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<H2><A NAME="sec:5.4"><SPAN class="sec-nr">5.4</SPAN> <SPAN class="sec-title">Constraints</SPAN></A></H2>
<P><font size=-1>XPCE</font> allows the user to specify constraints
between pairs of objects. In the example above we would like the text to
be centered relative to the bitmap. This may be achieved using:
<PRE class="code">
10 ?- get(@ic, member, bitmap, Bitmap),
get(@ic, member, text, Text),
new(_, constraint(Bitmap, Text, identity(center_x))).
</PRE>
<P>Each time either the bitmap or the text changes this constraint will
invoke <B><CODE><-</CODE>center_x</B> on the changed object and <B><CODE>-></CODE>center_x</B>
with the return value on the other object. Class <A class="" href="summary.html#class:spatial">spatial</A>
defines more general geometrical constraints between graphicals.
<P>Constraints are high-level, but potentially expensive means to
specify graphical relations. An alternative is the redefinition of the
<B><CODE>-></CODE>geometry</B> method of (compound) graphical
objects. See <A class="sec" href="udc.html">chapter 7</A>.
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