
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</informalexample>
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<note>
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<para>An actor must still be set to reactive so that its
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signals can be routed to a click action.</para>
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</note>
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<para>Create a handler function (note the function
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signature is different from the one for the press or
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releas signal handler):</para>
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<informalexample>
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<programlisting>
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void
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clicked_cb (ClutterClickAction *action,
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ClutterActor *actor,
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gpointer user_data)
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{
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/* display the number of the clicked button (equivalent
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* to the number returned by clutter_event_get_button())
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*/
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g_debug ("Button %d clicked", clutter_click_action_get_button (action));
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}
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</programlisting>
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</informalexample>
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<para>Connect the signal to the handler:</para>
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<informalexample>
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<programlisting>
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g_signal_connect (action,
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"clicked",
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G_CALLBACK (clicked_cb),
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NULL);
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</programlisting>
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</informalexample>
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<para>The <link linkend="events-buttons-example-2">example
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code</link> gives a bit more detail about how to use click
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actions.</para>
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</section>
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</section>
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<section>
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<title>Full examples</title>
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<example id="events-buttons-example-1">
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<title>Examining properties of a <type>ClutterButtonEvent</type></title>
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<programlisting>
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<xi:include href="examples/events-buttons.c" parse="text">
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<xi:fallback>a code sample should be here... but isn't</xi:fallback>
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</xi:include>
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</programlisting>
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</example>
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<example id="events-buttons-example-2">
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<title>Using <type>ClutterClickAction</type> to capture
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button events on an actor</title>
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<programlisting>
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<xi:include href="examples/events-buttons-click.c" parse="text">
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<xi:fallback>a code sample should be here... but isn't</xi:fallback>
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</xi:include>
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</programlisting>
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</example>
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<example id="events-buttons-example-3">
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<title>Using button and pointer events for drawing</title>
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<para>This code was inspired by
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<ulink url="http://git.clutter-project.org/cluttersmith/">ClutterSmith</ulink>
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</para>
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<programlisting>
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<xi:include href="examples/events-buttons-lasso.c" parse="text">
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<xi:fallback>a code sample should be here... but isn't</xi:fallback>
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</xi:include>
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</programlisting>
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</example>
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</section>
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</section>
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</chapter>
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