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<meta name="AUTHOR" content="Jim Bublitz">
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<h1>Tutorial - Second Program</h1>
<h3>KMainWindow</h3>
<p>
In this tutorial, we turn the simple "Hello, World" program into a full-fledged
KDE program (but not one that does anything useful).
</p>
<p>
Most KDE programs have at least two features in common: the have a main window
that manages all of the other elements that make up the graphical interface of
the program, and they have an event loop, that handles the communication between
the program elements and between the user and the program (mouse clicks and keyboarding).
</p>
<p>
First, we change the objects we imports from <b><i>kdeui</i></b>:
</p>
<pre style="background-color : #f0f8ff">
from PyKDE4.kdeui import KApplication, KMainWindow
</pre>
<p>
We're no longer importing the <i>KGuiItem</i> or <i>KMessageBox</i> we used in the first
program.
</p>
<p>
Next, we create our own customized main window by subclassing (inheriting from) <i>KMainWindow</i>:
</p>
<pre style="background-color : #f0f8ff">
class MainWindow (KMainWindow):
def __init__ (self):
KMainWindow.__init__ (self)
</pre>
<p>
When you derive a new class from an existing class, it's important to call the constructor or
<i>__init__</i> of the parent class. In this case, <i>KMainWindow.__init__</i> doesn't
require any arguments, so we just pass <i>self</i> to it.
</p>
<p>
Notice that our main window (<i>MainWindow</i>) does absolutely nothing but create itself.
</p>
<p>
In order to display our main window and have our program operate, we add just three lines to
the end of our program:
</p>
<pre style="background-color : #f0f8ff">
mainWindow = MainWindow ()
mainWindow.show ()
app.exec_ ()
</pre>
<p>
The first line:
</p>
<pre style="background-color : #f0f8ff">
mainWindow = MainWindow ()
</pre>
<p>
creates an instance of our <i>MainWindow</i> class. The next line:
</p>
<pre style="background-color : #f0f8ff">
mainWindow.show ()
</pre>
<p>
causes our main window to be displayed. The final line:
</p>
<pre style="background-color : #f0f8ff">
app.exec_ ()
</pre>
<p>
starts <i>KApplication</i>'s event loop. In the previous program we didn't have an event
loop running, but the program could still respond to mouse clicks - that's because KDE
dialogs run their own private event loop behind the scenes.
</p>
<p>
Note also that in PyKDE, the call to run the event loop is <i>exec_</i> - the extra underscore
at the end of the name is necessary because otherwise the name would clash with Python's
built-in <i>exec</i> keyword.
</p>
<p>
<i>app.exec_</i> runs continuously, handling all the events that make up the actions of our
program - it doesn't return until program execution ends. Once app.exec_ begins to run,
nothing else in our program will occur unless initiated by some event or action,
either from the user, some external source, or some event we create, like a timer timing-out.
</p>
<p>
That means that usually all initialization and setup takes place before we start the event loop.
</p>
<h3>What now?</h3>
<p>
In PyKDE for KDE3, this would be a good start for a complete program. In KDE4, programs operate
a little differently, and most probably won't use <i>KMainWindow</i> directly any longer - much of
the functionality that was in <i>KMainWindow</i> in KDE3 has been moved into another class.
We'll look at that class - <i>KXmlGuiWindow</i> - in the next tutorial.
</p>
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