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<H3><A NAME="SECTION000297000000000000000">
2.9.7 Models</A>
</H3>
<P>
The <tt class="module">kiwi.model</tt> module <tt class="class">Model</tt> classes. As discussed
before, the <tt class="class">Model</tt> class provides a mechanism for updating the
user interface based on changed performed in the model instance.
<P>
This is a powerful mechanism that allows you to guarantee that the
interface is always up-to-date with relation to an instance; it should
be used with care, however, because the update semantics are not trivial
to understand - just imagine having custom callbacks that alter the
same attribute as the proxy callback and you will understand why it is
easy to get in recursive loops. This tip I can offer you, therefore:
avoid altering the model in custom callbacks, and only do it in special
situations.
<P>
You can also attach multiple proxies to the same model and have them
update automatically; the next section will discuss this further. Before
passing on to that, however, I need to describe a special <tt class="class">Model</tt>
called <tt class="class">PickledModel</tt>. To demonstrate by example, I will revisit
the first example we saw in this text, the Person editor from section
<A href="person.html#person">2</A>. Let's see the code again:
<P>
<BR>
<PRE CLASS="verbatim">#!/usr/bin/env python
import gtk
from kiwi.model import PickledModel
from kiwi.ui.delegates import ProxyDelegate
# define the class that holds our application data
class Person(PickledModel):
pass
person = Person.unpickle()
# create and run a proxy interface attached to person
view = ProxyDelegate(person, gladefile="Person",
widgets=["address", 'name', 'phone'],
delete_handler=gtk.main_quit)
view.focus_topmost()
view.show_and_loop()
# save changes done to the instance
person.save()
</PRE>
<P>
Looking at the code, you might notice three special issues:
<P>
<OL>
<LI>The class our model inherits from is <tt class="class">FrameWork.PickledModel</tt>.
</LI>
<LI>We actually get our model instance by making a call to
<code>Person.unpickle()</code>, passing the class <tt class="class">Person</tt> as
an argument.
</LI>
<LI>We call <code>person.save()</code> at the end of the program.
</LI>
</OL>
<P>
<tt class="class">PickledModel</tt> offers a very cheap way to offer persistence for
your model instances. If you are familiar with the ZODB, think of
<tt class="class">PickledModel</tt> as a cheapo implementation that allows you to save
and restore an instance. To use it:
<P>
<OL>
<LI>Have your model class inherit from PickledModel.
<P>
</LI>
<LI>Use the <code>PickledModel.unpickle</code> which is a class method on
your model. it takes a filename as the optional second parameter (if omitted,
it uses the name of the class, with an extension of ".pickle").
The method will try and load the model state from a pickle file,
and if it doesn't exist, will create a new model and return it to you.
If the pickle file is damaged, it will save it with the extension ".err"
(avoiding it being erased by an unwitting <code>save()</code> call).
<P>
</LI>
<LI>Use the <code>save()</code> method to save the pickle when you are
ready. You may specify a filename to it; otherwise, it uses the default
filename specified through the <code>set_filename()</code> method. Note that
<code>PickledModel.unpickle</code> makes the filename it tried to used the
default by calling <code>set_filename()</code> internally.
</LI>
</OL>
<P>
The <span class="file">diary3.py</span> example in <code>examples/diary/</code> uses pickling in a
more involved manner and is recommended as a reference.
<P>
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<span class="release-info">Release 1.9.22, documentation updated on August, 2006.</span>
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