Now that we know the theory behind this, let's clarify by walking through the example helloworld.py program.
Lines 9-76 define the
that contains all the callbacks as object methods and the object instance
initialization method. Let's examine the callback methods.
Lines 13-14 define the
method that will be called when the button is "clicked". When called the
method, prints "Hello World" to the console. We ignore the object instance,
the widget and the data parameters in this example, but most callbacks use
data is defined with a default value of
None because PyGTK will not pass a data value if it is
not included in the
connect() call; this would
trigger an error since the callback is expecting three parameters and may
receive only two. Defining a default value of None allows the callback to be
called with two or three parameters without error. In this case the data
parameter could have been left out since the
hello() method will always be called with just two
parameters (never called with user data). The next example will use the
data argument to tell us which button was
def hello(self, widget, data=None): print "Hello World"
The next callback (lines 16-26) is a bit special. The "delete_event" occurs when the window manager sends this event to the application. We have a choice here as to what to do about these events. We can ignore them, make some sort of response, or simply quit the application.
The value you return in this callback lets GTK+ know what action to take. By returning TRUE, we let it know that we don't want to have the "destroy" signal emitted, keeping our application running. By returning FALSE, we ask that "destroy" be emitted, which in turn will call our "destroy" signal handler. Note the comments have been removed for clarity.
def delete_event(widget, event, data=None): print "delete event occurred" return False
destroy() callback method (lines
28-30) causes the program to quit by calling
gtk.main_quit() . This function tells GTK+ that it is
to exit from
gtk.main() when control is returned to
def destroy(widget, data=None): print "destroy signal occurred" gtk.main_quit()
Lines 32-71 define the
instance initialization method
__init__() that creates
the window and widgets used by the program.
Line 34 creates a new window, but it is not displayed until we direct GTK+ to show the window near the end of our program. The window reference is saved in an object instance attribute (self.window) for later access.
self.window = gtk.Window(gtk.WINDOW_TOPLEVEL)
Lines 41 and 46 illustrate two examples of connecting a signal
handler to an object, in this case, the
window. Here, the
"delete_event" and "destroy" signals are caught. The first is emitted when
we use the window manager to kill the window, or when we use the
call. The second is emitted when, in the "delete_event" handler, we return
self.window.connect("delete_event", self.delete_event) self.window.connect("destroy", self.destroy)
Line 49 sets an attribute of a container object (in this case
window) to have a blank area along the inside of it
10 pixels wide where no widgets will be placed. There are other similar
methods that we will look at in Chapter 18, Setting Widget Attributes
Line 52 creates a new button and saves a reference to it in
self.button. The button will have the label "Hello World"
self.button = gtk.Button("Hello World")
In line 57 we attach a signal handler to the button so when it
emits the "clicked" signal, our
method is called. We are not passing any data to
hello() so we just pass
the data. Obviously, the "clicked" signal is emitted when we click the
button with our mouse pointer. The user data parameter value
None is not required and could be removed. The callback
would then be called with one less parameter.
self.button.connect("clicked", self.hello, None)
We are also going to use this button to exit our program. Line
62 illustrates how the "destroy" signal may come from either the window
manager, or from our program. When the button is "clicked", same as above,
it calls the
hello() callback first, and then the
following one in the order they are set up. You may have as many callbacks
as you need, and all will be executed in the order you connected
Since we want to use the
destroy() method that accepts one argument (the
widget to be destroyed - in this case the
window), we use
connect_object() method and pass it the
reference to the window. The
method arranges to pass the
window as the first
callback argument instead of the button.
destroy() method is called it will cause the
"destroy" signal to be emitted from the window which will in turn cause the
to be called to end the program.
self.button.connect_object("clicked", gtk.Widget.destroy, self.window)
Line 65 is a packing call, which will be explained in depth later on in Chapter 4, Packing Widgets . But it is fairly easy to understand. It simply tells GTK+ that the button is to be placed in the window where it will be displayed. Note that a GTK+ container can only contain one widget. There are other widgets, described later, that are designed to layout multiple widgets in various ways.
Now we have everything set up the way we want it to be. With all the signal handlers in place, and the button placed in the window where it should be, we ask GTK+ (lines 66 and 69) to "show" the widgets on the screen. The window widget is shown last so the whole window will pop up at once rather than seeing the window pop up, and then the button forming inside of it. Although with such a simple example, you'd never notice.
Widgets also have a
hide() that is the
show(). It doesn't actually
destroy the widget, but it removes the widget renderining from your
display. This can be reversed with another
Lines 73-75 define the
which calls the
def main(self): gtk.main()
Lines 80-82 allow the program to run automatically if called
directly or as an argument of the python interpreter. Line 81 creates an
instance of the
HelloWorld class and saves a
reference to it in the
hello variable. Line 82 calls the
method to start the GTK+ event processing loop.
if __name__ == "__main__": hello = HelloWorld() hello.main()
Now, when we click the mouse button on a GTK+ button, the widget
emits a "clicked" signal. In order for us to use this information, our
program sets up a signal handler to catch that signal, which dispatches the
function of our choice. In our example, when the button we created is
hello() method is called with the
None argument, and then the next handler for this signal
is called. The next handler calls the widget
destroy() function with the window as its argument
thereby causing the window to emit the "destroy" signal, which is caught,
and calls our
Another course of events is to use the window manager to kill
the window, which will cause the "delete_event" to be emitted. This will
call our "delete_event" handler. If we return
the window will be left as is and nothing will happen. Returning
FALSE will cause GTK+ to emit the "destroy" signal that
HelloWorld "destroy" callback to be
called, exiting GTK.