Gtk development-using-glade-3

GTK Development
Using Glade 3
GTK+ is a toolkit, or a collection of libraries, which
developers can use to develop GUI applications for
Linux, OSX, Windows, and any other platform on
which GTK+ is available.

Original Article by Micah Carrick

Table of Contents
Part 1 - Designing a User Interface Using Glade 3 ___________________________________________1

Quick Overview of GTK+ Concepts ______________________________________________________________1

Introduction to Glade3 _________________________________________________________________________3

Getting Familiar with the Glade Interface_______________________________________________________3

Manipulating Widget Properties________________________________________________________________5

Specifying Callback Functions for Signals _______________________________________________________6

Adding Widgets to the GtkWindow _____________________________________________________________9

How Packing Effects the Layout ______________________________________________________________ 13

Editing the Menu (or Toolbar) ________________________________________________________________ 15

Final Touches to the Main Window ___________________________________________________________ 18

Part 2 - Choosing a Programming Language for GT K+ Dev elopm ent _____________ 19

Which is the BEST Language? ________________________________________________________________ 19

Language Choice Considerations _____________________________________________________________ 19

A Look at Python vs. C________________________________________________________________________ 20

Part 3 __________________________________________________________________________________________ 22

Setting Up Your Development Environment___________________________________________________ 22

GtkBuilder and LibGlade _____________________________________________________________________ 24

The Minimal Application _____________________________________________________________________ 25

Compiling and Running the Application ______________________________________________________ 27

Stepping Through the Code ___________________________________________________________________ 29
Including the GTK+ Library _______________________________________________________________ 29
Initializing the GTK+ Library ______________________________________________________________ 29
Building the Interface with GtkBuilder ____________________________________________________ 30
Getting References to Widgets From GtkBuilder _________________________________________ 32
Connecting Callback Functions to Signals _______________________________________________ 33
Showing the Application Window _________________________________________________________ 36
In Summary _______________________________________________________________________________ 37

Part 1| Designing a User Interface Using Glade 3

In part 1 of the GTK+ and Glade3 GUI Programming Tutorial series, we will be
designing the graphical user interface (GUI) for a GTK+ text editor application (shown
left) which will be used throughout these tutorials. This GUI design will be created using
the Glade Interface Designer and is completely independent of the programming
language used to implement the design, which will come in subsequent tutorials.

Quick Overview of GTK+ Concepts

If you have no experience with GTK+, you may struggle with some of the
concepts I am going to cover. Although I am going to attempt to teach some of these
concepts on the fly, it would serve you well to read up on these ideas further, perhaps
after working through part 1 of this tutorial. Understanding the fundamental concepts of
GTK+ will be instrumental in your ability to effectively use Glade.

First of all, GTK+ is not a programming language. GTK+ is a toolkit, or a
collection of libraries, which developers can use to develop GUI applications for Linux,
OSX, Windows, and any other platform on which GTK+ is available. It can be thought
of in the same terms as MFC or the Win32 API on Windows, Swing and SWT in Java,
or Qt (the "other" Linux GUI toolkit used by KDE).

Although GTK+ itself is written in C, there are a multitude of language "bindings"
allowing programmers to develop GTK+ applications in the language of their choice
including C++, Python, Perl, PHP, Ruby, and many others.

GTK+ is based on 3 main libraries: Glib, Pango, and ATK, however, we primarily
work with GTK+ and let GTK+ do it's magic with those 3 libraries. GLib wraps most of
the standard C library functions for portability (allowing your code to run on Windows
and Linux if desired). We use GLib a lot when working in C or C++, which I will explain
more thoroughly when implementing our design using C. Higher-level languages such
as Python and Ruby won't have to worry about GLib as they have their own standard
libraries which provide similar functionality.

GTK+ and associated libraries implement an object oriented approach through
GObject. How this works isn't important just yet, and different programming languages
will reveal this to you differently, however, it's important to understand that GTK+
uses object orientation (yes, even in C).

Every piece of a GTK+ GUI is comprised of one or more "widgets" which are
objects. All widgets will be derived from a base widget called GtkWidget. For example,
an application's window is a widget called GtkWindow. The toolbar within that window

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is a widget called GtkToolbar. Although a GtkWindow is also a GtkWidget, a
GtkWidget is not neccesarily a GtkWindow. Child widgets are derived from their parent
objects to extend the functionality of that object. These are standard OOP (object
oriented programming) concepts (hint: Google search "object oriented programming" if
this is a new concept).

We can look at any widget in the GTK+ reference documentation to see which
objects it is derived from. In the case of GtkWindow, it looks something like this:

GObject
+----GInitiallyUnowned
+----GtkObject
+----GtkWidget
+----GtkContainer
+----GtkBin
+----GtkWindow

As you can see, a GtkWindow is derived from GtkBin which is derived from
GtkContainer, and so on. For your first application, you don't need to worry about
anything above the GtkWidget object. The reason this heirarchy is so important is
because when you're looking for functions, properties, and signals for any particular
widget, you need to realize that the functions, properties, and signals of it's parent
objects apply to it as well. In part 2, this will become even more apparent when writing
code for this example application.

We also begin to see a naming convention emerge. This is pretty handy. We can
easily tell what library an object or function is from. All objects beginning with Gtk are
from GTK+. Later, we'll see things like GladeXML which is part of Libglade or GError
which is part of GLib. All objects (and thus Widgets) are incamel case. The functions
which manipulate these objects are in lower-case with underscores for spaces. For
example, gtk_window_set_title() is a function to set the title property of a GtkWindow
object.

All the reference documentation you will need is available online
from library.gnome.org/devel/references, however, it is much easier to
use Devhelp which is likely available as a package for your distribution. Devhelp allows
you to browse and search the API documentation for the libraries you have installed on
your system (assuming you install that libraries documentation as well).

More information on GTK+ and Glib:

 Foundations of GTK+ Development (book)

 GTK+ 2.0 Tutorial

 GTK+ - The GIMP Toolkit

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 GLib Reference Manual

 GTK+ Reference Manual

 GTK+ 2.0 Tutorial (C Programming)

 PyGTK+ 2.0 Tutorial (Python Programming)

Introduction to Glade3

Glade is a RAD (Rapid Application Development) tool for designing GTK+
applications. Glade is a GTK+ application itself. It is simply a piece of software
developers use to simplify the process of laying out an application's interface. Glade
creates what will hereforth be refered to a s a "glade file". A glade file is actually an
XML file which describes the heirachy of the widgets comprising the interface.

Glade originally generated C code to build the GUI (and you'll still find examples
and tutorials doing this). This was later discouraged in favor of using a library,
Libglade, to build the interface at run time. And finally, as of Glade3, the old method
has become deprecated. That means the ONLY thing glade does is allow you to
generate a glade file which describes how the GUI is going to be built. This allows
more flexibility with the developer, prevents having to re-compile applications when a
minor interface change is needed, and allows more programming languages to be
used with Glade.

Glade3 has had significant changes since previous versions such as Glade2.
Glade3 has been available for some time and you shouldn't have any problems
obtaining it. The package manager for your distribution (yum, aptitude, etc.) should
have Glade3 available. You should note however, that the package will have 3 in it.
Where 'glade' may be the name for the old package, 'glade-3' or 'glade3' will be the
package name for the new version on which this tutorial is based. Glade is also
available from source at glade.gnome.org.

Getting Familiar with the Glade Interface

Start up Glade and let's get familiar with it's interface. I will be referring to various
aspects of Glade by the names described here. On the left hand side is the "Palette".
The Palette is like that of a graphics editing application. It is a palette of GtkWidgets
which you can use to design your application. In the middle area (which is empty when
you first start Glade) is the "Editor". This is where you see your design in progress. On
the right hand side is the "Inspector" on top and the widget "Properties" below that.
The Inspector shows your design as a tree allowing you to access and view the
heirarchy of the widgets making up your design. We manipulate various properties of
widgets in the Properties tabs, including specifying callback functions for signals
(explained later).

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So, the verfy first thing we're going to do, is create a Toplevel widget and save
our file. To do this, Click on the GtkWindow icon in the Palette under the
'Toplevels' section. You should notice a gray box show up inside the Editor area of
Glade. This is the workable area of a GtkWindow. The titlebar, close button, etc. will be
added to the widget by the window manager (ie: GNOME) so we don't see it while
editing. We will always start with a toplevel widget in Glade, typically a GtkWindow.

Before going further, save the file as "tutorial.glade".

Now the file you just saved, "tutorial.glade", is an XML file. If you were to open it up in a
text editor, it would look something like this:

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE glade-interface SYSTEM "glade-2.0.dtd">

<glade-interface>
<widget class="GtkWindow" id="window1">
<property name="events">GDK_POINTER_MOTION_MASK |
GDK_POINTER_MOTION_HINT_MASK | GDK_BUTTON_PRESS_MASK |
GDK_BUTTON_RELEASE_MASK</property>
<child>
<placeholder/>
</child>
</widget>
</glade-interface>

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As you can see, it's just a simple XML file. In part2 we will be using C with
Libglade to parse this XML file and generate the UI at run-time. Being XML, this could
just as easily be parsed by a Python program or any other language. As we continue to
work in Glade, this file will be updated to describe our interface in this XML format any
time we save. Exit out of your text editor and return to Glade.

Manipulating Widget Properties

The Editor of Glade now shows an empty GtkWindow widget. We are going to
manipulate some of the widget's properties. If you look in the Properties pane of
Glade, you will see 4 tabs: 'General', 'Packing', 'Common', and 'Signals'. Let's talk
about the first 2 tabs. GtkWidgets typically have various properties which manipulate
how they function and/or how they are displayed on the screen.

If you look at the reference documentation for a GtkWidget and scroll down to the
"Properties" section, you'll see a list of the properties for a GtkWindow. These are
typically the properties which appear in the 'General' tab of the Glade properties pane
and will vary from widget to widget. The name property exists for every widget in Glade
and is what we will use to reference the widget when it comes time to write code for
the application. Change the 'name' property of this GtkWindow from "window1" to
"window". Then, add the text "GTK+ Text Editor" to the 'Window Title' property.

We'll discuss the 'Packing' tab in a bit, but first, let's look at the 'Common' tab.
This tab also contains properties which belong to our GtkWindow, however, we don't
see them in the reference documentation for GtkWindow. This is because this is where
we set properties which areinherited from parent objects. Looking at the reference
documentation for a GtkWidget in the section called "Object Hierarchy", you'll see the
objects from which GtkWindow is derived. Click on the GtkContainer link to jump to
the reference documentation for a GtkContainer. You'll notice that GtkContainer has a

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property called "border-width" and we have a property in Glade for "Border width" at
the bottom of the 'Common' tab. We'll learn more about what a container widget is
later, however, this demonstrates how important that object heirarchy is. Since many
widgets are derived from GtkContainer, Glade puts it's properties into the 'Common'
tab.

In the "Object Hierarchy" section of the reference documentation for a
GtkContainer you'll see that it is derived from a GtkWidget. Now click the GtkWidget
link in to jump to the reference documentation for a GtkWidget. The GtkWidget has a
bunch of properties, many of which are also shown in the 'Common' tab of Glade's
Properties pane. These are properties which are common to all GTK+ widgets since all
GTK+ widgets are derivitives of GtkWidget.

Specifying Callback Functions for Signals

Objects emit a "signal" when something that might be useful to the programmer
happens. These are similiar to "events" from Visual Basic. If a user does anything
within your GUI, chances are they are emitting signals. As a programmer, you choose
which signals you want to capture and perform a task, and connect a callback function
to that signal.

The first signal we'll learn, and the one which you'll use in just about every GTK+
application you write, is the "destroy" signal emitted by GtkObject. This signal is
emitted whenever a GtkObject is destroyed. This is important, because when the user
closes the window through the little 'x' up in the title bar or any other means, the widget
is destroyed. We want to capture this signal and exit our application properly. This is
better illustrated when we write code for this GUI, however, for now, let's just specify
the function we want to call when the "destroy" signal is emitted for our GtkWindow.

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Look at the 'Signals' tab in the Glade Properties pane. You see a tree view where
GtkWindow and each of the objects from which it is derived are listed. If you expand
the GtkObject, you'll see all the signals emitted by GtkObject. These correspond to the
reference documentation for a GtkObject in the "Signals" section.

Under the 'Handler' column, click the gray text "<Type here>" to begin editing.
Select 'on_window_destroy' from the drop down and then hit ENTER. We can type
anything we want here, however, glade provides a drop-down list of some of the mroe
common callback function naming conventions. How this value is used depends on
how the programmer connects signals in the code, however, for this tutorial, we want
the GtkWindow's "destroy" signal to be associated with the handler string
"on_window_destroy". We'll look at this closer in Part 2.

At this point, we actually have a working GUI. We could write a few lines of code
in C, Python, Ruby, or any number of programming languages which would show our
empty window and then properly terminate when we clicked the "x" in the titlebar. For
this tutorial however, I will be showing you how to build the entire GUI in Glade3 before
writing any code. However, to satisfy any possible curiosity, if you would like to see
what a simple program looks like that would implement this Glade interface so far...

In C

/*
First run tutorial.glade through gtk-builder-convert with this
command:
gtk-builder-convert tutorial.glade tutorial.xml

Then save this file as main.c and compile it using this command
(those are backticks, not single quotes):

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gcc -Wall -g -o tutorial main.c `pkg-config --cflags --libs gtk+-
2.0` -export-dynamic

Then execute it using:
./tutorial
*/
#include <gtk/gtk.h>

void
on_window_destroy (GtkObject *object, gpointer user_data)
{
gtk_main_quit ();
}

int
main (int argc, char *argv[])
{
GtkBuilder *builder;
GtkWidget *window;

gtk_init (&argc, &argv);

builder = gtk_builder_new ();
gtk_builder_add_from_file (builder, "tutorial.xml", NULL);
window = GTK_WIDGET (gtk_builder_get_object (builder, "window"));
gtk_builder_connect_signals (builder, NULL);

g_object_unref (G_OBJECT (builder));

gtk_widget_show (window);
gtk_main ();

return 0;
}

In Python (note: you must set the 'visible' property of 'window' to "Yes" in the
'Common' properties tab in Glade)

#!/usr/bin/env python

# First run tutorial.glade through gtk-builder-convert with this
command:

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# gtk-builder-convert tutorial.glade tutorial.xml
# Then save this file as tutorial.py and make it executable using
this command:
# chmod a+x tutorial.py
# And execute it:
# ./tutorial.py

import pygtk
pygtk.require("2.0")
import gtk

class TutorialApp(object):
def __init__(self):
builder = gtk.Builder()
builder.add_from_file("tutorial.xml")
builder.connect_signals({ "on_window_destroy" : gtk.main_quit
})
self.window = builder.get_object("window")
self.window.show()

if __name__ == "__main__":
app = TutorialApp()
gtk.main()

Again, I'm not going to go over the details of the code used to implement this GUI
in this part of the tutorial, but instead focus on using Glade3. However, you can see
that implementing an interface designed in Glade is just a few lines of code in the
language of your choosing!

Adding Widgets to the GtkWindow

If you recall from the reference documentation for a GtkWindow, the GtkWindow
is a derivative of GtkContainer. Widgets derived from GtkContainer are container
widgets, meaning they can contain other widgets. This is another fundamental concept
of GTK+ programming. If you come from a Windows programming background, you
may be expecting to just drop a bunch of widgets onto the window and drag them
around into the position you want them. But this is not how GTK+ works --and for good
reason.

GTK+ widgets "packed" into various containers. Containers can be packed into
containers into containers and so forth. There are various packing properties which
effect how space is allocated for widgets packed into containers. Through these

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packing properties and nesting containers, we can have complex GUI designs without
having to write code to handler the resizing and re-positioning of our widgets.

This is probably one of the more difficult concepts for a new GTK+ developer, so
let's just see it in action!

The GtkWindow is a derivative of the container GtkBin which is a container that
contains only one child widget. But this text editor is going to have 3 main sections; a
menu bar, a text editing area, and a status bar. Therefore, we use a fundamental
GTK+ widget, the GtkVBox. The GtkVBox (vertical box) is a container widget which
can contain any number of child widgets stacked up vertically like rows (GtkHBox is
the horizontal equivelant).

Click on the GtkVBox icon in the Glade Palette under the 'Containers' section.
You'll notice that the 'Select' toolbar button on the top of the Glade window is no longer
depressed and your mouse cursor is the GtkVBox icon with a plus (+) sign when
hovering over the Glade Editor. This means you are ready to drop the GtkVBox
somewhere. Click in gray area of the Editor which is the empty space of the
GtkWindow.

A dialog box pops up asking you for the 'Number of items'. In this case we want 3
rows, so leave it as 3 and click 'OK'.

You should see the GtkWindow in the Editor divided into 3 rows now. These are
the 3 empty child widgets of the GtkVBox we just added. You should also notice that
the 'Select' toolbar at the top of Glade is once again depressed--allowing you to select
widgets within the Editor.

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Next, click on the GtkMenuBar icon in the Glade Palette under 'Containers'.
Drop this one in the top row of the GtkVBox you just added.

Now click on the GtkScrolledWindow icon in the Glade Palette under
'Containers'. Drop this one in to the middle row of the GtkVBox. When you do that, it
may not seem like anything has happened. However, you should notice that that
middle row looks selected. It's not--the GtkScrolledWindow is.The reason you don't
see anything, is because a GtkScrolledWindow doesn't have any initial visible
components. It's a container which will provide scroll bars when it's child widget gets
too large. We'll need this for our text editing widget.

11

Click the GtkTextView icon in the Glade Palette under 'Control and Display'.
Drop this one right on top of that GtkScrolledWindow (the middle row). We have now
just added the GtkTextView to the GtkScrolledWindow which was added to the
GtkVBox.

Finally, click on the GtkStatusbar icon in the Glade Palette under 'Control and
Display' and drop it into the bottom row.

And there you have it; the basic layout of our GTK+ text editor. If you look at the
Glade Inspector you will see the parent-child relationship of our design.

The Inspector will come in handy. You cannot always click on a widget in the
Editor as it might not be visible. For example, you cannot click on the
GtkScrolledWindow we added, because you can only see it's child, the GtkTextView.

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Therefore, if you need to change the properties of "scrolledwindow1", you will have to
select it in the Inspector.

I mentioned earlier how "packing" is an often frustrating concept to new GTK+
developers. Therefore, I'm going to show you first hand how various packing effects
the layout of your design.

How Packing Effects the Layout

When you look at the interface we've designed so far, you may take for granted
how "smart" Glade was. How did it know we didn't want to make the status bar taller?
Moreover, if you resize the window, how does it know that the text editing area grows
to fill the new vertical space? Well, Glade guessed. It applied default packing options
which are often what we want--but not always.

The best way to learn about packing is to play around with packing properties in
Glade as you can see the effects in real time. First, a quick description of the
applicable properties. Once you get a feel for GTK+, you may want to read more on
packing and space allocation.

 homogeneous: A property of the container widget which when set, tells
GTK+ to allocate the same amount of space for each child.
 expand: A property of the child being packed specifying if it should recieve
extra space when the parent grows.
 fill: A property of the child being packed specifying whether any extra space
should be given to the child or used as padding around the child.

Let's look at the default packing for our design. The GtkScrolledWindow has
"expand"=TRUE which means it recieves extra space when the parent grows, and it
has "fill"=TRUE which means it uses that extra space it recieves. This is how we want
it to work.

Widget Property Value

GtkVBox "vbox1" homogeneous FALSE

GtkMenuBar "menubar1" expand FALSE

fill TRUE

GtkScrolledWindow expand TRUE
"scrolledwindow1"

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fill TRUE

GtkStatusbar "statusbar1" expand FALSE

fill TRUE

Now, let's see what homogeneous does. Select the GtkVBox in the Glade
Inspector and change it's "Homogeneous" property under 'General' properties tab to
"Yes". Now the parent, "vbox1", allocates the same amount of space to each of it's
children. Since all 3 child widgets have "fill"=TRUE, they fill up this extra space
allocated to them.

Set the "Homogeneous" property back to "No".

Click on the GtkScrolledWindow "scrolledwindow1" in the Glade Inspector and
set the "Expand" property in the 'Packing' properties tab to "No". Now none of the child
widgets will recieve the extra space when the GtkVBox grows. I've highlighted the 3
children in the image below to illustrate this. Each of the child widgets is it's initially
requested size. The extra space allocated to the GtkVBox is simply unused since none
of it's children want it (but still belongs to the GtkVBox).

14

Now set the "Expand" property of the GtkScrolledWindow back to "Yes" and
change the "Fill" property to "No" instead. This time, the extra space is allocated to the
GtkScrolledWindow since "expand"=TRUE, however, the GtkScrolledWindow doesn't
use the space it was allocated since "fill"=FALSE.

Set the "Fill" property back to "Yes" to restore our original packing properties.

I know it seems odd at first, but as you continue to work in Glade, you'll start to
pick up on how these packing properties work and once you've conquored that part of
the learning curve, you'll be amazed at how little work you have to do related to the
position and size of your GUI's elements.

Editing the Menu (or Toolbar)

Glade3 comes with a new Menu and Toolbar editor. Although we aren't using a
GtkToolbar in this tutorial, the process is very similar to that of the GtkMenubar. We
will use the Glade3 Menu Editor to remove many of the items we won't be using and to
specify signal handlers for the menu items we will be using.

Although you can manipulate the properties and signals of GtkMenuItems from
the standard Glade properties pane and can remove items from the Glade Inspector,
the Glade3 menu editor provides a simpler way to edit your application's m enu.

Select the GtkMenuBar by clicking it in the Glade Editor or in the Glade Inspector.
Then right-click and select 'Edit...' from the popup menu. This will launch the menu
editor.

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The menu editor contains properties just like Glade's main Properties pane and
signals at the bottom just like the 'Signals' tab of Glade's main Properties pane. The
main difference in the editor is the tree view on the left. It allows you easily add and
remove menu items and drag them around. Go ahead and remove the one labeled
"_View". This is the only menu item which Glade generates that we won't be using in
this tutorial.

For the remaining menu items we will need to do a few things. We need to
rename them so that we have a clean, legible way to reference them in our source
code. Then, we'll make some changes as a work around to a bug that might effect
some readers using GTK+ 2.12.9 or below (Bug #523932), and finally we'll specify
signal handlers. The steps for each of the menu items will be the same, so I'll just walk
you through the 'New' menu item. Remember, this is all done in the menu editor but
can also be done using the Inspector and Properties pane in glade.

1. Click on the new menu item 'gtk-new'
2. Change the 'Name' property under 'Menu Item' to "new_menu_item"
3. Change the 'Label' property under 'Properties' to "_New" (note the
underscore, that's an accellerator)
4. Change the 'Stock Item' property under 'Properties' to "None"
5. Click on another menu item such as 'gtk-open' and then click back to 'gtk-
new' to refresh the properties (Bug #533503)
6. Change the 'Stock Image' under 'Internal Image Properties' to the 'New'
image
7. Specify a handler for the "activate" signal callled
"on_new_menu_item_activate" (This is done just like before, where we click
the tree view to get a drop down list of possible signal names)

16

Now repeat those steps for each of the menu items: 'gtk-open', 'gtk-save', etc.
Below are screen shots of the before and after on the 'New' menu item:

Glade Menu Editor before changes

17

Glade Menu Editor after changing 'New' menu item

Final Touches to the Main Window

It's never very easy to understand references to things like "textview1",
"textview2", etc. when you're coding. So now that you know how to set properties,
change the names of the following widgets (remember, the name is in the 'General' tab
of the Properties pane):

1. Rename "textview1" to "text_view"
2. Rename "statusbar1" to "statusbar"

And just to make it look a little nicer when the scroll bars are visible, let's add a a
border and shadow to the GtkScrolledWindow

1. Change the "Shadow Type" to "Etched In' in the 'General' properties tab for
"scrolledwindow1"
2. Change the "Border Width" to 1 in the 'Common' properties tab for
"scrolledwindow1"
3. Change the "Left Margin" to 2 in the 'General' properties for "text_view"
4. Change the "Right Margin" to 2 in the 'General' properties for "text_view"

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Part 2| Choosing a Programming Language for GTK+
Development
In part 1 of the GTK+ and Glade3 GUI Programming Tutorial series, we designed a
graphical user interface (GUI) for a GTK+ text editor application. After doing so, we were
left with an XML file (tutorial.glade) which described our application's user interface.

In this part of the GTK+ and Glade3 GUI Programming Tutorial series I will be
discussing the various programming languages available for GTK+ development.
Subsequent parts of this tutorial will cover both C programming and Python
programming. With my help, you will have to make the decision of which one you want to
learn how to use (or both!).

Which is the BEST Language?

Let's get this out of your system now. This is a question for which you can spend
the rest of your life reading answers to--and each will be different. The problem is that
this is the wrong question to ask as the answer is different for every person in each
different circumstance. Each language comes with it's advantages and it's drawbacks.
The question to ask: Which language is well suited for me on this particular project?

Language Choice Considerations

The important thing to remember when starting in with a language, is to keep an
open mind about other languages. You may start GTK+ programming with language X,
and later switch to language Y once you know and understand how it's benefits are
suited to your task. The GTK+ concepts will remain the same from language to
language.

1. Experience Level
How experienced you are with programming in general as well as how
much time, patience, and devotion you are willing to spend are important
factors in choosing a language. People without any programming
experience have to learn fundamental programming concepts as well as
the syntax and features of a new language. An experienced programmer
can pick up a new language very quickly in comparison and can focus
more on what the language has to offer as opposed to it's learning curve.
Furthermore, if you're already a PHP expert, perhaps starting GTK+
development with PHP might appeal to you. Maybe you took a course on
C++ in college and want to start there. Maybe you only worked with Visual
Basic but are ready to take the plunge and learn C.

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2. Activity and Community Support
GTK+ is written in C. Other languages are available through "language
bindings" which "wrap" the functionality. How active the project is which
provides said bindings is an important factor. You want to choose a
language that is up-to-date with new releases of GTK+ and bug fixes (all
languages I've mentioned are pretty well up to date). Furthermore, a
strong user-base and thus large community will be important as you get
most of your support from the community. The more people using a
particular language for GTK+, the more information there will be readily
available.
3. Efficient Programmer vs. Efficient Program
There's often a trade-off between how easy the program is to use and how
efficient the program is in terms of speed and how much you can do with it
on a lower-level. For many applications, the difference in efficiency of any
two languages is negligable--and a new programmer would never even
notice. For this reason, the increase in productivity is often the deciding
factor. As and example, if I needed to write a program which allowed me to
simply interface with some command-line utitlity through a GUI, I would
likely choose Python or Ruby. However, if I were going to develop a
sophisticated, powerful IDE, I would likely choose C or C++. In fact, you
can even use several languages in one project! You could write the
memory or processor intensive routines in C or C++ and do the rest in
Python or Ruby.
4. Language Sexiness
That's right-- how a language looks and feels is often a factor. You spend
a lot of time staring at that code. How it flows on the screen, how it reads,
and the overall development process in a particular language might appeal
to you more than another. You should enjoy the programming you're
doing. It's great having options isn't it!

A Look at Python vs. C

I have chosen to fork this tutorial into 2 languages based on the above criteria. It
is my humble opinion that C and Python fit the above criteria best. Both have very,
very strong community support and are being used for a large portion of the projects
developed for Linux and especially GNOME. Furthermore, they sit on sort of opposite
ends of the spectrum with regard to the efficiency vs. productivity debate. You could
even follow this tutorial down both paths and compare the 2 languages yourself.

If you have no programming experience, or perhaps just a little experience with
something like Visual Basic or PHP, I recommend starting with Python. Even if you are

20

an experienced programmer with C, C++, or Java experience you may want to learn
Python. It's an exciting modern language, fun to program with, and incredible quick to
learn and use. For Rapid Application Development in Linux, Glade and Python make a
great team. Learn more about the Python GTK+ binding PyGTK at www.pygtk.org.

If you're an experienced programmer or dedicated student, it may be worth your
while to learn C or C++ for GTK+ development--especially if you're already familiar
with C or C++. Learning GTK+ in C makes switching to another language such as
Python a breeze. Furthermore, you'll have more options for contributing to existing
projects. Personally, I do the majority of my GTK+ development in C despite the extra
time it takes.

21

Part 3| Writing a Basic Program to Implement
the Glade File
In this part of the GTK+ and Glade3 GUI Programming Tutorial series, I will s how
you a very basic program that will parse the Glade file we created in Part 1 and display
the main window for our GTK+ text editor. In this part of the tutorial, I will be discussing
the GTK+ concepts first and then show the code in both Python and C (in different
colors). If you have chosen to work in one language or the other, you can opt to skip
over the code explanation for the language you are not going to use.

Setting Up Your Development Environment

To work with GTK+ and complete this step of the tutorial you will need a text
editor, a terminal window, the GTK+ development libraries, and optionally Devhelp, the
developer's help reference. If you're new to the Linux world, welcome to lots of options.
There isn't one particular editor or IDE that is the "standard". Most developers actually
work with their favorite text editor and a terminal window. Although there are some "full
featured" IDEs out there, you may want to stick with a plain text editor and terminal at
this point so as not to be overwhelmed or tripped up by features and automated tasks
the IDE might perform.

I do my work using Gedit, the default GNOME text editor. There is a plugins
package available for Gedit which contains a terminal plugin and I have written Gedit
Symbol Browser Plugin which allows you to quickly jump to functions in your source
code. Below are Gedit screenshots from my system while working on this tutorial (click
to see full-size).

22

Working in C

Working in Python

The development libraries you will need depend both on your distribution and
whether you want to work in Python or C. Although the process can vary greatly

23

depending on your platform and distribution, I can give you the information you need to
get started. If you have problems installing any of the packages you need, post your
problem or question at the GTK+ Forums or a forum for your distribution.

When it comes to development libraries with Linux, you can often get all the
packages you need using your distributions package manager to resolve
dependencies. For example, on Ubuntu you can likely just issue the command: 'sudo
aptitude install libgtk-2.0-dev'. This command will install the GTK+ development
package, it's dependencies, their dependencies, and so on.

It's important to install the "development packages". These are suffixed with "-
dev" in Ubuntu/Debian and "-devel" in Redhat/Fedora. The development packages
include header files and other includes that allow you to build applications which use a
particular library. Just remember, "package" allows you to run applications using that
library where "package-dev" or "package-devel" allows you to write applications using
that library.

Another prefix you will see on packages is "-doc" such as "libgtk2.0-doc". This will
be the documentation for that library and once installed will allow you to browse the
documentation using Devhelp: the GNOME developer's help browser.

If you're programming with C you should install the following packages with their
dependencies: build-essential, libgtk2.0-dev, libgtk2.0-doc, libglib2.0-doc, devhelp
(package names may vary depending on distribution, these are for Ubuntu).

If you're programming with Python you should install the following packages with
their dependencies: python2.5-dev python2.5-doc, python2.5-gtk2, python-gtk2-doc,
python-gobject-doc, devhelp (package names may vary depending on distribution,
these are for Ubuntu).

GtkBuilder and LibGlade

If you recall, the file we created with Glade in part 1 of this tutorial series
(tutorial.glade) was an XML filedescribing our GUI. The actual GUI will be built by our
program. Therefore, the program will have to open and parse the XML file and create
instances of the widgets described within. There is already a library written to perform
this task--2 libraries in fact.

LibGlade was originally the library used to parse the glade file and create the
widgets described within. At the time of writing, this will still be the more commonplace
method used in other tutorials and books. However, GTK+ 2.12 included an object
called GtkBuilder which is essentially the same thing and is built right in to GTK+. As
this is intended to eventually replace Libglade, we will be using GtkBuilder in this
tutorial. However, as you learn and look at code elsewhere on the internet, keep in
mind that anywhere you see LibGlade being used, GtkBuilder can be used instead.

24

Since (at the time of writing) GtkBuilder is relatively new, Glade does not yet
support saving in the GtkBuilder format. The GtkBuilder format is still an XML file, but
with a slightly different schema. That means that in order to use GtkBuilder on a glade
file, we must first convert it to the GtkBuilder format. GTK+ 2.12 provides a conversion
script for this process, and will already be installed on your system at this point.

You can read some of the common questions I get about all this
Libglade/GtkBuilder stuff at Libglade to GtkBuilder F.A.Q..

So we now convert the glade XML file tutorial.glade to the GtkBuilder XML file
tutorial.xml with the following command:

gtk-builder-convert tutorial.glade tutorial.xml

The file 'tutorial.xml' is the file we will actually parse in our program, however, we
still need tutorial.glade when we want to make changes using Glade. This is only
necessary until Glade supports the GtkBuilder format in a later version (They are
aiming to have this ready by Glade 3.6 which you can follow along Bug #490678).

The Minimal Application

We're finally ready to write some code! Let's just recap what we've done so far.

1. Using Glade, we created tutorial.glade which describes our user interface.
2. We've selected which language we will use to write our program; Python, C,
or both.
3. We have a text editor and a terminal window available.
4. We have installed the development libraries we need to program GTK+
applications.
5. Using gtk-builder-convert, we converted tutorial.glade to tutorial.xml for use
with GtkBuilder.

Now, before we start digging in to all the details of what each line of code does,
we are going to write a minimal application just to ensure everything works and get
acquainted with the development process. So, open up your text editor and type in the
following...

If you're programming in C

#include <gtk/gtk.h>

void
{

25

gtk_main_quit();
}

int
main (int argc, char *argv[])
{
GtkBuilder *builder;
GtkWidget *window;



window = GTK_WIDGET (gtk_builder_get_object (builder,
"window"));

gtk_main ();

return 0;
}

Save this file as 'main.c' in the same directory as 'tutorial.xml'

If you're programming in Python

import sys
import gtk

class TutorialTextEditor:

def on_window_destroy(self, widget, data=None):
gtk.main_quit()

def __init__(self):



26

builder.connect_signals(self)

if __name__ == "__main__":
editor = TutorialTextEditor()
editor.window.show()
gtk.main()

Save this file as 'tutorial.py' in the same directory as 'tutorial.xml'

Compiling and Running the Application


Since C is a compiled language, we need to use the gcc compiler to compile our
source code into a binary application. In order for gcc to know where the GTK+
libraries are that it needs to link to and what compiler flags to use, we use a program
called pkg-config. When we installed the GTK+ development package, a package-
config file named 'gtk+-2.0.pc' was installed on our system. This file tells the pkg-config
program which version of the GTK+ libraries are installed and where the include files
live on our system. To illustrate this, type the following command in your terminal:

pkg-config --modversion gtk+-2.0

The output should show the version of GTK+ you have installed. On my system it
shows '2.12.0'. Now let's look at what compiler flags are needed to build a GTK+
application on my system:

pkg-config --cflags gtk+-2.0

The output of that command shows a bunch of -I switches which are specifying
include paths for the compiler to use. This will tell gcc where to look for include files
when we use '#include' in our application. The very first one on my system is '-
I/usr/include/gtk-2.0'. That means that when I use '#include <gtk/gtk.h>' in my code,
gcc will be able to find '/usr/include/gtk-2.0/gtk/gtk.h'.

Anytime you use a '#include <library/header.h>' style include that is not part of
the standard C library in your code, there should be a '-I/path/to/library' style option
passed to gcc. These libraries can be installed in different locations based on
distribution, operating system, or user preference. Good thing we have pkg-config to
handle all of this for us!

Let's compile our application so far. Issue this command in the terminal (make
sure you are in the same directory in which both 'main.c' and 'tutorial.xml' reside:

27

gcc -Wall -g -o tutorial main.c -export-dynamic `pkg-config --cflags
--libs gtk+-2.0`

The '-Wall' option tells gcc to show all warnings. The '-g' option will generate
debugging information which will be useful should you have a bug in your application
and need to step through the code using a debugger such as gdb. The option '-o
tutorial' tell gcc to generate the output executable into a file named 'tutorial'. 'main.c' is
the file gcc will compile. The '-export-dynamic' has to do with how we connect signals
to callback functions which will be discussed when we step through the code. And
finally, the pkg-config command appears.

Notice how it is enclosed in backticks (those are not single quotes). The backtick
is usually to the left of the '1' key on th e keyboard with the tilde character (~). This is
telling our shell to first execute the command 'pkg-config --cflags --libs gtk+-2.0' and
put the output of that command into the current command. So if you execute 'pkg-
config --cflags --libs gtk+-2.0' on your system and then paste it's output onto the end of
that gcc command, it would be virtually the same thing. By using pkg-config to append
the include paths and library paths to our compile command, we can use the same
command to compile our program on any system, regardless of where those libraries
are installed.

After your application compiles, there should be a new executable file named
'tutorial' which you execute using:

./tutorial

When you do so, you are going to see several warnings from GTK, something
along the lines of " Gtk-WARNING **: Could not find signal handler 'xxxxxx'". Don't
worry about those for now. Those are telling use that we specified a signal handler in
our glade file which we did not yet write a function for. I'll address these when we step
through the code. But you should have seen your GTK+ Text Editor window show up,
and clicking the 'x' in the window titlebar should properly terminate the application.

If for some reason you were not able to get the application to compile or execute,
post your error messages and any other information in the GTK+ Forums.


Since Python is an Interpreted Language we don't need to compile our program.
We simply invoke the Python interpreter, which we actually do with the first line in our
source code. So all we need to do to run our Python program, is change the
permissions so that the file is executable and then run it. Change the permissions
using:

chmod a+x tutorial.py

28

And now you can run it using:

./tutorial.py

you should have seen your GTK+ Text Editor window show up, and clicking the
'x' in the window titlebar should properly terminate the application.

If for some reason you were not able to get the application to compile or execute,
post your error messages and any other information in the GTK+ Forums.

Stepping Through the Code

Note: You should be looking up each new function in the GTK+ reference
documentation as I introduce them. Get to know that documentation, it will be your
best friend. Install Devhelp, use it! I will provided a link to the online reference
documentation each time I introduce a new function in case you were unable to install
Devhelp.

Including the GTK+ Library


Hopefully you know enough about C programming to understand the first
line '#include <gtk/gtk.h>'. If you don't, you should probably go back and work
through a basic C programming tutorial before continuing with this one. By
including gtk.h, we are indirectly including a multitude of header files. In fact, with
only a few exceptions, we are including all of the GTK+ library and it's
dependencies including GLib. If you want to know exactly what is being included
just take a look at that file! Essentially, when you're looking through the reference
manuals, you have access to most of the functions beginning with gtk_, g_, gdk_,
pango_, and atk_.


Hopefully you know enough about Python programming to understand the
first two lines '#import sys' and '#import gtk'. If you don't, you should probably go
back and work through a basic Python programming tutorial before continuing
with this one. We now have access to all gtk.x classes.

Initializing the GTK+ Library

Python implicitly initializes the GTK+ library for you. In C however, we must
initialize the GTK+ library before ANY call to a GTK+ function!

29



Looking in 'main()' we see that we initialize GTK+ before anything else using
the gtk_init() function.

Building the Interface with GtkBuilder

In a GTK+ application written entirely through code, that is, without the
assistance of Glade or another interface designer, we would have to
programatically create each widget, set the various properties of that widget, and
add it to it's parent widget where applicable. Each of these steps could require
several lines of code for each widget. That can be tedious. Just think about the
interface we created in part 1. There are over 20 widgets defined (including all
the menu items). To create all those widgets through pure code could exceed a
hundred lines of lines of code once all the properties were applied!

Good thing we're using Glade and GtkBuilder. With just 2 lines of code,
GtkBuilder will open and parse tutorial.xml, create all the widgets defined within,
apply their properties, and establish the widgets' parent-child relationships. Once
that is done we can then ask builder for the references to the widgets we want to
further manipulate or otherwise reference.

30



The first variable we declared in main() was a pointer to a GtkBuilder object. We
initialize that pointer usinggtk_builder_new(). This function will create a new GtkBuilder
object and return the pointer to that object which we are storing in the 'builder' variable.
Just about all GTK+ objects will be created in this fashion.

The builder object at this point hasn't built any UI elements yet. We can
use gtk_builder_add_from_file() to parse our XML file 'tutorial.xml' and add it's
contents to the builder object.

We are passing NULL as the third parameter to gtk_builder_add_from_file()
because we are not going to learn about GError just yet. So we do not have any error
checking yet and if the tutorial.xml file is not found or some other error occurs, our
program will crash, but we'll address that later.

31

You will notice that after calling gtk_builder_new() to create a new builder object,
all the other gtk_builder_xxx functions take that builder object as the first parameter.
This is how GTK+ implements object oriented programming in C, and will be consistent
with all GTK+ objects (compare that with how Python, a natural OOP language
implements the same thing below).



When we initialize the TutorialTextEditor class with 'editor = TutorialTextEditor()'
the class's initialization method, '__init__', is called. The first thing this method does is
initialize a new gtk.Builder class withgtk.Builder(). The builder instance is local to the
__init__ method because once we build our UI, we will no longer need the builder
object.

The builder object at this point hasn't built any UI elements yet. We
use gtk.Builder.add_from_file() to parse our XML file 'tutorial.xml' and add it's contents
to the builder object.

Getting References to Widgets From GtkBuilder

Once the builder has created all of our widgets we will want to get references to
some of those widgets. We only need references to some of the widgets because
some of them have already done their job and need no further manipulation. For
example, the GtkVBox which holds our menu, text view, and statusbar has already
done it's job of laying out our design and our code does not need to access it. So, we
need to get a reference to any widget we will manipulate during the lifetime of our
application and store it in a variable. At this point in the tutorial, we only need to
reference the GtkWindow named "window" so that we can show it.


window = GTK_WIDGET (gtk_builder_get_object (builder, "window"));

A couple things are happening here. First, let's look at gtk_builder_get_object().
The first parameter is the object from which we want to get an object. Again, this is
how OOP is implemented in C. The second parameter is the name of the object we
want to get a pointer to. This corresponds to the 'name' we specified for the widget in
Glade during part 1. If you recall, we named the main application GtkWindow 'window'.
So, that's what we pass to gtk_builder_get_object().

The gtk_builder_get_object() function returns a pointer to a GObject and we are
storing this pointer in 'window' which we declared at the beginning of main() as a

32

pointer to a GtkWidget. Moreover, we know that the object we are trying to get was a
GtkWindow. This is why I placed so much emphasis on the 'Object Hierarchy' of
widgets and GTK+ objects. If you look at the Object Hierarchy for a GtkWindow you
will see that GtkWidget is one of it's ancestors as is GObject. Therefore, a
GtkWindow is a GObject and it is a GtkWidget. This is a fundamental OOP concept
and critical to working with GTK+.

So, the GTK_WIDGET() wrapped around the call to gtk_builder_get_object() is a
convenience macro used for casting. You can cast a GTK+ widget into any of it's
ancestors using one of these casting macros. All GTK+ objects will have them
available. So, 'GTK_WIDGET(something)' is the same as '(GtkWidget*)something'.
We're casting the pointer to a GObject returned from the call to
gtk_builder_get_object() to a pointer to a GtkWidget as that's what 'window' was
declared as.

Finally, the reason we declared window as a pointer to a GtkWidget in the
beginning of main() rather than as a pointer to a GtkWindow is due to convention. We
could have declared it as a GtkWindow* and that would have still been correct. All
GTK+ widgets are derived from a GtkWidget so we can always declare a variable
pointing to any GTK+ widget as such. Many functions take GtkWidget* as a paramter
and many functions return GtkWidget* and thus it usually makes sense to declare your
variables as such and simply cast them to the specific widget where applicable (which
you'll see later).



We are using gtk.Builder.get_object() to get the object named "window" from the
builder. This corresponds to the 'name' we specified for the widget in Glade during part
1. If you recall, we named the main application's GtkWindow 'window'. So, that's what
we pass to get_object(). We assign the returned object to self.window so that we have
access to the application's window anywhere within the TutorialTextEditor() class.

Connecting Callback Functions to Signals

In part 1 we specified "handlers" for various "signals" in our interfac e. If you
recall, GTK+ emits signals for various events that occur. This is a fundamental concept
of GUI programming. Our application needs to know when the user does something so
that it can respond to that action. As we'll see soon, our application just sits around in a
loop waiting for something to happen. We will be using GtkBuilder to connect the
signal handlers we defined using Glade with callback functions in our code. GtkBuilder
will look at our code's symbols and connect the appropriate handlers for us.

33

In part 1 we specified a handler named 'on_window_destroy' for the "destroy"
signal of the GtkWindow named 'window'. Therefore, GtkBuilder will expect to find a
function or method named 'on_window_destroy'. The "destroy" signal is emitted when
a GtkObject is destroyed. As we'll see in the next bit of code, our application is going to
sit in an infinite loop waiting for events to happen. When the user closes the window
(such as clicking the 'x' in the titlebar), our application will need to break out of the loop
and terminate. By connecting a callback to the "destroy" signal of the GtkWindow we
will know when to terminate. Therefore, this is a signal you will use in almost every
GTK+ application you write.

Note: The method being used to connect callbacks to signals in this example is
equivalent to using glade_xml_signal_autoconnect() function when using LibGlade
instead of GtkBuilder.



When we call gtk_builder_connect_signals() we pass the builder object as the
first parameter as always. The second parameter allows us to pass user data
(anything we want) to our callback function. This will be important later, but for now
we'll just pass NULL. This function uses GModule, a part of GLib used to dynamically
load modules, to look at our applications symbol table (function names, variable
names, etc.) to find the function name that matches the handler name we specified in
Glade.

In Glade we specified a handler for the GtkWindow's "destroy" signal called
'on_window_destroy'. So, gtk_builder_connect_signals() is looking for a function
named 'on_window_destroy' that matches the signature of the callback function for the
"destroy" signal. If you recall from part 1, the "destroy" signal belonged to GtkObject.
Therefore, we find the prototype for the callback function in the manual for GtkObject
under the 'signals' section: GtkObject "destroy" Signal. This tells us what the prototype
for our callback function should look like.

Based on the prototype specified in the manual, I wrote the following function:

void
{
gtk_main_quit();
}

So now gtk_builder_connect_signals() will find this function and see that it both
matches the name of the handler we specified in Glade and has a compatible

34

signature (takes the same arguments) as that specified for the "destroy" signal and
makes the connection. Now our function on_window_destroy() will be called when the
GtkWindow 'window' is destroyed.

In on_window_destroy() we just call gtk_main_quit() to properly terminate our
application. This function will break out of the main loop which I will talk about more
when we get there in just a bit.

Right after the call to gtk_builder_connect_signals() there was a call
to g_object_unref().


This is because we are no longer going to use the GtkBuilder object. We used it
to construct our widgets and then we obtained pointers to the widgets we needed to
reference. So now we can free all the memory it used up with XML stuff.

You'll also noticed that we are using one of those casting macros to cast
(GtkBuilder*) to (GObject*). We must do this because g_object_unref() takes a
GObject* as a parameter. Since a GtkBuilder is derived from a GObject (as are all
widgets) this is perfectly valid.


builder.connect_signals(self)

In Glade we specified a handler for the GtkWindow's "destroy" signal called
'on_window_destroy'. So,gtk.Builder.connect_signals() is looking for a method named
'on_window_destroy' that matches the signature of the callback method for the
"destroy" signal. If you recall from part 1, the "destroy" signal belonged to GtkObject.
Therefore, we find the prototype for the callback function in the manual for gtk.Object
under the 'signals' section: gtk.Object "destroy" Signal. This tells us what the prototype
for our callback method should look like.

Based on the prototype specified in the manual, I wrote the following method:

def on_window_destroy(self, widget, data=None):
gtk.main_quit()

So now builder.connect_signals() will find this method and see that it both
matches the name of the handler we specified in Glade and has a compatible
signature (takes the same arguments) as was specified for the "destroy" signal and
makes the connection. Now our method on_window_destroy() will be called when the
GtkWindow 'window' is destroyed.

35

In on_window_destroy() we just call gtk.main_quit() to properly terminate our
application. This function will break out of the main loop which I will talk about more
when we get there in just a bit.

Showing the Application Window

Before we enter the GTK+ main loop (discussed next), we want show our
GtkWindow widget as our app doesn't do much good if it's not even visible.



Calling gtk_widget_show() sets the Widget's GTK_VISIBLE flag telling GTK+ to
show the widget (which will happen within the GTK+ main loop discussed next).


editor.window.show()

Calling gtk.Widget.show() tells GTK+ to show the widget (which will happen
within the GTK+ main loop discussed next).

Entering the GTK+ Main Loop

The main loop in GTK+ is an infinite loop which performs all of the "magic". This
is how GUI programming works. Once we build our GUI and setup our program, we
enter the GTK+ main loop and just wait for an event to occur which we care about
(such as closing the window). A lot is happening inside this main loop, however, for a
beginner you can simply think of it as an infinate loop in which GTK+ checks the state
of things, updates the UI, and emits signals for events.

After entering the main loop, our application isn't doing anything (but GTK+ is).
When the user resizes the window, minimizes it, clicks on it, presses keys, etc. GTK+
is checking each of these events and emitting signals for them. However, our
application is only connected to one signal currently, the "destroy" signal of 'window'.
When the window is closed and the "destroy" signal is emitted, then the GTK+ main
loop will turn over control to the handler function we have connected to that signal
which breaks us out of the GTK+ main loop thus allowing our application to terminate.


gtk_main ();


36

gtk.main()

In Summary

1. Application uses GtkBuilder to create the GUI from XML file.
2. Application gets reference to main window widget.
3. Application connects 'on_window_destroy' handler to the "destroy" signal.
4. Application flags the window to be shown.
5. Application enters GTK+ main loop (window is shown).
6. User clicks the 'x' in the titlebar as a result of which GTK+ main loop emits
the "destroy" signal.
7. Handler 'on_window_destroy' breaks out of GTK+ main loop.
8. Application terminates normally.

37

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