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<H3><A NAME="SECTION000262000000000000000">
2.6.2 Advanced Controllers</A>
</H3>
<P>
While the previous section on Controllers offered a nice example with
the Faren application, it does present some issues. First, the interface
could use some extra niceness (deleting text from the entry does
nothing!). Second, the controller has to invoke methods on its view's
instance variables (<code>self.view.celsius.set_text()</code> for example),
which is a sign of high coupling. One good way to solve these issues is
to provide a subclass of BaseView that does some extra setup and
provides an API for the Controller (<span class="file">faren2.py</span>):
<P>
<BR>
<PRE CLASS="verbatim">#!/usr/bin/env python
import gtk
from kiwi.controllers import BaseController
from kiwi.ui.views import BaseView
class FarenControl(BaseController):
def convert_temperature(self, temp):
celsius = (temp - 32) * 5/9.0
farenheit = (temp * 9/5.0) + 32
return farenheit, celsius
def on_quitbutton__clicked(self, *args):
self.view.hide_and_quit()
# use changed instead of insert_text, since it catches deletes too
def after_temperature__changed(self, entry, *args):
temp = view.get_temp()
if temp == None:
self.view.clear_temp()
else:
farenheit, celsius = self.convert_temperature(float(temp))
self.view.update_temp(farenheit, celsius)
class FarenView(BaseView):
widgets = ["quitbutton", "temperature", "celsius", "farenheit",
"celsius_label" , "farenheit_label", "temperature_label"]
def __init__(self):
BaseView.__init__(self, gladefile="faren",
delete_handler=self.quit_if_last)
def get_temp(self):
return self.temperature.get_text() or None
def update_temp(self, farenheit, celsius):
self.farenheit.set_text("%.2f" % farenheit)
self.celsius.set_text("%.2f" % celsius)
def clear_temp(self):
self.farenheit.set_text("")
self.celsius.set_text("")
view = FarenView()
ctl = FarenControl(view)
view.show()
gtk.main()
</PRE>
<P>
This (much longer, sure, but with a nicer division) example shows us
using a separate View class, which presents an API for manipulating the
View interface widgets, consisting of the following methods:
<P>
<UL>
<LI><code>get_temp()</code>: Gets the temperature from the gtk.Entry in the
interface, and returns it. Returns None if the entry is empty.
</LI>
<LI><code>update_temp()</code>: Updates the contents of the two labels that
represent the temperature (in farenheit and in celsius).
</LI>
<LI><code>clear_temp()</code>: Clears the contents of the two labels that
represent the temperature.
</LI>
</UL>
<P>
The Controller also uses <code>changed</code> instead of <code>insert_text</code>,
so it supports both insertion and deletion.
<P>
This is basically what you need to know about the controller. While it
is useful in a number of applications, if your interface has a number of
widgets and rich interaction, you will notice that your View API will
start looking like a exact copy of the widget methods. This is highly
undesirable (as are accessor functions, for the same reason): it means
the Controller is tightly coupled to the View, which is not what we
want. And <B>herein lies the main problem with the View/Controller
split</B>: if your interface is simple (as it is in our example), it's a
nice way to separate things; however, if your interface is complex and
has lots of widgets (I'd say a good rule of thumb is over 5 interactive
widgets) you will end up with a lot of low-level View/Controller message
passing.
<P>
(Another potential problem with the split is that neither the Controller
nor the View are very reusable, which defeats part of the goals of
modularity). The next section describes UI Delegates, which are
combinations of View and Controller that simplify things somewhat.
<P>
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<span class="release-info">Release 1.9.22, documentation updated on August, 2006.</span>
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