<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"><title>2. Handling key events</title><link rel="stylesheet" type="text/css" href="style.css"><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot"><link rel="home" href="index.html" title="The Clutter Cookbook"><link rel="up" href="events.html" title="Chapter 3. Events"><link rel="prev" href="events.html" title="Chapter 3. Events"><link rel="next" href="events-mouse-scroll.html" title="3. Detecting mouse scrolling on an actor"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">2. Handling key events</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="events.html">Prev</a> </td><th width="60%" align="center">Chapter 3. Events</th><td width="20%" align="right"> <a accesskey="n" href="events-mouse-scroll.html">Next</a></td></tr></table><hr></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="events-handling-key-events"></a>2. Handling key events</h2></div></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="idm140200513092848"></a>2.1. Problem</h3></div></div></div><p>You want to respond to key presses on an actor.</p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="idm140200513091536"></a>2.2. Solutions</h3></div></div></div><p>There are two possible solutions:</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p><span class="emphasis"><em>Solution 1:</em></span> Connect a callback to the
actor; inside the callback, manually analyse which key and
modifier(s) were pressed and react accordingly.</p></li><li class="listitem"><p><span class="emphasis"><em>Solution 2:</em></span> Use an actor's
<span class="type">ClutterBindingPool</span> to declaratively assign
actions to specific key and modifier combinations.</p></li></ol></div><p>Each solution is covered below.</p><div class="section"><div class="titlepage"><div><div><h4 class="title"><a name="idm140200508454672"></a>2.2.1. Solution 1</h4></div></div></div><p>Connect the <span class="emphasis"><em>key-press-event</em></span>
signal for an actor to a callback; then examine the event
in the callback to determine which key and modifiers were
pressed.</p><p>First, connect an actor's
<span class="emphasis"><em>key-press-event</em></span> signal to a callback:</p><div class="informalexample"><pre class="programlisting">g_signal_connect (actor, "key-press-event", G_CALLBACK (_key_press_cb), NULL);</pre></div><p>Then, in the callback, check which key was pressed and which
modifiers were down at the same time. For example, this callback
checks for a press on the up arrow key and whether
the <span class="keycap"><strong>Shift</strong></span> and/or <span class="keycap"><strong>Ctrl</strong></span>
key were down:</p><div class="informalexample"><pre class="programlisting">static gboolean
_key_press_cb (ClutterActor *actor,
ClutterEvent *event,
gpointer user_data)
{
guint keyval = clutter_event_get_key_symbol (event);
ClutterModifierType state = clutter_event_get_state (event);
gboolean shift_pressed = (state & CLUTTER_SHIFT_MASK ? TRUE : FALSE);
gboolean ctrl_pressed = (state & CLUTTER_CONTROL_MASK ? TRUE : FALSE);
if (CLUTTER_KEY_Up == keyval)
{
if (shift_pressed & ctrl_pressed)
g_debug ("Up and shift and control pressed");
else if (shift_pressed)
g_debug ("Up and shift pressed");
else
g_debug ("Up pressed");
/* The event was handled, and the emission should stop */
return CLUTTER_EVENT_STOP;
}
/* The event was not handled, and the emission should continue */
return CLUTTER_EVENT_PROPAGATE;
}</pre></div><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>Clutter provides a range of key value definitions
(like <code class="constant">CLUTTER_KEY_Up</code>, used above). These are
generated from the list in the
<a class="ulink" href="http://cgit.freedesktop.org/xorg/proto/x11proto/plain/keysymdef.h" target="_top">X.Org source code</a>
(replace "XK" with "CLUTTER_KEY" in the definitions there to get the
Clutter equivalents; alternatively, look at the
<code class="filename">clutter-keysyms.h</code> header file for the
list).</p><p><code class="constant">CLUTTER_SHIFT_MASK</code>,
<code class="constant">CLUTTER_CONTROL_MASK</code> and other modifiers are
defined in the <span class="type">ClutterModifierType</span> enum.</p></div></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a name="idm140200509683552"></a>2.2.2. Solution 2</h4></div></div></div><p>Assign actions to an actor's <span class="type">ClutterBindingPool</span>.
A binding pool stores mappings from a key press (either a single key
or a key plus modifiers) to actions; an action is simply a callback
function with a specific signature.</p><p>While this approach is trickier to implement, it is more
flexible and removes the drudgery of writing branching code to
handle different key presses. See the
<a class="link" href="events-handling-key-events.html#events-handling-key-events-discussion" title="2.3. Discussion">Discussion</a>
section for more details.</p><p>To use this approach with an actor which will receive key press
events, first get that actor's binding pool. In the example below,
we're using the binding pool for the default
<span class="type">ClutterStage</span>:</p><div class="informalexample"><pre class="programlisting">ClutterBindingPool *binding_pool;
GObjectClass *stage_class;
stage_class = CLUTTER_STAGE_GET_CLASS (stage);
binding_pool = clutter_binding_pool_get_for_class (stage_class);</pre></div><p>Next, install actions into the binding pool. For example, to
install an action bound to the up arrow key, which calls the
<code class="function">_move_up()</code> function when that key is pressed,
you would do:</p><div class="informalexample"><pre class="programlisting">clutter_binding_pool_install_action (binding_pool,
"move-up", /* identifier */
CLUTTER_KEY_Up, /* up arrow pressed */
0, /* no modifiers pressed */
G_CALLBACK (_move_up),
NULL, /* no user data passed */
NULL);</pre></div><p>Another example, binding up arrow +
<span class="keycap"><strong>Shift</strong></span> + <span class="keycap"><strong>Ctrl</strong></span> to an action
which calls <code class="function">_move_up_shift_control()</code> when
activated:</p><div class="informalexample"><pre class="programlisting">clutter_binding_pool_install_action (binding_pool,
"move-up-shift-control",
CLUTTER_KEY_Up,
CLUTTER_SHIFT_MASK + CLUTTER_CONTROL_MASK,
G_CALLBACK (_move_up_shift_control),
NULL,
NULL);</pre></div><p>The function called when an action is activated looks
like this (for <code class="function">_move_up()</code>):</p><div class="informalexample"><pre class="programlisting">static void
_move_up (GObject *instance,
const gchar *action_name,
guint key_val,
ClutterModifierType modifiers,
gpointer user_data)
{
g_debug ("Up pressed");
}</pre></div><p>Then bind the <span class="emphasis"><em>key-press-event</em></span> signal
for the actor (in our case, the stage) to a callback:</p><div class="informalexample"><pre class="programlisting">g_signal_connect (stage,
"key-press-event",
G_CALLBACK (_key_press_cb),
NULL);</pre></div><p>Finally, inside the callback, pass control to the actor's
binding pool rather than dissecting the key press event
yourself:</p><div class="informalexample"><pre class="programlisting">static gboolean
_key_press_cb (ClutterActor *actor,
ClutterEvent *event,
gpointer user_data)
{
ClutterBindingPool *pool;
pool = clutter_binding_pool_find (G_OBJECT_TYPE_NAME (actor));
return clutter_binding_pool_activate (pool,
clutter_event_get_key_symbol (event),
clutter_event_get_state (event),
G_OBJECT (actor));
}</pre></div><p>Now, when a key + modifiers that have been bound to an action
are pressed on the actor, the appropriate action is activated.</p></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a name="events-handling-key-events-discussion"></a>2.3. Discussion</h3></div></div></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a name="idm140200512094416"></a>2.3.1. Pros and cons of Solution 1 and Solution 2</h4></div></div></div><p>Solution 1 is the simplest (in terms of the amount of code you
have to write for simple cases), but could quickly turn into a mess if
you need many conditions or want to capture many key combinations.
Also, if multiple actors need to respond to key press events, you'll
need similar event dissection code in each callback.</p><p>Solution 2 is more complicated to implement, but scales better
if you have many different key combinations on multiple actors.
The binding pool protects you from the minutiae of detecting which
keys were pressed, leaving you to concentrate on the
triggered actions instead. This could simplify your control
logic.</p><p>In addition, Solution 2 lets you write a single callback to
handle all key press events for all actors. This callback could then
use <code class="function">clutter_binding_pool_find()</code>
(as in the example code) to determine which binding pool to
activate (depending on which actor received the key press
event).</p><p>Finally, a binding pool allows you to block and unblock actions.
This means you can make the response to a key press event conditional
on application state. For example, let's say you wanted the up arrow
key to move an actor, but only when the actor is at the bottom
of the stage. To implement this, you could disable the up arrow key
action in the binding pool initially; then, once the actor reaches the
bottom of the stage, enable the up arrow key action again. While this
is possible with Solution 1, you would have to implement more of the
state management code yourself.</p></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a name="idm140200508424176"></a>2.3.2. Other useful things to know about key press events</h4></div></div></div><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>A <span class="type">ClutterKeyEvent</span> contains only a
<span class="emphasis"><em>single</em></span> key value, plus possibly one
or more modifier keys (like <span class="keycap"><strong>Shift</strong></span>,
<span class="keycap"><strong>Ctrl</strong></span>, <span class="keycap"><strong>Alt</strong></span> etc.).
There are no functions in the Clutter API which return
events for tracking near-simultaneous presses on multiple
keys.</p></li><li class="listitem"><p>By default, the stage receives all key events.
To make another actor receive key events, use
<code class="function">clutter_stage_set_key_focus()</code>:</p><div class="informalexample"><pre class="programlisting">/*
* stage is a ClutterStage instance;
* actor is the ClutterActor instance which should receive key events
*/
clutter_stage_set_key_focus (stage, actor);</pre></div></li></ul></div></div></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="events.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="events.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="events-mouse-scroll.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 3. Events </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 3. Detecting mouse scrolling on an actor</td></tr></table></div></body></html>