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<H3><A NAME="SECTION000294000000000000000">
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2.9.4 Customizing Proxies and Models</A>
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From our example, proxy and instance appear to be completely coupled -
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even the names of the components and attributes are tied. This is a
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precise interpretation of the situation, but in my opinion, this
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coupling is not only unavoidable, it is the essence of UI architecture.
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Treating the interface as a completely separate entity from the object
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it manipulates is, honestly, a bad idea, because <B>the interface is a
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representation of the object</B>, and as such, essentially coupled to it.
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Would it make sense to remove the <code>url</code> attribute from the model
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and <I>not</I> remove it from the interface in question?
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Because the UI is really a representation, however, there are times
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where the contents of the widget and its attached proxy attribute must
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differ in some way. Often, it is a matter of cardinality: more than one
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widget defines a single proxy attribute, or vice-versa; at other times,
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the data format in the widget does not match the format in the model
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(think of dates, represented by strings in the interface, but stored as
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DateTime objects in the object)<A NAME="tex2html7"
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HREF="#foot294"><SUP>7</SUP></A>. The Kiwi Proxy was
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designed to cater to these different requirements, using accessor
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functions when available.
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Accessors provide an easy way to translate the model value to the
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interface value: a pair of <code>get_*()</code> and <code>set_*()</code> functions
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implemented in the model that perform internal manipulation of its
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variables, removing the need to directly manipulate the instance
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variable. You can define accessors for as few or as many model
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To make the process clearer, it is worth discussing how the model and
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the UI are updated. The heuristics for updating a model are:
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<LI>If a change happens in the Proxy's widget <code>X</code>, it looks at
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the model attached to it.
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<LI>If the model offers a <code>set_X()</code> method, it is called, with
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the new value as its only parameter.
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<LI>If not, it manipulates the model directly by using
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<code>setattr()</code>, which is the equivalent of <code>model.X = value</code>. If
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<code>Proxies.set_attr_warnings(True)</code> has been called, a warning like
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the following will be printed:
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<div class="verbatim"><pre>
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Kiwi warning: could not find method set_title in model
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<__main__.NewsItem instance at 0x82011ac>, using setattr()
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<LI>The model is updated (If multiple proxies are attached to the
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model, special things happen, additionally, as you will see in section
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<A href="multipleproxies.html#multipleproxies">2.9</A>).
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The heuristics for updating the interface widget are:
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<LI>On startup, the proxy queries the model for the value for
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attribute <code>X</code>.
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<LI>It tries first using an accessor method <code>get_X()</code>
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(which should return a single value). If the accessor does not exist, it
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will attempt to access the model's variable directly (using
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<code>getattr()</code>). As with <code>setattr()</code> above, a warning will be
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printed if attribute warnings are enabled.
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<LI>The interface is updated with this initial value, and normal
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event processing begins.
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<LI>If a model's state for <code>X</code> is altered (<code>item.X =
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"foo"</code>), a notification is sent to the proxy, with the attribute that
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was altered. If a callback calls <code>self.update("X")</code>, a notification
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is sent to the proxy, with the attribute that was altered.
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<LI>The proxy receives the notification; it gets the value of <code>X</code>
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from the model using the same method as in step 2, and updates the
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widget contents accordingly.
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Summarizing: if you would like to customize the connection between model
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and proxy for an attribute, implement accessor functions
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(<code>get_foo()</code> and <code>set_foo()</code>) for it. If you would like to
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verify that no direct instance manipulations are happening, use the
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module function <code>set_attr_warnings()</code> and check the output
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printed to the console's standard error.
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Let's extend our previous example to provide an accessor and explain how
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things work out (<span class="file">newsform3.py</span>).
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<div class="verbatim"><pre>
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class NewsItem(FrameWork.Model):
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"""An instance representing an item of news.
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Attributes: title, author, url"""
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def set_url(self, url):
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"""sets the url, prefixing "http://" if missing"""
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if len(url) > len(http) and string.find(url, http) != 0:
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In this example, we provide an accessor for setting the url (a "setter")
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prefixed by "http://". The accessor is called when the entry's text
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changes (for each character inserted or deleted), which is why I have to
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check for the length of the url typed in. Note that I don't provide a
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<code>get_url()</code> method (a "getter"), which means the raw url would be
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loaded from the instance into the interface. In this specific case, this
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is not an issue, because data is only loaded from the instance at
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instantiation time and when the model attribute is changed. However, for
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most cases both setter and getter need to convert to and from the model
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<BR><HR><H4>Footnotes</H4>
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<DT><A NAME="foot294">... object)</A><A
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HREF="node20.html#tex2html7"><SUP>7</SUP></A></DT>
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<DD> Apart from these reasons, some
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people insist that it is bad or wrong to manipulate the instance
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directly, and that accessors should always be used.
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<span class="release-info">Release 1.9.12, documentation updated on August, 2006.</span>
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