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<h3 class="section">1.3 Common Features of Widgets</h3>

<p>A <i>widget</i> is a lisp symbol which has a function binding. The

first argument is always a keyword and is called the <i>option</i>.

The argument pattern for the remaining arguments depends on the

<i>option</i>. The most common <i>option</i> is <code>:configure</code> in

which case the remaining arguments are alternating keyword/value

pairs, with the same keywords being permitted as at the creation

of the widget.

</p>

<p>A <i>widget</i> is created by means of a <i>widget constructor</i>, of

which there are currently 15, each of them appearing as the title of a

section in <a href="Widgets.html#Widgets">Widgets</a>. They live in the <code>"TK"</code> package, and for

the moment we will assume we have switched to this package. Thus for

example <code>button</code> is such a widget constructor function. Of course

this is lisp, and you can make your own widget constructors, but when

you do so it is a good idea to follow the standard argument patterns

that are outlined in this section.

</p>

<pre class="example">(button '.hello)

==> .HELLO

</pre></div>

<p>creates a <i>widget</i> whose name is <code>.hello</code>. There is a parent child

hierarchy among widgets which is implicit in the name used for the

widget. This is much like the pathname structure on a Unix or Dos

file system, except that <code>'.'</code> is used as the separator rather

than a <code>/</code> or <code>\</code>. For this reason the widget instances

are sometimes referred to as <i>pathnames</i>. A child of the

parent widget <code>.hello</code> might be called <code>.hello.joe</code>, and

a child of this last might be <code>.hello.joe.bar</code>. The parent of

everyone is called <code>.</code> . Multiple top level windows are created

using the <code>toplevel</code> command (see <a href="toplevel.html#toplevel">toplevel</a>).

</p>

<p>The widget constructor functions take keyword and value pairs, which

allow you to specify attributes at the time of creation:

</p>

<pre class="example">(button '.hello :text "Hello World" :width 20)

==>.HELLO

</pre></div>

<p>indicating that we want the text in the button window to be

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<code>Hello World</code> and the width of the window to be 20 characters

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wide. Other types of windows allow specification in centimeters

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<code>2c</code>, or in inches (<code>2i</code>) or in millimeters <code>2m</code>

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or in pixels <code>2</code>. But text windows usually have their

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dimensions specified as multiples of a character width and height.

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This latter concept is called a grid.

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</p>

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<p>Once the window has been created, if you want to change the

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text you do NOT do:

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</p><div class="example">

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<pre class="example">(button '.hello :text "Bye World" :width 20)

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</pre></div>

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<p>This would be in error, because the window .hello already exists.

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You would either have to first call

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</p>

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<pre class="example">(destroy '.hello)

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</pre></div>

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<p>But usually you just want to change an attribute. <code>.hello</code> is

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actually a function, as we mentioned earlier, and it is this function

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that you use:

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</p>

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<pre class="example">(.hello :configure :text "Bye World")

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</pre></div>

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<p>This would simply change the text, and not change where the window had

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been placed on the screen (if it had), or how it had been packed

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into the window hierarchy. Here the argument <code>:configure</code> is

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called an <i>option</i>, and it specifies which types of keywords can

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follow it. For example

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</p>

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<pre class="example">(.hello :flash)

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</pre></div>

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<p>is also valid, but in this case the <code>:text</code> keyword is not permitted

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after flash. If it were, then it would mean something else besides

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what it means in the above. For example one might have defined

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</p>

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<pre class="example">(.hello :flash :text "PUSH ME")

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</pre></div>

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<p>so here the same keyword <code>:text</code> would mean something else, eg

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to flash a subliminal message on the screen.

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<p>We often refer to calls to the widget functions

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as messages. One reason for this is that they actually turn into

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messages to the graphics process <samp>gcltksrv</samp>. To actually see these

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messages you can do

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</p><div class="example">

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<pre class="example">(debugging t).

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</pre></div>

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