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<span id="Linked-Variables"></span><div class="header">
<p>
Next: <a href="tkconnect.html" accesskey="n" rel="next">tkconnect</a>, Previous: <a href="Lisp-Functions-Invoked-from-Graphics.html" accesskey="p" rel="prev">Lisp Functions Invoked from Graphics</a>, Up: <a href="General.html" accesskey="u" rel="up">General</a> [<a href="wm.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>]</p>
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<hr>
<span id="Linked-Variables-1"></span><h3 class="section">1.7 Linked Variables</h3>
<p>It is possible to link lisp variables to <b>TK</b> variables. In general
when the <b>TK</b> variable is changed, by for instance clicking on a
radiobutton, the linked lisp variable will be changed. Conversely
changing the lisp variable will be noticed by the <b>TK</b> graphics side, if
one does the assignment in lisp using <code>setk</code> instead of
<code>setq</code>.
</p>
<div class="example">
<pre class="example">(button '.hello :textvariable '*message* :text "hi there")
(pack '.hello)
</pre></div>
<p>This causes linking of the global variable <code>*message*</code> in lisp
to a corresponding variable in <b>TK</b>. Moreover the message that is in
the button <code>.hello</code> will be whatever the value of this global
variable is (so long as the <b>TK</b> side is notified of the change!).
</p>
<p>Thus if one does
</p>
<div class="example">
<pre class="example">(setk *message* "good bye")
</pre></div>
<p>then the button will change to have <i>good bye</i> as its text.
The lisp macro <code>setk</code> expands into
</p>
<div class="example">
<pre class="example">(prog1 (setf *message* "good bye") (notice-text-variables))
</pre></div>
<p>which does the assignment, and then goes thru the linked variables
checking for those that have changed, and updating the <b>TK</b> side should
there be any. Thus if you have a more complex program which might
have done the assignment of your global variable, you may include
the call to <code>notice-text-variables</code> at the end, to assure that
the graphics side knows about the changes.
</p>
<p>A variable which is linked using the keyword <code>:textvariable</code> is
always a variable containing a string.
</p>
<p>However it is possible to have other types of variables.
</p>
<div class="example">
<pre class="example">(checkbutton '.checkbutton1 :text "A button" :variable '(boolean *joe*))
(checkbutton '.checkbutton2 :text "A button" :variable '*joe*)
(checkbutton '.checkbutton3 :text "Debugging" :variable '(t *debug*)
:onvalue 100 :offvalue -1)
</pre></div>
<p>The first two examples are the same in that the default variable type
for a checkbutton is <code>boolean</code>. Notice that the specification of a
variable type is by <code>(<i>type</i> variable)</code>. The types which are
permissible are those which have coercion-fucntions, See <a href="Return-Values.html">Return Values</a>. In the first example a variable <code>*joe*</code> will be linked, and
its default initial value will be set to nil, since the default initial
state of the check button is off, and the default off value is nil.
Actually on the <b>TK</b> side, the corresponding boolean values are <code>"1"</code>
and <code>"0"</code>, but the <code>boolean</code> type makes these become <code>t</code>
and <code>nil</code>.
</p>
<p>In the third example the variable *debug* may have any lisp value (here
<i>type</i> is <code>t</code>). The initial value will be made to be <code>-1</code>,
since the checkbutton is off. Clicking on <code>.checkbutton3</code> will
result in the value of <code>*debug*</code> being changed to 100, and the light
in the button will be toggled to on, See <a href="checkbutton.html">checkbutton</a>. You may
set the variable to be another value besides 100.
</p>
<p>You may also call
</p>
<div class="example">
<pre class="example">(link-text-variable '*joe* 'boolean)
</pre></div>
<p>to cause the linking of a variable named *joe*. This is done
automatically
whenever the variable is specified after one of the keys
</p>
<div class="example">
<pre class="example">:variable :textvariable.
</pre></div>
<p>Just as one must be cautious about using global variables in lisp, one
must be cautious in making such linked variables. In particular note
that the <b>TK</b> side, uses variables for various purposes. If you make a
checkbutton with pathname <code>.a.b.c</code> then unless you specify a
<code>:variable</code> option, the variable <code>c</code> will become associated to
the <b>TK</b> value of the checkbutton. We do NOT link this variable by
default, feeling that one might inadvertently alter global variables,
and that they would not typically use the lisp convention of being of
the form <code>*c*</code>. You must specify the <code>:variable</code> option, or
call <code>link-variable</code>.
</p>
<hr>
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<p>
Next: <a href="tkconnect.html" accesskey="n" rel="next">tkconnect</a>, Previous: <a href="Lisp-Functions-Invoked-from-Graphics.html" accesskey="p" rel="prev">Lisp Functions Invoked from Graphics</a>, Up: <a href="General.html" accesskey="u" rel="up">General</a> [<a href="wm.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>]</p>
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