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|
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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Widget Concepts
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<div class="SECT1">
<h1 class="SECT1">
<a name="Z57">Widget Concepts</a>
</h1>
<p>
This section discusses concepts that apply to all widgets,
including memory management and certain special states
widgets can be in. It's a "conceptual" section; however,
the concepts are very important to practical topics covered
later in the book.
</p>
<div class="SECT2">
<h2 class="SECT2">
<a name="WIDGETLIFECYCLE">Widget Life Cycle</a>
</h2>
<p>
Widget resource and memory management is mostly
automatic. However, there are a couple of "gotchas" to
keep in mind if you're doing more complicated things.
</p>
<div class="FIGURE">
<a name="FL-WIDGETDESTROY"></a>
<div class="FUNCSYNOPSIS">
<a name="FL-WIDGETDESTROY.SYNOPSIS"></a>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="FUNCSYNOPSISINFO">
#include <gtk/gtkwidget.h>
</pre>
</td>
</tr>
</table>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">gtk_widget_destroy</tt></code>(GtkWidget*
<tt class="PARAMETER"><i>widget</i></tt>);</code>
</p>
</div>
<p>
<b>Figure 21. Widget Destruction</b>
</p>
</div>
<p>
A widget can be destroyed at any time by calling <tt
class="FUNCTION">gtk_widget_destroy()</tt> (shown in <a
href="z57.html#FL-WIDGETDESTROY">Figure 21</a>);
destroying a widget frees any associated memory and other
resources. If the widget is inside a container, it is
automatically removed from the container before it's
destroyed. It's worth noting that <tt class="FUNCTION">
gtk_widget_destroy()</tt> is simply another name for <tt
class="FUNCTION">gtk_object_destroy()</tt>; <span class=
"STRUCTNAME">GtkObject</span>s have "virtual destructors"
so <tt class="FUNCTION">gtk_object_destroy()</tt> will
always do the right thing.
</p>
<p>
Internally, a reference count is maintained for all
widgets (actually, all <span class="STRUCTNAME">
GtkObject</span>s). Objects begin their life with a
reference count of 1, even though they have not yet been
referenced. At this stage the object is said to be <i
class="FIRSTTERM">floating</i> and is flagged as such. It
is possible to remove the object's initial reference;
this is called <i class="FIRSTTERM">sinking</i> the
floating object and will destroy the object if the
floating reference was the only one.
</p>
<p>
Containers first reference and then sink any floating
widgets that are added to them. By sinking a widget, a
container "takes ownership" of it for resource management
purposes. Thus, the reference count of the widget remains
1, but the object is no longer flagged as floating. When
a widget is removed from a container---or the container
is destroyed---the reference count is decremented to 0.
When an object's reference count reaches 0, it is
destroyed.
</p>
<p>
In practice, this means that you only have to destroy
toplevel widgets; any widgets that are inside a container
will be destroyed along with the container.
</p>
<p>
There's a danger here, however. Sometimes you want to
remove a widget from a container; perhaps some element of
your interface is optional or only appears under certain
circumstances. When you remove the widget (using <tt
class="FUNCTION">gtk_container_remove()</tt>), it will be
unreferenced, its reference count will drop to 0, and it
will be destroyed. To avoid this situation, you should
add a reference to the widget before you remove it. <a
href="z57.html#FL-REFCOUNTS">Figure 22</a> lists the
functions to manipulate reference counts.
</p>
<div class="FIGURE">
<a name="FL-REFCOUNTS"></a>
<div class="FUNCSYNOPSIS">
<a name="FL-REFCOUNTS.SYNOPSIS"></a>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="FUNCSYNOPSISINFO">
#include <gtk/gtkobject.h>
</pre>
</td>
</tr>
</table>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">gtk_object_ref</tt></code>(GtkObject* <tt
class="PARAMETER"><i>object</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">gtk_object_unref</tt></code>(GtkObject*
<tt class="PARAMETER"><i>object</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">gtk_object_sink</tt></code>(GtkObject* <tt
class="PARAMETER"><i>object</i></tt>);</code>
</p>
</div>
<p>
<b>Figure 22. Reference Counting</b>
</p>
</div>
<p>
<tt class="FUNCTION">gtk_object_ref()</tt> and <tt class=
"FUNCTION">gtk_object_unref()</tt> have widget-specific
variants (<tt class="FUNCTION">gtk_widget_ref()</tt>,
etc.) but the object and widget versions are completely
synonymous. The widget-specific versions are leftovers
from earlier versions of GTK+.
</p>
<p>
So to safely remove a widget from a container, you might
do this:
</p>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="PROGRAMLISTING">
gtk_widget_ref(widget);
gtk_container_remove(container, widget);
</pre>
</td>
</tr>
</table>
<p>
The widget now has one reference, held by your code. At
some point you'll need to release the reference,
destroying the widget. (It would make sense to do so
after re-adding the widget to some other container, for
example.)
</p>
<p>
It's worth pointing out that removing widgets from
containers is uncommon; in general it's faster to simply
hide the widget with <tt class="FUNCTION">
gtk_widget_hide()</tt>, then <tt class="FUNCTION">
gtk_widget_show()</tt> it at some later time.
</p>
<p>
<tt class="FUNCTION">gtk_object_sink()</tt> is used
almost exclusively in widget implementations, when you
expect to be the primary "owner" of an object. If an
object is not "floating", <tt class="FUNCTION">
gtk_object_sink()</tt> has no effect. To claim ownership
of a widget, do this:
</p>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="PROGRAMLISTING">
gtk_widget_ref(widget);
gtk_object_sink(GTK_OBJECT(widget));
</pre>
</td>
</tr>
</table>
<p>
This code adds one reference to the widget; if the widget
was "floating," it also subtracts one reference. If the
widget was not floating, <tt class="FUNCTION">
gtk_widget_sink()</tt> has no effect.
</p>
<p>
It's important to understand the details because in some
cases they can be important. But most of the time, you
can get by with a few simple rules:
</p>
<ul>
<li>
<p>
You must destroy any toplevel widgets when you are
done with them, but child widgets are destroyed
automatically.
</p>
</li>
<li>
<p>
If you want to remove a widget from a container
without destroying it, you must first add a reference
to the widget.
</p>
</li>
<li>
<p>
If you add a reference to a widget, you are
responsible for unreferencing the widget again when
you're done with it.
</p>
</li>
</ul>
</div>
<div class="SECT2">
<h2 class="SECT2">
<a name="SEC-REALIZINGSHOWING">Realizing, Mapping, and
Showing</a>
</h2>
<p>
Fully understanding GTK+ requires some minimal
understanding of the X Window System. This book assumes
you have a user-level understanding---you know what an X
server is, that X is network transparent, what a window
manager does, and so on. A few more details are needed to
write programs, however.
</p>
<p>
One detail is particularly important: the X Window System
maintains a tree of <i class="FIRSTTERM">windows</i>.
"Window" in this sense refers to an X window, not a <tt
class="CLASSNAME">GtkWindow</tt>---<tt class=
"CLASSNAME">GtkWindow</tt> is a GTK+-specific concept, a
widget that corresponds to an application's toplevel X
window. An X window is not the user-visible concept
"window" represented by <tt class="CLASSNAME">
GtkWindow</tt>; rather, it's an abstraction used by the X
server to partition the screen. The "background"
displayed by your X server is the <i class="FIRSTTERM">
root window</i>; the root window has no parent.
Application windows are typically near-children of the
root window; most window managers create a child of the
root window to hold the window's titlebar and other
decorations, and place the application window inside.
Window managers have total control over application
windows---they can reposition them, reparent them, and
iconify them at will. Application windows can in turn
contain subwindows, which are controlled by the
application. Note that GTK+ uses the GDK library, rather
than using X directly; in GDK, there is a thin X window
wrapper called <span class="STRUCTNAME">GdkWindow</span>.
Don't confuse <span class="STRUCTNAME">GdkWindow</span>
and <tt class="CLASSNAME">GtkWindow</tt>.
</p>
<p>
An X window, or a <span class="STRUCTNAME">
GdkWindow</span>, gives the X server hints about the
structure of the graphics being displayed. Since X is
network transparent, this helps reduce network traffic.
The X server knows how to show windows on the screen;
hide them; move them around (keeping children in position
relative to their parents); capture events such as mouse
movements on a per-window basis; and so on. A <span
class="STRUCTNAME">GdkWindow</span> is also the
fundamental unit for drawing graphics---you can't draw to
"the screen" as a whole, you must draw on a <span class=
"STRUCTNAME">GdkWindow</span>.
</p>
<p>
Most GTK+ widgets have a corresponding <span class=
"STRUCTNAME">GdkWindow</span>. There are exceptions, such
as <tt class="CLASSNAME">GtkLabel</tt>; these are
referred to as "no window widgets," and are relatively
lightweight. Widgets with no associated <span class=
"STRUCTNAME">GdkWindow</span> draw into their parent's
<span class="STRUCTNAME">GdkWindow</span>. Some
operations, such as capturing events, require a <span
class="STRUCTNAME">GdkWindow</span>; thus they are
impossible on no-window widgets.
</p>
<p>
Widgets pass through a number of states related to their
<span class="STRUCTNAME">GdkWindow</span>:
</p>
<ul>
<li>
<p>
A widget is said to be <i class="FIRSTTERM">
realized</i> if its corresponding <span class=
"STRUCTNAME">GdkWindow</span> has been created.
Widgets are realized via <tt class="FUNCTION">
gtk_widget_realize()</tt>, and unrealized via <tt
class="FUNCTION">gtk_widget_unrealize()</tt>. Since
an X window must have a parent, if a widget is
realized its parent must also be.
</p>
</li>
<li>
<p>
A widget is <i class="FIRSTTERM">mapped</i> if <tt
class="FUNCTION">gdk_window_show()</tt> has been
called on its <span class="STRUCTNAME">
GdkWindow</span>. This means the server has been
asked to display the window on the screen; obviously
the <span class="STRUCTNAME">GdkWindow</span> must
exist, implying that the widget is realized.
</p>
</li>
<li>
<p>
A widget is <i class="FIRSTTERM">visible</i> if it
will automatically be mapped when its parent is
mapped. This means that <tt class="FUNCTION">
gtk_widget_show()</tt> has been called on the widget.
A widget can be rendered invisible by calling <tt
class="FUNCTION">gtk_widget_hide()</tt>; this will
either unschedule the pending map, or unmap the
widget (hide its <span class="STRUCTNAME">
GdkWindow</span>). Since toplevel widgets have no
parent, they are mapped as soon as they are
shown.
</p>
</li>
</ul>
<p>
In typical user code, you only need to call <tt class=
"FUNCTION">gtk_widget_show()</tt>; this implies realizing
and mapping the widget as soon as its parent is realized
and mapped. It's important to understand that <tt class=
"FUNCTION">gtk_widget_show()</tt> has no immediate
effect, it merely schedules the widget to be shown. This
means you don't have to worry about showing widgets in
any particular order; it also means that you can't
immediately access the <span class="STRUCTNAME">
GdkWindow</span> of a widget. Sometimes you need to
access the <span class="STRUCTNAME">GdkWindow</span>; in
those cases you'll want to manually call <tt class=
"FUNCTION">gtk_widget_realize()</tt> to create it. <tt
class="FUNCTION">gtk_widget_realize()</tt> will also
realize a widget's parents, if appropriate. It's uncommon
to need <tt class="FUNCTION">gtk_widget_realize()</tt>;
if you find that you do, perhaps you are approaching the
problem incorrectly.
</p>
<p>
Destroying a widget automatically reverses the entire
sequence of events, recursively unrealizing the widget's
children and the widget itself.
</p>
<p>
<a href="z57.html#FL-SHOWREALIZE">Figure 23</a>
summarizes the functions discussed in this section.
</p>
<div class="FIGURE">
<a name="FL-SHOWREALIZE"></a>
<div class="FUNCSYNOPSIS">
<a name="FL-SHOWREALIZE.SYNOPSIS"></a>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="FUNCSYNOPSISINFO">
#include <gtk/gtkwidget.h>
</pre>
</td>
</tr>
</table>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">gtk_widget_realize</tt></code>(GtkWidget*
<tt class="PARAMETER"><i>widget</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">
gtk_widget_unrealize</tt></code>(GtkWidget* <tt
class="PARAMETER"><i>widget</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">gtk_widget_map</tt></code>(GtkWidget* <tt
class="PARAMETER"><i>widget</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">gtk_widget_unmap</tt></code>(GtkWidget*
<tt class="PARAMETER"><i>widget</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">gtk_widget_show</tt></code>(GtkWidget* <tt
class="PARAMETER"><i>widget</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">gtk_widget_hide</tt></code>(GtkWidget* <tt
class="PARAMETER"><i>widget</i></tt>);</code>
</p>
</div>
<p>
<b>Figure 23. Showing/Realizing Widgets</b>
</p>
</div>
<p>
<a href="z57.html#ML-STATES">Figure 24</a> summarizes
macros for querying the states discussed in this section.
</p>
<div class="FIGURE">
<a name="ML-STATES"></a>
<div class="FUNCSYNOPSIS">
<a name="ML-STATES.SYNOPSIS"></a>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="FUNCSYNOPSISINFO">
#include <gtk/gtkwidget.h>
</pre>
</td>
</tr>
</table>
<p>
<code><code class="FUNCDEF"><tt class="FUNCTION">
GTK_WIDGET_NO_WINDOW</tt></code>(<tt class=
"PARAMETER"><i>widget</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF"><tt class="FUNCTION">
GTK_WIDGET_REALIZED</tt></code>(<tt class=
"PARAMETER"><i>widget</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF"><tt class="FUNCTION">
GTK_WIDGET_MAPPED</tt></code>(<tt class=
"PARAMETER"><i>widget</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF"><tt class="FUNCTION">
GTK_WIDGET_VISIBLE</tt></code>(<tt class=
"PARAMETER"><i>widget</i></tt>);</code>
</p>
</div>
<p>
<b>Figure 24. Widget Predicates</b>
</p>
</div>
</div>
<div class="SECT2">
<h2 class="SECT2">
<a name="Z58">Other Widget Concepts</a>
</h2>
<p>
This section describes a few other concepts associated
with the <tt class="CLASSNAME">GtkWidget</tt> base class,
including <i class="FIRSTTERM">sensitivity</i>, <i class=
"FIRSTTERM">focus</i>, and <i class="FIRSTTERM">widget
states</i>.
</p>
<div class="SECT3">
<h3 class="SECT3">
<a name="SEC-SENSITIVITY">Sensitivity</a>
</h3>
<p>
Widgets can be <i class="FIRSTTERM">sensitive</i> or <i
class="FIRSTTERM">insensitive</i>; insensitive widgets
do not respond to input. (On other platforms, this is
termed "ghosted" or "inactive").
</p>
<p>
<tt class="FUNCTION">gtk_widget_set_sensitive()</tt>
(<a href="z57.html#FL-SETSENSITIVE">Figure 25</a>)
changes a widget's sensitivity.
</p>
<div class="FIGURE">
<a name="FL-SETSENSITIVE"></a>
<div class="FUNCSYNOPSIS">
<a name="FL-SETSENSITIVE.SYNOPSIS"></a>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="FUNCSYNOPSISINFO">
#include <gtk/gtkwidget.h>
</pre>
</td>
</tr>
</table>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">
gtk_widget_set_sensitive</tt></code>(GtkWidget* <tt
class="PARAMETER"><i>widget</i></tt>, gboolean <tt
class="PARAMETER"><i>setting</i></tt>);</code>
</p>
</div>
<p>
<b>Figure 25. Changing Sensitivity</b>
</p>
</div>
<p>
By default sensitivity is set to <span class=
"STRUCTNAME">TRUE</span>. A widget is only "really"
sensitive if all its parents are sensitive; that is,
you can make an entire container full of widgets
(in)sensitive by setting the sensitivity of the
container. The "real" sensitivity of a widget,
including its parent's state, can be tested with the
<tt class="FUNCTION">GTK_WIDGET_IS_SENSITIVE()</tt>
macro. The sensitivity of the widget itself, which only
matters if the widget's parent is sensitive, can be
queried using <tt class="FUNCTION">
GTK_WIDGET_SENSITIVE()</tt>. These are in <a href=
"z57.html#ML-ISSENSITIVE">Figure 26</a>.
</p>
<div class="FIGURE">
<a name="ML-ISSENSITIVE"></a>
<div class="FUNCSYNOPSIS">
<a name="ML-ISSENSITIVE.SYNOPSIS"></a>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="FUNCSYNOPSISINFO">
#include <gtk/gtkwidget.h>
</pre>
</td>
</tr>
</table>
<p>
<code><code class="FUNCDEF"><tt class="FUNCTION">
GTK_WIDGET_IS_SENSITIVE</tt></code>(<tt class=
"PARAMETER"><i>widget</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF"><tt class="FUNCTION">
GTK_WIDGET_SENSITIVE</tt></code>(<tt class=
"PARAMETER"><i>widget</i></tt>);</code>
</p>
</div>
<p>
<b>Figure 26. Sensitivity</b>
</p>
</div>
</div>
<div class="SECT3">
<h3 class="SECT3">
<a name="SEC-FOCUSWIDGET">Focus</a>
</h3>
<p>
Within each toplevel window, one widget at a time may
have the <i class="FIRSTTERM">keyboard focus</i>. Any
key events received by the toplevel window are
forwarded to the focused widget. This is important
because typing something on the keyboard should have
only one effect---changing only one text entry field,
for example.
</p>
<p>
Most widgets will give some visual indication that they
have the current focus. Using the default GTK+ theme,
the focused widget is typically surrounded by a thin
black frame. The user can move the focus between
widgets, using the arrow keys or the Tab key. Focus can
also move to a widget if the user clicks it.
</p>
<p>
The concept of focus is important for keyboard
navigation. For example, pressing Enter or the space
bar "activates" many widgets if they have the focus;
you can move between buttons with the Tab key, and
press one with Space, for example.
</p>
</div>
<div class="SECT3">
<h3 class="SECT3">
<a name="SEC-GRABS">Grabs</a>
</h3>
<p>
Widgets can <i class="FIRSTTERM">grab</i> the pointer
and keyboard away from other widgets. This essentially
means that the widget becomes "modal"; input goes only
to that widget, and the focus can't be changed to
another widget. A typical reason to grab input is to
create a modal dialog; if a window has the grab,
interaction with other windows is blocked. Note that
there is another, GDK-level "grab"; a GDK keyboard or
pointer grab occurs on an X-server-wide basis---that
is, other applications are unable to receive keyboard
or mouse events. A widget grab is a GTK+ concept; it
only grabs events away from other widgets in the same
application.
</p>
</div>
<div class="SECT3">
<h3 class="SECT3">
<a name="Z59">Default</a>
</h3>
<p>
Each window may have at most one <i class="FIRSTTERM">
default</i> widget. For example, dialogs typically have
a default button which is activated when the user
presses the Enter key.
</p>
</div>
<div class="SECT3">
<h3 class="SECT3">
<a name="SEC-WIDGETSTATES">Widget States</a>
</h3>
<p>
Widgets have <i class="FIRSTTERM">states</i> which
determine their appearance:
</p>
<ul>
<li>
<p>
<i class="FIRSTTERM">Normal</i>: just like it
sounds.
</p>
</li>
<li>
<p>
<i class="FIRSTTERM">Active</i>: a button is
currently pressed in, or a check box is currently
checked, for example.
</p>
</li>
<li>
<p>
<i class="FIRSTTERM">Prelight</i>: the mouse is
over the widget (and typically clicking would have
some effect). Buttons "highlight" when you move
over them, for example.
</p>
</li>
<li>
<p>
<i class="FIRSTTERM">Selected</i>: the widget is in
a list or other set of alternatives, and is the
currently selected option.
</p>
</li>
<li>
<p>
<i class="FIRSTTERM">Insensitive</i>: the widget is
"ghosted," inactive, or unresponsive; it will not
respond to input.
</p>
</li>
</ul>
<p>
The exact meaning and visual representation of a given
state depends on the particular widget and the current
theme. You can access the state of the widget <tt
class="FUNCTION">GTK_WIDGET_STATE()</tt> (<a href=
"z57.html#ML-WIDGETSTATE">Figure 27</a>). This macro
returns one of the constants <span class="STRUCTNAME">
GTK_STATE_NORMAL</span>, <span class="STRUCTNAME">
GTK_STATE_ACTIVE</span>, <span class="STRUCTNAME">
GTK_STATE_PRELIGHT</span>, <span class="STRUCTNAME">
GTK_STATE_SELECTED</span>, or <span class="STRUCTNAME">
GTK_STATE_INSENSITIVE</span>.
</p>
<div class="FIGURE">
<a name="ML-WIDGETSTATE"></a>
<div class="FUNCSYNOPSIS">
<a name="ML-WIDGETSTATE.SYNOPSIS"></a>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="FUNCSYNOPSISINFO">
#include <gtk/gtkwidget.h>
</pre>
</td>
</tr>
</table>
<p>
<code><code class="FUNCDEF"><tt class="FUNCTION">
GTK_WIDGET_STATE</tt></code>(<tt class=
"PARAMETER"><i>widget</i></tt>);</code>
</p>
</div>
<p>
<b>Figure 27. State Accessor</b>
</p>
</div>
</div>
</div>
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