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<h1 class="chapter">Chapter 2. Geometry Management</h1>
<div class="htmltoc"><h4 class="tochead">Contents:</h4>
<p> <a href="ch02_01.htm#mastperltk-chp-2-sect-1">The pack Geometry Manager</a><br />
<a href="ch02_02.htm">The grid Geometry Manager</a><br />
<a href="ch02_03.htm">The place Geometry Manager</a><br />
<a href="ch02_04.htm">The form Geometry Manager</a><br />
<a href="ch02_05.htm">Geometry Management Summary</a><br /></p></div>
<p><a name="INDEX-123" /></a>To
display widgets on the screen, they must be passed to a
<em class="firstterm">geometry manager</em>. The geometry manager
controls the position and size of the widgets in the display window.
Several geometry managers are available with Perl/Tk:
<tt class="literal">pack</tt>, <tt class="literal">place</tt>,
<tt class="literal">grid</tt>, and <tt class="literal">form</tt>.
</p>
<p><a name="INDEX-124" /></a>All the geometry managers are invoked as
methods on the widget, but they all have their own methodologies and
arguments to change where and how the widgets are put on the screen:
</p>
<blockquote><pre class="code">$widget1->pack(); $widget2->place(); $widget3->grid(); $widget4->form( );</pre></blockquote>
<p><a name="INDEX-125" /></a><a name="INDEX-126" /></a>When you organize the widgets in
your window, it is often necessary to separate groups of widgets to
get a certain look and feel. For instance, when you use
<tt class="literal">pack</tt>, it is difficult to have widgets stacked both
horizontally and vertically without grouping them in some fashion. We
use a Frame widget or another window (a Toplevel widget) to group
widgets inside a window.
</p>
<p><a name="INDEX-127" /></a>We create our first window
by calling <tt class="literal">MainWindow</tt>. The MainWindow is a special
form of a Toplevel widget. For more detailed information on how to
create/configure Frame and Toplevel widgets, see <a href="ch11_01.htm">Chapter 11, "
Frame, MainWindow,and Toplevel Widgets"</a>.
</p>
<p><a name="INDEX-128" /></a>With the
exception of <tt class="literal">place</tt>, differences between the
geometry managers make it difficult (not entirely impossible, but
definitely not recommended) to use more than one geometry manager
within the same area. In
<tt class="literal">$mw</tt>, we can display many types of widgets, but if
we start using <tt class="literal">pack</tt>, we should continue to use
<tt class="literal">pack</tt> on all the widgets contained directly in
<tt class="literal">$mw</tt>. Don't switch to <tt class="literal">grid</tt>
in the middle, because the two geometry managers will get into a
<em class="emphasis">race</em> <em class="emphasis">condition</em>: one will
create its layout, which affects the geometry calculations of the
other, which affects the layout of the first, causing it to recompute
its geometries, ad infinitum. However, let's assume our
MainWindow contains a Frame, which in turn contains other widgets. We
could use <tt class="literal">pack</tt> to pack the Frame inside the
MainWindow and then we could use <tt class="literal">grid</tt> to manage
the widgets inside the Frame. See <a href="ch02_01.htm#mastperltk-chp-2-fig-1">Figure 2-1</a>.
</p>
<a name="mastperltk-CHP-2-FIG-1" /></a><div class="figure"><img src="figs/mptk.0201.gif" alt="Figure 2-1" width="257" height="181" /></div><h4 class="objtitle">Figure 2-1. Frame within a window that uses a different geometry manager</h4>
<p><a name="INDEX-129" /></a>Although the different geometry managers
have their own strengths and weaknesses, <tt class="literal">pack</tt> is
the most commonly used, so we'll discuss it first and in the
most detail. The <tt class="literal">grid</tt> geometry manager has been
improved greatly with the release of Tk 8.0 and subsequent porting to
Perl. The <tt class="literal">place</tt> geometry manager is the most
tedious to use, because you have to determine exact coordinates
(relative or absolute) for every single widget. Finally, the
<tt class="literal">form</tt> geometry manager is like a combination of
<tt class="literal">pack</tt> and <tt class="literal">place</tt>.
</p>
<div class="sect1"><a name="mastperltk-CHP-2-SECT-1" /></a>
<h2 class="sect1">2.1. The pack Geometry Manager</h2>
<p><a name="INDEX-130" /></a>Remember
when you were a child and you had those wooden puzzles to put
together? Each piece in the puzzle had exactly one place where it
could go and there weren't any overlaps allowed between pieces.
</p>
<p><a name="INDEX-131" /></a>With the <tt class="literal">pack</tt> geometry
manager, our windows are similar to the wooden puzzle, because
widgets cannot overlap or cover each other, partially or completely
(see <a href="ch02_01.htm#mastperltk-chp-2-fig-2">Figure 2-2</a>). If a Button is packed in a
certain space on the window, the next Button (or any widget) will
have to fit around the already packed Button. Luckily, our windows
will be dealing only with rectangular shapes instead of funny-shaped
puzzle pieces.
</p>
<a name="mastperltk-CHP-2-FIG-2" /></a><div class="figure"><img src="figs/mptk.0202.gif" alt="Figure 2-2" width="377" height="181" /></div><h4 class="objtitle">Figure 2-2. Overlap error</h4>
<p>The order in which you pack your widgets is very important because it
directly affects what you see on the screen. Each Frame or Toplevel
maintains a list of items that are displayed within it. This list has
an order to it: if widget A is packed before widget B, then widget A
will get preference if space becomes scarce. This will become clear
as we go through some examples. You will often get a different look
to your window just by packing the widgets in a different order.
</p>
<p>If you don't care what the window looks like and how the
widgets are put in it, you can use <tt class="literal">pack
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