GtkAda User's Guide

Version 1.2.12

Document revision level 1.102.2.1

Date: 2001/04/20 07:14:33

E. Briot, J. Brobecker, A. Charlet


Table of Contents


Copyright (C) 1998-2000, Emmanuel Briot, Joel Brobecker, Arnaud Charlet

Copyright (C) 2000-2001, ACT-Europe

This document may be copied, in whole or in part, in any form or by any means, as is or with alterations, provided that (1) alterations are clearly marked as alterations and (2) this copyright notice is included unmodified in any copy.

Introduction: What is GtkAda ?

GtkAda is a high-level portable graphical toolkit, based on the gtk+ toolkit, one of the official GNU toolkits. It makes it easy to create portable user interfaces for multiple platforms, including most platforms that have a X11 server and Win32 platforms.

Although it is based on a C library, GtkAda uses some advanced Ada95 features like tagged types, generic packages, access to subprograms, exceptions, etc... to make it easier to use and design interfaces. For efficiency reasons, it does not use controlled types, but takes care of all the memory management for you in other ways.

As a result, this library provides a secure, easy to use and extensible toolkit.

Compared to the C library, GtkAda provides type safety (especially in the callbacks area), and object-oriented programming. As opposed to common knowledge, it requires less type casting than with in C. Its efficiency is about the same as the C library through the use of inline subprograms.

GtkAda comes with a complete integration to the graphical interface builder Glade(1). This makes it even easier to develop interfaces, since you just have to click to create a window and all the dialogs. Ada code can then be generated with a single click.

GtkAda also has a full support for OpenGL, and comes with a very thin binding to the OpenGL (Mesa) libraries. You can thus create graphical applications that display 3D graphics, and display them in a GtkAda window, as with any other 2D graphics. This manual does not document OpenGL at all, see any book on OpenGL, or the specification that came with your OpenGL library, for more information.

The following Internet sites will always contain the latest public packages for GtkAda, gtk+ and Glade.

http://libre.act-europe.fr/GtkAda

http://www.gtk.org

http://glade.gnome.org

The scheme used for GtkAda's version numbers is the following: the major version number is the same as for the underlying gtk+ library (e.g 1.2 or 1.3). Thus, stable versions of GtkAda will have even major numbers, and development versions (potentially less stable, although we are trying to make sure that no big problem exists in those versions) will have odd version numbers. The minor version numbers depends on GtkAda's release number.

This documentation is distributed with GtkAda version 1.2.12. At this stage, every widget found in gtk+ 1.2 has been implemented, and the test program found in the gtk release has been fully coded in Ada (have a look at the testgtk/ directory in the distribution).

This toolkit was tested on the following systems:

with the latest version of the GNAT compiler, developed and supported by Ada Core Technologies (see http://www.gnat.com).

If you manage to use it on other systems (which should probably be straightforward - just recompile GtkAda), please let us know so that we can add to the above list.

This version of GtkAda is known to be compatible with gtk+ 1.2.2 up to version 1.2.8. This release is no longer compatible with older versions of gtk+ (up to 1.0).

This version of GtkAda is compatible with Glade version 0.5.9. Due to some modification in the output format of Glade, this release will not work with older versions. It is also not guaranteed to work with more recent versions. Please update your version of Glade.

This document does not describe all the widgets available in GtkAda, nor does it try to explain all the subprograms. The GtkAda Reference Manual provides this documentation instead, as well as the GtkAda sources spec files themselves, whose extension is `.ads'.

No complete example is provided in this documentation. Instead, please refer to the examples that you can find in the `testgtk/' and `examples/' directory in the GtkAda distribution, since these are more up-to-date (and more extensive). They are heavily commented, and are likely to contain a lot of information that you might find interesting.

If you are interested in getting support for GtkAda (including priority bug fixes, early releases, help in using the toolkit, help in designing your interface, on site consulting...), please contact ACT Europe (mailto:sales@act-europe.fr) or Ada Core Technologies (mailto:sales@gnat.com).

Getting started with GtkAda

This chapter describes how to start a new GtkAda application. It explains the basic features of the toolkit, and shows how to compile and run your application.

It also gives a brief overview of the extensive widget hierarchy available in GtkAda.

How to build and install GtkAda

This section explains how to build and install GtkAda on your machine. It is Unix-oriented, since GtkAda is distributed in binary format on Windows machines, and comes with all the dependent packages, including the gtk+ libraries and Glade. If you are a Windows-user, you should skip this section.

On Unix systems, you first need to install the glib and gtk+ libraries. Download the compatible packages from the gtk+ web site (http://www.gtk.org, compile and install it. When compiling glib, make sure that the support for multithreading is enabled (this is the default, and can be enforced with the --enable-threads and --with-threads command line switches.

Change your PATH environment variable so that the script gtk-config, which indicates where gtk+ was installed and what libraries it needs is automatically found by GtkAda. You will no longer need this script once GtkAda is installed, unless you develop part of your application in C.

OpenGL support will not be activated in GtkAda unless you already have the OpenGL libraries on your systems. You can for instance look at Mesa, which is free implementation.

Optionally, you can also install the Glade interface builder. Get the compatible package from the Glade web site, compile and install it. The official version already knows about Ada (at least enough to call GtkAda's own programs), so no patch is needed.

You can finally download the latest version of GtkAda from the web site. Untar and uncompress the package, then simply do the following steps:

> ./configure
> make install

As usual with the configure script, you can specify where you want to install the GtkAda libraries by using the --prefix switch.

If you have some OpenGL libraries installed on your system, you can make sure that configure finds them by specifying the --with-GL-prefix switch on the command line. configure should be able to automatically detect the libraries however.

You must then make sure that the system will be able to find the dynamic libraries at run time if your application uses them. Typically, you would do one of the following:

Organization of the GtkAda package

In addition to the full sources, the GtkAda package contains a lot of heavily commented examples. If you haven't been through those examples, we really recommend that you look at them and try to understand them, since they contain some examples of code that you might find interesting for your own application.

How to compile an application with GtkAda

This section explains how you can compile your own applications. The procedure is system-dependent, and thus is divided into two subsections.

Unix systems

On Unix systems, a script called gtkada-config is automatically created when you build GtkAda. This script is copied in a subdirectory `bin/' in the installation directory.

The easiest and recommended way to build a GtkAda application is to use the gnatmake program distributed with GNAT, that takes care of all the dependencies for you. Use the gtkada-config to specify where GtkAda and gtk+ libraries have been installed.

> gnatmake <main-file> `gtkada-config`

Note the use of back-ticks around gtkada-config, which force the shell to evaluate the script and put the output on the command line.

However, on complex systems, gnatmake might not be enough. Users frequently like to create Makefiles. The script gtkada-config remains useful in that case, since you can call it from your Makefile (same syntax as above with the back-ticks) to create variables like FLAGS and LIBS. See the switches of gtkada-config below for more information.

The script gtkada-config understands the following command line switches (chosen to be compatible with the ones set by gtk-config):

Windows systems

Things are somewhat easier on Windows systems. You don't have access to the gtkada-config script. However, the good news is that you also don't have to specify which libraries to use or where to find them.

The only thing you should specify on the gnatmake command line is where the GtkAda spec files are found, as in:

> gnatmake <main-file> -Ic:\GtkAda\lib

if GtkAda was installed in `c:\GtkAda'.

No static libraries are provided currently, so you can't create a static version of your application and you will have to use the dynamic libraries.

Architecture of the toolkit

The gtk+ toolkit has been designed from the beginning to be portable. It is made of three libraries, gtk, gdk and glib.

Glib is a non-graphical library, that includes support for lists, h-tables, threads, etc... It is a highly optimized, platform-independent library. Since most of its contents are already available in Ada (or in the `GNAT.*' hierarchy in the GNAT distribution), GtkAda does not include a binding to it, except for a few required packages. These are the `Glib.*' packages in the GtkAda distribution.

Gdk is the platform-dependent part of gtk+. Its implementation is completely different on win32 systems and X11 systems, although the interface is of course the same. It provides a set of functions to draw lines, rectangles and pixmaps on the screen, manipulate colors, etc... It has a complete equivalent in GtkAda, through the `Gdk.*' packages.

Gtk is the top level library. It is platform independent, and does all its drawing through calls to Gdk. This is where the high-level widgets are defined. It also includes support for callbacks. Its equivalent in the GtkAda libraries are the `Gtk.*' packages. It is made of a fully object-oriented hierarchy of widgets (see section Widgets Hierarchy).

Since your application only calls GtkAda, it is fully portable, and can be recompiled as-is on other platforms.

+---------------------------------------------+
|             Your Application                |
+---------------------------------------------+
|                 GtkAda                      |
|               +-----------------------------+
|               |            GTK              |
|       +-------+-----------------------------+
|       |           GDK                       |
+-------+--------------+--+-------------------+
|          GLIB        |  | X-Window / Win32  |
+----------------------+  +-------------------+

Although the packages have been evolving a lot since the first versions of GtkAda, the specs are stabilizing now. We will try as much as possible to provide backward compatibility whenever possible.

Since GtkAda is based on gtk+ we have tried to stay as close to it as possible while using high-level features of the Ada95 language. It is thus relatively easy to convert external examples from C to Ada.

We have tried to adopt a consistent naming scheme for Ada identifiers:

WARNING: all the generic packages allocate some memory for internal structures, and call internal functions. This memory is freed by gtk itself, by calling some Ada functions. Therefore the generic packages have to be instanciated at library level, not inside a subprogram, so that the functions are still defined when gtk needs to free the memory.

WARNING Before any other call to the GtkAda library is performed, Gtk.Main.Init must be invoked first. Most of the time, this procedure is invoked from the main procedure of the application, in which case no use of GtkAda can be done during the application elaboration.

Widgets Hierarchy

All widgets in GtkAda are implemented as tagged types. They all have a common ancestor, called Gtk.Object.Gtk_Object. All visual objects have a common ancestor called Gtk.Widget.Gtk_Widget.

The following table describes the list of objects and their inheritance tree. As usual with tagged types, all the primitive subprograms defined for a type are also known for all of its children. This is a very powerful way to create new widgets, as will be explained in section Creating new widgets in Ada.

Although gtk+ was written in C its design is object-oriented, and thus GtkAda has the same structure. The following rules have been applied to convert from C names to Ada names: a widget Gtk_XXX is defined in the Ada package Gtk.XXX, in the file `gtk-xxx.ads'. This follows the GNAT convention for file names. For instance, the Gtk_Text widget is defined in the package Gtk.Text, in the file `gtk-text.ads'.

Note also that most of the documentation for GtkAda is found in the spec files themselves.

It is important to be familiar with this hierarchy. It is then easier to know how to build and organize your windows. Most widgets are demonstrated in the `testgtk/' directory in the GtkAda distribution.

hierarchy

            Hierarchy of widgets in GtkAda

Hierarchical composition of a window

Interfaces in GtkAda are built in layers, as in Motif. For instance, a typical dialog is basically a Gtk_Window, that in turn contains a Gtk_Box, itself divided into two boxes and a Gtk_Separator, and so on.

boxes

Altough this may seem more complicated than setting absolute positions for children, this is the simplest way to automatically handle the resizing of windows. Each container that creates a layer knows how it should behave when it is resized, and how it should move its children. Thus almost everything is handled automatically, and you don't have to do anything to support resizing.

If you really insist on moving the children to a specific position, look at the Gtk_Fixed widget and its demo in `testgtk/'. But you really should not use this container, since you will then have to do everything by hand.

All the containers are demonstrated in `testgtk/', in the GtkAda distribution. This should help you understand all the parameters associated with the containers. It is very important to master these containers, since using the appropriate containers will make building interfaces a lot easier.

If you look at the widget hierarchy (see section Widgets Hierarchy), you can see that a Gtk_Window inherits from Gtk_Bin, and thus can have only one child. In most cases, the child of a Gtk_Window will thus be a Gtk_Box, which can have any number of children.

Some widgets in GtkAda itself are built using this strategy, from the very basic Gtk_Button to the more advanced Gtk_File_Selection.

For example, by default a Gtk_Button contains a Gtk_Label, which displays the text of the button (like "OK" or "Cancel").

However, it is easy to put a pixmap in a button instead. When you create the button, do not specify any label. Thus, no child will be added, and you can give it your own. See `testgtk/create_pixmap.adb' for an example on how to do that.

Signal handling

In GtkAda, the interaction between the interface and the core application is done via signals. Most user actions on the graphical application trigger some signals to be `emitted'.

A signal is a message that an object wants to broadcast. It is identified by its name, and each one is associated with certain events which happen during the widget's lifetime. For instance, when the user clicks on a Gtk_Button, a "clicked" signal is emitted by that button.More examples of signals can be found in the GtkAda reference manual.

It is possible to cause the application to react to such events by `connecting' to a signal a special procedure called a `handler' or `callback'. This handler will be called every time that signal is emitted, giving the application a chance to do any processing it needs. More than one handler can be connected to the same signal on the same object; the handlers are invoked in the order they were connected.

Depending on their type, the widgets define zero (e.g Gtk_Label), one or several different signals. Of course, the signals, as every thing else, are inherited from the parents, thus every widget answers to a lot of signals.

The easiest way to find out what signals can be emitted by a widget is to look at the GtkAda reference manual. Hopefully, every widget will finally be documented there. That document explains when the signal is emitted, and the general form that handlers should have (although you can always add a User_Data is you wish).

In the meantime, you can also look directly at the C header files, distributed with the gtk+ library. Each widget is described in its own C file and has two C structures associated with it. One of them is the "class" structure, and contains a series of pointers to functions. Each of these functions has the same name as the signal name.

For instance, consider the following extract from gtkbutton.h:

struct _GtkButtonClass
{
  GtkBinClass        parent_class;
  
  void (* pressed)  (GtkButton *button);
  void (* released) (GtkButton *button);
  void (* clicked)  (GtkButton *button);
  void (* enter)    (GtkButton *button);
  void (* leave)    (GtkButton *button);
};

This means that the Gtk_Button widget redefines five new signals, called respectively "pressed", "release", ...

The profile of the handler can also be deduced from those pointers: The handler has the same arguments, plus an optional User_Data parameter that can be used to pass any kind of data to the handler. When the User_Data parameter is used, the value of this data is specified when connecting the handler to the signal. It is then given back to the handler when the signal is raised.

Therefore, the profile of a handler should look like:

procedure Pressed_Handler
  (Button    : access Gtk_Button_Record'Class;
   User_Data : ...);

The callback does not need to use all the arguments. It is legal to use a procedure that "drops" some of the last arguments. The User_Data is a special case, since you decide at connection time whether you want it or not. If you decided to have it then you callback must accept it, and this is checked by the compiler.

Any number of arguments can be dropped as long as those arguments are the last ones in the list and you keep the first one. For instance, the signal "button_press_event" normally can be connected to a handler with any of the following profiles:

--  with a user_data
procedure Handler
  (Widget    : access Gtk_Widget_Record'Class;
   Event     : Gdk.Event.Gdk_Event;
   User_Data : ...);
procedure Handler
  (Widget    : access Gtk_Widget_Record'Class;
   User_Data : ...);

--  without a user_data
procedure Handler
  (Widget : access Gtk_Widget_Record'Class;
   Event  : Gdk.Event.Gdk_Event);
procedure Handler (Widget : access Gtk_Widget_Record'Class);

Beware that adding new arguments is not possible, since no value would be provided for them. When connecting a handler GtkAda will not always verify that your handler does not have more arguments than expected, so caution is recommended (it only does so if you use the Gtk.Marshallers package, see below).

All the signal-handling work is performed by using the services provided by the Gtk.Handlers package. This package is completely self-documented, so please check the documentation for this package either in the GtkAda Reference Manual or in the specifications themselves.

Here is a commented example of how to use it; the complete example can be found in create_file_selection.adb (inside the testgtk/ directory). Let's imagine that an application opens a file selector to allow the user to select a file. GtkAda provides a high-level widget called Gtk_File_Selection which can be used in this case:

declare
   Window : Gtk_File_Selection;
begin
   Gtk.File_Selection.Gtk_New (Window, Title => "Select a file");
end;

When the "OK" button is pressed, the application needs to retrieve the file selected and then close the dialog. All we need to know in that procedure is what widget to operate on. This can be achieved by the following handler:

procedure OK (Files : access Gtk_File_Selection_Record'Class) is
begin
   Ada.Text_IO.Put_Line ("Selected " & Get_Filename (Files));
   --  Prints the name of the selected file.
   Destroy (Files);
   --  Destroys the file selector dialog
end Ok;

We now need to connect the object we created in the first part and this new callback we just defined. As documented in the reference manual, Gtk.Handlers defines four types of generic packages, depending on the arguments one expects in the callback and whether the callback returns a value or not. Note that you can not arbitrarily choose to return a value, this depends on the signal, as explained above.

In our case, since our callback does not return any value and does not have any User_Data (ie we don't need to pass it extra values, defined at connection time), the appropriate package is Gtk.Handlers.Callback.

We thus instantiate that package. Once again, remember that the generic packages in GtkAda need to be present in memory at all time, since they take care of freeing the memory they are using when finished. They must be instantiated at library level and not inside a subprogram.

package Files_Cb is new 
  Handlers.Callback (Gtk_File_Selection_Record);

As you can see in Gtk.Handlers, this package now provides a set of Connect subprograms than can be used to establish a permanent tie between a widget and a handler. It also provides a set of other subprograms which one can use to emit the signals manually, although most of the time the signals are simply emitted internally by GtkAda. We will not discuss the Emit_By_Name subprograms here.

The general form of handler, as used in Gtk.Handlers, expects some handlers that take two or three arguments: the widget on which the signal was applied, an array of all the extra arguments sent internally by GtkAda, and possibly some user data given when the connection was made.

This is the most general form of handler and it covers all the possible cases. However, it also expects the user to manually extract the needed values from the array of arguments. This is not always the most convenient solution. This is why GtkAda provides a second package related to signals, Gtk.Marshallers.

This package provides a set of functions that can be used as callbacks directly for GtkAda, and that will call your own handlers after extracting the required values from the array of arguments. Although this might sound a little bit complicated, this is in fact a very powerful mechanism, in fact similar to what is done internally by gtk+ in C. This does not add any overhead compared to C programs, since we do basically the same thing but at the Ada level.

A set of functions To_Marshaller is found in every generic package in Gtk.Handlers. They take a single argument, the name of the function you want to call, and return a handler that can be used directly in Connect.

The connection is then done with the following piece of code. Note that this can be done just after creating the widget, in the same block. As soon as it is created, a widget is ready to accept connections (although no signals will be emitted before the widget is shown on the screen).

Note the two following things: we use To_Marshaller since our handler does not accept the array of arguments as a parameter, and we use the special Object_Connect procedure. This means that the parameter to our callback (Files) will be the Slot_Object given in Object_Connect, instead of being the button itself.

Files_Cb.Object_Connect
  (Get_Ok_Button (Window),  --  The object to connect to the handler
   "clicked",               --  The name of the signal
   Files_Cb.To_Marshaller (Ok'Access),  --  The signal handler
   Slot_Object => Window);

Starting an application with GtkAda

You need to perform some initializations to start a GtkAda application:

--  predefined units of the library
with Gtk.Rc;
with Gtk.Main;
with Gtk.Enums;
with Gtk.Window;
...
--  My units
with Callbacks;
...
procedure Application is
   procedure Create_Window is ...

begin
   --  Set the locale specific datas (e.g time and date format)
   Gtk.Main.Set_Locale;

   --  Initializes GtkAda
   Gtk.Main.Init;

   --  Load the resources. Note that this part is optional.
   Gtk.Rc.Parse ("application.rc");

   --  Create the main window
   Create_Window;

   --  Signal handling loop
   Gtk.Main.Main;
end Application;

the Create_Window procedure looks like

   procedure Create_Window is
      Main_Window : Gtk.Window.Gtk_Window;
      ...
   begin
      Gtk.Window.Gtk_New
        (Window   => Main_Window,
         The_Type => Gtk.Enums.Window_Toplevel);

      --  From Gtk.Widget:
      Gtk.Window.Set_Title (Window => Main_Window, Title  => "Editor");

      --  Construct the window and connect various callbacks

      ...
      Gtk.Window.Show_All (Main_Window);
   end Create_Window;

Resource files

Resource files let you parametize aspects of the widgets in a GtkAda application without having to recompile it.

A resource file needs to be loaded (Gtk.Rc.Parse) before setting the corresponding window.

In this file, it is possible to specify the visual characteristics of the widgets (colors, fonts, ...). Under X, the xfontsel command allows you to easily select a font. The FontSelection widget is also a simple way to select fonts.

Here is an example of a resource file:

# application.rc
#
# resource file for "Application"

# Buttons style
style "button"
{
# BackGround Colors
#                  Red  Green  Blue
  bg[PRELIGHT] = { 0.0,  0.75, 0.0 } # Green when the mouse is on
                                     # the button
  bg[ACTIVE]   = { 0.75, 0.0,  0.0 } # Red on click
# ForeGround Colors
#                  Red  Green  Blue
  fg[PRELIGHT] = { 1.0,  1.0,  1.0 } # White when the mouse is on
                                     # the button
  fg[ACTIVE]   = { 1.0,  1.0,  1.0 } # White on click
}

# All the buttons will have the style "button"
widget_class "*GtkButton*" style "button"

# Text style
style "text"
{
  font = "-adobe-courier-medium-r-normal-*-15-*-*-*-*-*-*-*"
  text[NORMAL] = { 0.0, 0.0, 0.0 } # black
  fg[NORMAL]   = { 0.0, 0.0, 0.0 } # black
  base[NORMAL] = { 1.0, 1.0, 1.0 } # white : background color
}

# All Gtk_Text will have the "text" style
widget_class "*GtkText" style "text"

Memory management

GtkAda takes care of almost all the memory management for you. Here is a brief overview of how this works, you'll have to check the sources if you want more detailed information. Gtk+ (the C library) does its own memory management through reference counting, i.e. any widget is destroyed when it is no longer referenced anywhere in the application.

In GtkAda itself, a "user_data" is associated with each object allocated by a Gtk_New procedure. A "destroy" callback is also associated to be called when the object to which the user_data belongs is destroyed (see Gtk.Initialize_User_Data). Thus, every time a C object is destroyed, the equivalent Ada structure is also destroyed (see Gtk.Free_User_Data). Concerning widgets containing children, every container holds a reference to its children, whose reference counting is thus different from 0 (and generally 1). When the container is destroyed, the reference of all its children and grand-children is decremented, and they are destroyed in turn if needed. So the deallocation of a widget hierarchy is also performed automatically.

Tasking with GtkAda

The Glib library can be used in a task-safe mode by calling Gdk.Thread.Init before making any other Glib calls. In this mode Glib automatically locks all internal data structures as needed. This does not mean that two tasks can simultaneously access, for example, a single hash table, but they can access two different hash tables simultaneously. If two different tasks need to access the same hash table, the application is responsible for locking itself (e.g by using protected objects).

When Glib is initialized to be task-safe, GtkAda is task aware. There is a single global lock that you must acquire with Gdk.Threads.Enter before making any Gdk/Gtk call, and which you must release with Gdk.Threads.Leave afterwards.

Thus, Gtk.Main.Main should be called with the lock acquired (see example below), ensuring that all the functions executed in the task that started the main loop do not need to protect themselves again.

Beware that the GtkAda main loop (Gtk.Main.Main) can only be be run inside one specific task. In other words, you cannot call Gtk.Main.Main from any task other than the one that started the outer level main loop.

Note that Gdk.Threads assumes that you are using a tasking run time that maps Ada tasks to native threads. For example under Linux with GNAT, you need to change the default tasking run time for that purpose.

A minimal main program for a tasking GtkAda application looks like:

with Gdk.Threads;
with Gtk.Main;
with Gtk.Enums; use Gtk.Enums;
with Gtk.Window; use Gtk.Window;

procedure GtkAda_With_Tasks is
   Window : Gtk_Window;
begin
   Gdk.Threads.Init;
   Gtk.Main.Init;

   Gtk_New (Window, Window_Toplevel);
   Show (Window);

   Gdk.Threads.Enter;
   Gtk.Main.Main;
   Gdk.Threads.Leave;
end GtkAda_With_Tasks;

Callbacks require a bit of attention. Callbacks from GtkAda (signals) are made within the GtkAda lock. However, callbacks from Glib (timeouts, IO callbacks, and idle functions) are made outside of the GtkAda lock. So, within a signal handler you do not need to call Gdk.Threads.Enter, but within the other types of callbacks, you do.

Object-oriented features

GtkAda has been designed from the beginning to provide a full Object oriented layer over gtk+. This means that features such as type extension, dynamic dispatching, ... are made available through the standard Ada language.

This section will describe both how things work and how you can extend existing widgets or even create your own.

General description of the tagged types

Why should I use object-oriented programing ?

Every widget in GtkAda is a tagged type and has a number of primitive subprograms which are known to all its children.

This means that, as opposed to what you see in C code, you don't have (well most of the time you don't) to explictly cast types and, even when you have to, Ada always makes sure that the conversion is valid.

Thus your programs are much safer and most errors are found at compile time, as is usually the case with Ada.

For instance, if you create a table, put some widgets in it, and then, later in your program, try to access those widgets, then you do not need to know beforehand what their type is, when and by whom they were created, ... You simply ask for the children of the table, and you get in return a tagged type that contains all the information you need. You can even use dynamic dispatching without ever having to cast to a known type.

This makes GtkAda a very powerful tool for designing graphical interfaces.

If you think one of the standard widgets is nice, but would be even better if it was drawing itself in a slighlty different way, of if it could contain some other data that you need in your application, there is a very simple way to do it: just create a new type that extends the current one (see the section section Using tagged types to extend Gtk widgets below.

Maybe you want to create your own brand new widget, that knows how to draw itself, how to react to events, ... and you want to be able to reuse it anytime you need ? Once again, using the standard Ada features, you can simply create a new tagged type and teach it how to interact with the user. See the section section Creating new widgets in Ada below.

Type conversions from C to Ada widgets

There are basically three kinds of widgets that you can use with GtkAda:

GtkAda will always be able to find and/or create a valid tagged type in the first case, no matter if you explicitly created the widget or if it was created automatically by gtk+. For instance, if you created a widget in Ada, put it in a table, and later on extracted it from the table, then you will still have the same widget.

There are two issues: if the widget was explictly created by you, or at least by GtkAda, then it will always be and remain associated with a correct Ada type.

If the widget was created implicitly (for instance every time you create a Gtk_Button, a Gtk_Label is also created for the text displayed), then GtkAda will only be able to create the corresponding type by default for the following widgets: Gtk_Label, Gtk_Button, Gtk_Item, Gtk_List_Item, Gtk_Menu_Item, Gtk_Check_Menu_Item, Gtk_Radio_Menu_Item, Gtk_Tearoff_Menu_Item and Gtk_Tree_Item. For other widgets, it will instead create a Gtk_Widget, and you will have to either call Gtk.Unchecked_Cast to convert it back to the type you expect, or use Gtk.Type_Conversion as described below. Here is an example of use of Gtk.Unchecked_Cast:

declare
   Stub : Gtk_Window_Record;
begin
   Window := Gtk.Unchecked_Cast (Widget, Stub);
end;

In the third case (imported C widgets), GtkAda is not, by default, able to create the corresponding Ada type.

The solution we suggest to solve these two issues is to 'with' the Gtk.Type_Conversion unit. In that case, every standard widget, no matter who created them, will always be correctly converted to an appropriate Ada type. So, basically, if you put the following in your main unit:

with Gtk.Type_Conversion;

begin
   Gtk.Main.Init;
   Gtk.Type_Conversion.Init;
   ...
end

then you can safely get the children of any widget (table, boxes, ...) and be sure you have the right Ada type. You won't need to explictly convert your widget to something else.

However, 'with'ing this unit means that your application will depend on every package of GtkAda, which is a little bit heavier, and explains why this is not the default. We do recommend you use it if it is not extremely important whether your application depends on all the packages of GtkAda.

The case of imported C widgets is a little bit trickier. Since GtkAda does not know anything about them when it is built, it can't magically convert the C widgets to Ada widgets. This is your job to teach GtkAda how to do the conversion.

We thus provide a 'hook' function which you need to modify. This function is defined in the package Gtk.Type_Conversion. It is a function that takes a string with the name of the C widget (ex/ "GtkButton"), and expects a newly allocated pointer in return. If you don't know this type either, simply return null.

Using tagged types to extend Gtk widgets

Since version 0.6 of this toolkit, it is possible to associate your own data with existing widgets simply by creating new types. This section will show you a simple example, but you should rather read the source code in testgtk/ where we used this feature instead of using user_data as is used in the C version.

type My_Button_Record is new Gtk_Button_Record with record
    --  whatever data you want to associate with your button
end record;
type My_Button is access all My_Button_Record'Class;

With the above statements, your new type is defined. Every function available for Gtk_Button is also available for My_Button. Of course, as with every tagged type in Ada, you can create your own primitive functions with the following prototype:

procedure My_Primitive_Func (Myb : access My_Button_Record);

To instanciate an object of type My_Button in your application, do the following:

declare
   Myb : My_Button;
begin
   Myb := new My_Button_Record;
   Initialize (Myb);   --  from Gtk.Button
end;

The first line creates the Ada type, whereas the Initialize call actually creates the C widget and associates it with the Ada type.

Creating new widgets in Ada

With GtkAda, you can now create widgets directly in Ada. These new widgets can be used directly, as if they were part of gtk itself.

Creating new widgets is a way to create reuseable components. You can apply to them the same functions as would for any other widget, such as Show, Hide, ...

This section will explain how to create two types of widgets: composite widgets and widgets created from scratch. Two examples are provided with GtkAda, in the directories `examples/composite_widget' and `examples/base_widget'. Please also refer to the gtk+ tutorial, which describes the basic mechanisms that you need to know to create a widget (even if the Ada code is really different from the C code...)

Creating composite widgets

A composite widget is a widget that does not do much by itself. Rather, this is a collection of subwidgets grouped into a more general entity. For instance, among the standard widgets, Gtk_File_Selection and Gtk_Font_Selection belong to this category.

The good news is that there is nothing special to know. Just create a new tagged type, extending one of the standard widgets (or even another of your own widgets), provide a Gtk_New function that allocates memory for this widget, and call the Initialize function that does the actual creation of the widget and the subwidgets. There is only one thing to do: Initialize should call the parent class's Initialize function, to create the underlying C widget.

The example directory `examples/composite_widget' reimplements the Gtk_Dialog widget as written in C by the creators of gtk+.

Creating widgets from scratch

First, an important note: please do not read this if this is your first time using GtkAda or if you don't really understand the signal mechanism. Creating a nice and working widget really takes a lot of messing with the low level signals.

Creating a widget from scratch is what you want to do if your widget should be drawn in a special way, should create and emit new signals, ... The example we give in `examples/base_widget' is a small target on which the user can click, and that sends one of two signals "bullseye" or "missed", depending on where the user has clicked.

See also the example in `examples/tutorial/gtkdial' for a more complex widget, that implements a gauge where the user can move the arrow to select a new value.

Once again, the only two functions that you must create are Gtk_New and Initialize. This time, Initialize has to do two things:

Parent_Package.Initialize (Widget);

--  The above line calls the Initialize function from the parent.
--  This creates the underlying C widget, which we are going to
--  modify with the following call:

Gtk.Object.Initialize_Class_Record
  (Widget, Signals, Class_Record);
--  This initializes the "class record" for the widget and
--  creates the signals.

In the above example, the new part is the second call. It takes three or four arguments:

Then of course Initialize should set up some signal handlers for the functions you want to redefine. Three signals are especially useful:

Support for Glade, the Gtk GUI builder

Introduction

GtkAda now comes with support for the GUI builder Glade (this is not the glade released with Gnat for distributed systems). Using Glade itself is straightforward: it is an intuitive point and click GUI builder. The main difference from other builders is that, since GtkAda builds a UI with blocks by default, you will not be asked to set the size and position of your windows by default. If you are looking for this kind of interaction you should consider using the Gtk_Fixed container. However, please read the GtkAda reference manual before considering using Gtk_Fixed.

Using Glade

Note that we only recommend using version 0.5.9 of Glade, as some previous versions are not compatible. Using this version you can directly create Ada files from Glade by selecting Ada95 as the language under project options. By doing so, you will tell Glade to call Gate internally when using the "Write Source Code" functionality. Glade saves your interface in a project file whose syntax is XML based. In the following sections, we will refer to this file as either "the XML file" or "the project file" interchangeably.

Taking advantage of Glade's options

In this XML file, Glade will save various options that Gate can take advantage of. In particular, going to the project options window, some General Options in the "C Options" section will also be used by Gate: Gettext support, if enabled, will generate an additional package called <project>_intl that will use Gtkada.Intl; The "Set Widget Names" option is also recognized and will generate additional calls to Set_Name for each widget.

Associating icons with buttons

Up to now, the only way to specify an icon for a button was to associate a pixmap file to it. As an alternative, if the name specified in the "Icon" field does not contain any dots, Gate will consider the name as a variable rather than a file name. It is up to you to provide the necessary definitions in order to compile properly the generated code.

Note for Linux users

Some Linux systems come with a precompiled version of Glade that will usually have support for Gnome. GtkAda and Gate currently do not provide support for Gnome (GtkAda instead provides portable alternatives to Gnome widgets, see e.g Gtkada.Canvas, Gtkada.Dialogs, Gtkada.Pixmaps), which means that you should not enable any Gnome specific features in Glade. In particular, you should disable Gnome support in the Project Options window, and make sure that you don't use the "Stock Button" property, nor the list of predefined Gnome icons provided with various widgets such as toolbar buttons and menu items.

Gate

Invoking Gate

Gate is the static version of our support. This tool takes the Glade XML file as an argument and generates a set of Ada files that, when compiled, will recreate the interface you just designed with Glade. To invoke it, simply call gate will a Glade project file This will generate Ada files in the directory set as the "Source Directory" project option, or otherwise in the current working directory.

Structure of the generated files

The main file

The main file is the name of the program name specified in the XML Glade file: <program name>.adb. It contains initialization and creation code for each top level widget contained in the XML file. This is intended as a convenient default to visualize your GUI, but you will usually want to modify this initialization.

Top level widget files

For each top level widget, Gate will generate a package <widget>_Pkg.Callbacks in the files <widget>_pkg-callbacks.ads and .adb. These packages contain all the GtkAda calls needed to create the widgets you designed within Glade. You will usually not need to modify these files yourself.

Main handler file

This file, called callbacks_<program name>.ads contains all the Handler package instantiations needed by your application. You will usually not need to modify it.

Top level widget signal files

These are the most important files created by Gate. Each is called <widget_name>_pkg-callbacks.adb. They contain stubs for all the callbacks related to a top level widget you declared in Glade. With the main file, this the only files you should modify yourself. Note that it is recommended that you structure your application such that the real code is put in separate packages (i.e not generated files) as much as possible, to avoid potential merging problems when your interface is significantly changed within Glade. See next section for more details.

Currently, Gate will generate callback stub procedures that can handle the expected number of arguments for each signal. This is done using a relatively low level mechanism explained in the reference manual (package Gtk.Handlers). Basically, callbacks expecting arguments will take a Gtk_Argument as their only parameter, and Gate will generate the appropriate variable declaration and conversion calls to provide the expected GtkAda arguments. You should not modify this code by hand unless you know what you are doing.

For example, given a signal delete_event, Gate will generate the following procedure body, giving access to the arguments of this callback: a Gtk_Window_Record (Object) and a Gdk_Event (Arg1)

function On_Main_Window_Delete_Event
  (Object : access Gtk_Window_Record'Class;
   Params : Gtk.Arguments.Gtk_Args) return Boolean
is
   Arg1 : Gdk_Event := To_Event (Params, 1);
begin
   return False;
end On_Main_Window_Delete_Event;

Modifying generated files

Note that you can easily go back to Glade any time, modify your interface, and have Gate re-generate a set of files. All your modifications will be kept in the new files. For that, Gate creates a directory .gate in the current directory. Please do not delete it if you want Gate to be able to keep your changes from one version to the next.

Also note that to keep track of your modifications, gate relies on either merge, or patch and diff being available on your system. If you don't have a working set of diff/patch, configure will simply replace them by null operations, which means that regenerated files will override the previous ones.

Under Win32 based systems, GtkAda now comes with a set of unix-like tools (including patch and diff) that provide the same functionality. Note that these tools are provided as is, and we recommend using your own supported set of utilities. In particular, these tools have known deficiencies under Windows 95 and Windows 98. Future versions of GtkAda will provide an alternate implementation of the shell script gate in Ada, in order to solve this portability issue in a cleaner way. You can find the complete package of the GNU utilities provided at http://www.weihenstephan.de/~syring/win32/UnxUtils.html.

In some cases, due to major changes in the project file, Gate may not be able to merge all the changes. In this case, it will notify you so, and will either keep the old and new sections in the files (if you are using merge), or files with a .rej extension will be generated to help you do the merge manually.

Adding Gate support for new widgets

This section is intended for developpers that wish to add support for new GtkAda widgets in Gate. Note that this section is not complete yet.

Implementing the "Generate" procedure

To provide support for a new widget, you first need to write a "Generate" procedure that will take care of generating the piece of code related to the widget's properties. Note that the parents properties do not have to be handled by this function.

Dynamic loading of XML files

Introduction to Libglade

Libglade is an external library provided by default on some systems (e.g Linux, Gnome) that supports dynamic loading of XML files and creation of widgets at run time. The advantage of using libglade is that you do not need to recompile your application to change your user interface. On the other hand, everything is done at run time, meaning less compile checks, and also the inability to take advantage of the object oriented features of GtkAda and Ada such as deriving from a widget creating using the GUI builder, to add fields and properties. See packages Glade and Glade.XML for more information.

Limitations

Gate currently support all Gtk+ widgets and properties available under Glade. But, to help you identify widgets that may not be supported (e.g Gnome widgets), Gate will generate a warning on the standard error:

$ gate warning.glade
Generating Ada files...

GtkAda-WARNING **: Unsupported widget GnomeCanvas (canvas1)
The following files have been created/updated in src:
[...]

and add a comment in the <widget>_pkg.adb file that looks like:

--  WARNING: Unsupported widget GnomeCanvas (canvas1)

This means that while generating the file Gate detected an unsupported widget (in this case GnomeCanvas) whose name is canvas1. If you get such a warning your file may or may not compile properly, but you won't get the complete widget hierarchy at run time.

Feel free to send us (see section How to report bugs) the XML file that causes this problem. We don't guarantee a rapid fix for each particular problem but receiving real examples of missing functionnalities will certainly help implementing them faster.

Binding new widgets

GtkAda comes with a Perl script to help you create a binding to a C widget (this is the script we have used ourselves). This will not fully automate the process, although it should really speed things up. You will probably need less than 15 min to create a new binding once you will get used to the way GtkAda works. Note that your C file should have the same format as is used by Gtk+ itself.

Here are the steps to create a new binding :

How to report bugs

GtkAda is becoming more and more stable due to its increasing use, but you may still find bugs while using it. We have tried to test it as much as possible, essentially by converting the testgtk.c file found in the gtk distribution, as well as with generating a significant number of interfaces using the GUI builder and Gate. We strongly suggest that you have a look at testgtk, which gives a lot of examples of how to use this toolkit.

There are two kinds of problems you can encounter:

If you are a supported user of GNAT, send mail to mailto:report@gnat.com to report errors, otherwise send mail to the authors (mailto:gtkada@ada.eu.org) explaining exactly what your are doing, what is the expected result and what you actually get. Please include the required sources to reproduce the problem, in a format usable by gnatchop (basically, insert all the required sources at the end of the mail). Please try to provide as small as possible a subset of your sources.

Of course, we will welcome any patch you can provide, so that this toolkit may be as useful as possible.

Bibliography

We recommand the following documents. Most of them were written with C in mind, but should be easily adapted after you've read the rest of this document.


Footnotes

(1)

Glade was written by Damon Chaplin


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