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<H2><A NAME="SECTION000270000000000000000">
2.7 UI Delegates</A>
</H2>
<P>
The UI Delegate classes, located in the <tt class="module">kiwi.ui.delegates</tt> module, use
multiple inheritance, being derived from both a View and a Controller.
This means they combine functionality from both into a single class: not
only does it specify the interface, it also handles the widget signals.
It's the "can't beat them, join them" solution to the V+C coupling
problem, and it's used in Swing, MFC and Bakery, a C++ framework for
GTK+.
<P>
The <tt class="class">Delegate</tt> class is a combination of <tt class="class">BaseView</tt> and
<tt class="class">BaseController</tt>. You can use it to create your interface
programatically and then hook up handlers to it, just like you would do
to individual views and controllers. A very simple (yes, I too get bored
of hand-coding PyGTK widgets) example follows, found in the distribution
as <span class="file">examples/framework/simple.py</span>:
<P>
<DIV ALIGN="CENTER">
<IMG
ALIGN="BOTTOM" BORDER="0"
SRC="img7.png"
ALT="\includegraphics[scale=0.905]{images/simple2.eps}">
</DIV>
<P>
<BR>
<PRE CLASS="verbatim">#!/usr/bin/env python
import gtk
from kiwi.ui.delegates import Delegate
class Hello(Delegate):
def __init__(self):
self.index = 0
self.text = ["I've decided to take my work back underground",
"To keep it from falling into the wrong hands."]
topwidget = gtk.Window()
topwidget.set_title("So...")
self.button = gtk.Button(self.text[self.index])
self.button.show()
topwidget.add(self.button)
Delegate.__init__(self, topwidget, delete_handler=gtk.main_quit)
# focus button, our only widget
self.focus_topmost()
def on_button__clicked(self, button):
self.index = self.index + 1
# Two clicks and we're gone
if self.index > 1:
self.hide_and_quit()
# the *handler's* return value disappears into GTK+
return
# update the label of the button
button.set_label(self.text[self.index])
app = Hello()
app.show_and_loop()
</PRE>
<P>
<DIV ALIGN="CENTER">
<IMG
ALIGN="BOTTOM" BORDER="0"
SRC="img8.png"
ALT="\includegraphics[scale=0.905]{images/simple3.eps}">
</DIV>
<P>
As you can see, the Delegate was the only class we needed to inherit
from and instantiate. This makes the amount of infrastructure code go
down significantly, and most single-window applications (and individual
dialogs in multi-window applications) can be implemented with single
Delegate and GladeDelegate subclasses. As an example for using the
GladeDelegate, below is a third version of our temperature conversion
application, <span class="file">examples/Faren/faren3.py</span>:
<P>
<BR>
<PRE CLASS="verbatim">#!/usr/bin/env python
import gtk
from kiwi.ui.delegates import Delegate
class Farenheit(Delegate):
widgets = ["quitbutton", "temperature", "celsius", "farenheit",
"celsius_label" , "farenheit_label", "temperature_label"]
gladefile = "faren"
def __init__(self):
Delegate.__init__(self, delete_handler=self.quit_if_last)
def convert_temperature(self, temp):
farenheit = (temp * 9/5.0) + 32
celsius = (temp - 32) * 5/9.0
return farenheit, celsius
def clear_temperature(self):
self.farenheit.set_text("")
self.celsius.set_text("")
# Signal handlers
def on_quitbutton__clicked(self, *args):
self.hide_and_quit()
def after_temperature__changed(self, entry, *args):
temp = entry.get_text().strip() or None
if temp is None:
self.clear_temperature()
else:
try:
farenheit, celsius = self.convert_temperature(float(temp))
except ValueError:
farenheit = celsius = float('nan')
self.farenheit.set_text("%.2f" % farenheit)
self.celsius.set_text("%.2f" % celsius)
delegate = Farenheit()
delegate.show()
gtk.main()
</PRE>
<P>
As you can see, using the delegates makes a lot of sense in many cases:
the view and controller split isn't essential, and you can end up
with a lot less code with the same functionality at no reduced
maintainability or readability, which is what we are aiming for. For
larger applications, or apps where a lot of very unusual functionality
is required, it may still make sense to use separate Controllers and
Views, but for normal development, stick to delegates; they simplify
things.
<P>
Note that there is also a <tt class="class">SlaveDelegate</tt>, which works like a
normal SlaveView with an attached Controller.
<P>
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<span class="release-info">Release 1.9.22, documentation updated on August, 2006.</span>
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