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Import Tix 8.4.3.5 (as of svn r86089)

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Zachary Ware
2017-05-22 16:16:49 -05:00
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<HEAD>
<TITLE> Public Variable Configuration Methods<A NAME=652>&nbsp;</A></TITLE>
</HEAD>
<BODY BGCOLOR="#ffffff" TEXT="#000000" LINK="#0000ff" VLINK="#000080">
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<HR> <A NAME=tex2html1313 HREF=subsubsectionstar3_8_5_4.html><IMG ALIGN=MIDDLE SRC="../gif/icons/next_motif.gif"></A> <A NAME=tex2html1311 HREF=subsection3_8_5.html><IMG ALIGN=MIDDLE SRC="../gif/icons/up_motif.gif"></A> <A NAME=tex2html1305 HREF=subsubsectionstar3_8_5_2.html><IMG ALIGN=MIDDLE SRC="../gif/icons/previous_motif.gif"></A> <A NAME=tex2html1315 HREF=tableofcontents3_1.html><IMG ALIGN=MIDDLE SRC="../gif/icons/contents_motif.gif"></A> <BR>
<B> Next:</B> <A NAME=tex2html1314 HREF=subsubsectionstar3_8_5_4.html> Configuration Methods and </A>
<B>Up:</B> <A NAME=tex2html1312 HREF=subsection3_8_5.html> Declaring and Using </A>
<B> Previous:</B> <A NAME=tex2html1306 HREF=subsubsectionstar3_8_5_2.html> Type Checker</A>
<HR> <P>
<A NAME=Contents>&nbsp;</A><H3><A NAME=SECTION00085300000000000000> Public Variable Configuration Methods<A NAME=652>&nbsp;</A></A></H3>
<P>
After a widget instance is created, the user can assign new values
to the public variables using the configure method. For example, the
following code changes the <tt>-direction</tt> variable of the <tt>.arr</tt> instance to <tt>n</tt>.
<P>
<blockquote> <P><tt> .arr configure -direction n
</tt>
<P></blockquote>
<P>In order for configuration to work, you have to define a
configuration method that does what the programmer expects. The
configuration method of a public variable is invoked whenever the
user calls the configure method to change the value of this
variable. The name of a configuration method must be the name of the
public variable prefixed by the creation command of the class and
<tt>:config</tt>. For example, the name configuration method for the
<tt>-direction</tt> variable of the TixArrowButton class is <tt>tixArrowButton:config-direction</tt>. The following code implements
this method:
<P>
<blockquote> <P><tt> proc tixArrowButton:config-direction {w value} {<BR>
upvar #0 $w data<BR>
<BR>
$data(w:button) config -bitmap @$value.xbm<BR>
}</tt>
<P></blockquote>
<P>Notice that when <tt>tixArrowButton:config-direction</tt> is called,
the <tt>value</tt> parameter contains the new value of the <tt>-direction</tt> variable but <tt>data(-direction)</tt> contains the <tt>old</tt> value. This is useful when the configuration method needs to
check the previous value of the variable before taking in the new
value.
<P>If a type checker is defined for a variable, it will be called
before the configuration method is called. Therefore, the
configuration method can assume that the type of the <tt>value</tt>
parameter is got is always correct.
<P>Sometimes it is necessary to override the value supplied by the
user. The following code illustrates this idea:
<P>
<blockquote> <P><tt> proc tixArrowButton:config-direction {w value} {<BR>
upvar #0 $w data<BR>
<BR>
if {$value == &quot;n&quot;} {<BR>
set value s<BR>
set data(-direction) $value<BR>
}<BR>
<BR>
$data(w:button) config -bitmap @$value.xbm<BR>
return $data(-direction)<BR>
}</tt>
<P></blockquote>
<P>Notice the above code always overrides values of <tt>n</tt> to <tt>s</tt>. If you need to override the value, you must do the following two
things:
<P>
<blockquote> <UL><P><LI><P>Explicitly set the instance variable inside the configuration
method (the <tt>set data(-direction) $value</tt> line).
<P><LI><P>Return the modified value from the configuration method.
</UL>
</blockquote>
<P>If you do not need to override the value, you don't need to return
anything from the configuration method. In this case, the Tix
Intrinsics will assign the new value to the instance variable for
you.
<P>
<HR>
</FONT>
</BODY>
<P><ADDRESS>
<A HREF=http://tix.sourceforge.net>http://tix.sourceforge.net</A><BR>
</ADDRESS>