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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> Chaining Methods<A NAME=6411>&nbsp;</A></TITLE>
</HEAD>
<BODY BGCOLOR="#ffffff" TEXT="#000000" LINK="#0000ff" VLINK="#000080">
<FONT FACE="Tahoma, Arial, Helvetica">
<HR> <A NAME=tex2html1234 HREF=subsubsectionstar3_8_4_3.html><IMG ALIGN=MIDDLE SRC="../gif/icons/next_motif.gif"></A> <A NAME=tex2html1232 HREF=subsection3_8_4.html><IMG ALIGN=MIDDLE SRC="../gif/icons/up_motif.gif"></A> <A NAME=tex2html1226 HREF=subsubsection3_8_4_1.html><IMG ALIGN=MIDDLE SRC="../gif/icons/previous_motif.gif"></A> <A NAME=tex2html1236 HREF=tableofcontents3_1.html><IMG ALIGN=MIDDLE SRC="../gif/icons/contents_motif.gif"></A> <BR>
<B> Next:</B> <A NAME=tex2html1235 HREF=subsubsectionstar3_8_4_3.html> The tixChainMethod call</A>
<B>Up:</B> <A NAME=tex2html1233 HREF=subsection3_8_4.html> Standard Initialization Methods</A>
<B> Previous:</B> <A NAME=tex2html1227 HREF=subsubsection3_8_4_1.html> The InitWidgetRec Method</A>
<HR> <P>
<A NAME=Contents>&nbsp;</A><H3><A NAME=SECTION00084200000000000000> Chaining Methods<A NAME=6411>&nbsp;</A></A></H3>
<P>
The above implementation is not sufficient because our
TixArrowButton class is derived from TixPrimitive. The class
derivation in Tix is basically an <em>is-a</em> relationship:
TixArrowButton <em>is a</em> TixPrimitive. TixPrimitive defines the
method <tt>tixPrimitive:InitWidgetRec</tt> which sets up the instance
variables of every instance of TixPrimitive. Since an instance of
TixArrowButton is also an instance of TixPrimitive, we need to make
sure that the instance variables defined by TixPrimitive are also
properly initialized. The technique of calling a method defined in a
superclass is called the <em>chaining</em> of a method. The following
implementation does this correctly:
<P>
<blockquote> <P><tt> proc tixArrowButton:InitWidgetRec {w} {<BR>
upvar #0 $w data<BR>
<BR>
tixPrimitive:InitWidgetRec $w<BR>
set data(count) 0<BR>
}</tt>
<P></blockquote>
<P>Notice that <tt>tixPrimitive:InitWidgetRec</tt> is called before anything
else is done. This way, we can define new classes by means of
successive refinement: we can first ask the superclass to set up the
instance variables, then we can modify some of those variables when
necessary and also define new variables.
<P>
<HR>
</FONT>
</BODY>
<P><ADDRESS>
<A HREF=http://tix.sourceforge.net>http://tix.sourceforge.net</A><BR>
</ADDRESS>