ch21_01.htm

by Randal L. Schwartz and Tom Phoenix ISBN 0-596-00132-0 Third Edition, published July 2001. (See

<a name="perlnut2-CHP-21-SECT-1.1.2" /></a><div class="sect3">
<h3 class="sect3">21.1.1.2. The cget method</h3>

<p><a name="INDEX-2679" /></a>For simply retrieving the value of an
option, <tt class="literal">configure</tt> returns more information than
you generally want. The <tt class="literal">cget</tt> method returns just
the current value.
</p>

</div>
</div>
<a name="perlnut2-CHP-21-SECT-1.2" /></a><div class="sect2">
<h3 class="sect2">21.1.2. Scrollbars</h3>

<p><a name="INDEX-2680" /></a><a name="INDEX-2681" /></a><a name="INDEX-2682" /></a><a name="INDEX-2683" /></a>Many
widgets have scrollbars associated with them. Scrollbars can be added
to a widget in two ways: with an independent Scrollbar widget or with
the <tt class="literal">Scrolled</tt> method when creating a widget. For
simple scrollbars, the <tt class="literal">Scrolled</tt> method is much
easier and therefore preferable.
</p>

<a name="perlnut2-CHP-21-SECT-1.2.1" /></a><div class="sect3">
<h3 class="sect3">21.1.2.1. Using the Scrolled method</h3>

<p><a name="INDEX-2684" /></a>Use the <tt class="literal">Scrolled</tt>
method to create both the widget and the scrollbar in a single
command. For example:
</p>

<blockquote><pre class="code">$mainwindow-&gt;Scrolled('Entry', -scrollbars =&gt; 'os'
        -textvariable =&gt; \$address)-&gt;pack;</pre></blockquote>

<p>This creates an Entry widget with an
"optional" scrollbar on the bottom.
The first argument to <tt class="literal">Scrolled</tt> is the type of
widget (in this case, an Entry widget). Then use the
<tt class="literal">-scrollbars</tt> option to list the location of the
scrollbar ("s" for the south, or
bottom, edge of the widget). Here, we specify an
"optional" scrollbar with
"o", meaning that the scrollbar
will appear only if needed.
</p>

<p>Any additional options to the <tt class="literal">Scrolled</tt> method are
taken as options to the widget itself. In this case,
we're setting the <tt class="literal">-textvariable</tt>
option to the Entry widget.
</p>

</div>

<a name="perlnut2-CHP-21-SECT-1.2.2" /></a><div class="sect3">
<h3 class="sect3">21.1.2.2. The Scrollbar widget</h3>

<p><a name="INDEX-2685" /></a><a name="INDEX-2686" /></a><a name="INDEX-2687" /></a>For more flexibility with a scrollbar,
you can use the Scrollbar widget. To do so, you need to create the
target widget to scroll. Set the <tt class="literal">-xscrollcommand</tt>
or <tt class="literal">-yscrollcommand</tt> option as appropriate,
configure the scrollbar to talk to the widget, and position the
scrollbar and target widget next to one another. For example:
</p>

<blockquote><pre class="code">$scrollbar = $mainwindow-&gt;Scrollbar(-orient =&gt; 'vertical');
$listbox = $mainwindow-&gt;Entry(-yscrollcommand =&gt; ['set' =&gt; $scrollbar]);
$scrollbar-&gt;configure(-command =&gt; ['yview' =&gt; $listbox]);
$scrollbar-&gt;pack(-side =&gt; 'right', -fill =&gt; 'y');
$listbox-&gt;pack(-side =&gt; 'left', -fill =&gt; 'both');</pre></blockquote>

<p>First, we create the scrollbar with vertical orientation (which is
actually the default). Next, we create the Listbox widget with the
<tt class="literal">-yscrollcommand</tt> option to define a callback when
the widget is scrolled vertically. The scrollbar is then configured
with a callback that says to inform the Listbox widget when it is
clicked vertically. Finally, the Scrollbar and Listbox widgets are
packed side-by-side. See further discussion of the Scrollbar widget
later in this chapter for more information.
</p>

</div>
</div>
<a name="perlnut2-CHP-21-SECT-1.3" /></a><div class="sect2">
<h3 class="sect2">21.1.3. Callbacks</h3>

<p><a name="INDEX-2688" /></a><a name="INDEX-2689" /></a><a name="INDEX-2690" /></a><a name="INDEX-2691" /></a>Many
widgets allow you to define a <em class="emphasis">callback</em>, which is
a command to execute when the widget is selected. For example, when
you press an exit button, the callback might be to a routine that
cleans up and quits the program. When you click on a radio button,
you might want to change the window to reflect the new preferences.
</p>

<p><a name="INDEX-2692" /></a>Widgets that support callbacks have a
<tt class="literal">-command</tt> option to provide the callback function.
In the "Hello World" example shown
previously, the callback is to <tt class="literal">sub {exit}</tt>. In that
example, the callback is called as an anonymous subroutine. You could
also use a reference to a subroutine (e.g.,
<tt class="literal">\&amp;routine</tt>). If you want to provide arguments
to a subroutine, you can call it as an anonymous list (e.g.,
<tt class="literal">[ \&amp;routine, $arg, $arg, ... ]</tt>).
</p>

</div>
<a name="perlnut2-CHP-21-SECT-1.4" /></a><div class="sect2">
<h3 class="sect2">21.1.4. Colors and Fonts</h3>

<p><a name="INDEX-2693" /></a><a name="INDEX-2694" /></a><a name="INDEX-2695" /></a><a name="INDEX-2696" /></a><a name="INDEX-2697" /></a>Tk was originally created for the X
Window System and is still primarily used in that environment. For
this reason, it has inherited the font and color scheme used for the
X Window System.
</p>

<p>Colors that can be used with Tk widgets are identified either by an
RGB value or by a name that has been associated with an RGB value. In
general, it is easier to use a color name than an explicit RGB value.
For a listing of the color names that are supported, see the
<em class="emphasis">rgb.txt</em> file in your X distribution or use the
<em class="emphasis">showrgb</em> command. (Most common color names are
supported, so you can say things such as
"red,"
"pink,"
"green," and even
"chartreuse" with confidence.)
</p>

<p>Fonts are another matter. Under the X Window System, fonts are named
things such as
<em class="emphasis">-adobe-helvetica-medium-o-normal -- 12-120-75-75-p-67-iso8859-1</em>.
Wildcards can make the fonts easier to use, but
they're still a mouthful. For a listing of fonts
available for a particular X server, use the
<em class="emphasis">xlsfonts</em> command. There are a few font
"aliases" that have been defined
for your convenience (such as <tt class="literal">fixed</tt>,
<tt class="literal">6x10</tt>, <tt class="literal">9x15</tt>, etc.), and you
might prefer to just stick to those.<a name="INDEX-2698" /></a><a name="INDEX-2699" /></a><a name="INDEX-2700" /></a> 
</p>

</div>
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