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<H3><A NAME="SECTION000291000000000000000">
2.9.1 Domain Objects (or Models)</A>
</H3>
<P>
Doing Object Oriented (OO) implementation implies the system you are
creating revolves around objects; in the case of Python applications, we
use the word `instances'. OO applications call the instances that
represent your problem's logical entities
<em class="citetitle"><a
href="http://c2.com/cgi/wiki?DomainObject"
title="'domain objects'"
>'domain objects'</a></em>. In a
mail application, for instance, you might have Person and Message domain
objects; in a sales application you could have Product and Sale.
<P>
Picking up from our last example, you could say <tt class="class">NewsItem</tt>
represented a class of domain objects, since its instances held data (or
`state') that represented a logical container relevant to the task we
had at hand, browsing news. This example was very simple, and there was
no mechanism coded into the <tt class="class">NewsItem</tt> class, but the concept is
the same.
<P>
One very common problem when coding a new UI to manipulate the
information in a domain object - at the fundamental level, in an
instance - is the amount of code that needs to be done to alter the
state of the instance. If you have a small form, with few controls in
it, it's merely a bother; if you have a complex form, with many controls
and lots of interaction, it can be a nightmare of accessors and message
passing. Each widget must have the appropriate signals connected, and
the handlers must either manipulate the instance directly, or call
accessors repeatedly. Changing the UI involves more coding and
complication.
<P>
So, in summary:
<P>
<UL>
<LI>I use the terms <B>object, model<A NAME="tex2html6"
HREF="#foot272"><SUP>6</SUP></A> and instance</B>
interchangeably to refer to a <B>domain object</B>.
</LI>
<LI>The problem at hand is propagating changes from the UI to the
domain objects attached to them.
<P>
This is the task Proxies attempt to simplify.
</LI>
</UL>
<P>
<BR><HR><H4>Footnotes</H4>
<DL>
<DT><A NAME="foot272">... model</A><A
HREF="node17.html#tex2html6"><SUP>6</SUP></A></DT>
<DD>A note on naming: in
MVC, one third of the triad is called Model, which (I believe) is short
for Application Model. In theory, the Application Model represents the
real-world behavior of your application, just like the domain object;
synonyms, basically. Some authors, however, defend that the Application
Model and the domain object should be separate entities. I don't think
this is necessarily true. From my experience, in most cases the two are
functionally equivalent (and separating them showed me that I ended up
doing was proxying calls from one to the other).
</DD>
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<span class="release-info">Release 1.9.22, documentation updated on August, 2006.</span>
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