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wxWidgets on the GNOME Desktop
wxWidgets is a C++ cross-platform GUI library, whose distintive feature is the use of native calls and native widgets on the respective platform, i.e. an application compiled for the Linux platform will use the GTK+ library for displaying the various widgets. There is also a version ("port") of wxWidgets which uses the Motif toolkit for displaying its widgets (this port is commonly referred to as wxMotif) and another one, which only uses X11 calls and which draws its widgets entirely itself, without using any outside library. This port is called wxX11 or sometimes more generally wxUniv (short for wxUniversal), since this widget set (implemented entirely within wxWidgets) is available wherever wxWidgets is available. Since this short overview is mainly about how to write wxWidgets applications for the GNOME desktop, I will focus on the GTK+ port, which is generally referred to as wxGTK.
wxGTK still supports the old version GTK+ 1.2, but it now defaults to the up-to-date version GTK+ 2.X, which is the basis for the current GNOME desktop. By way of using GTK+ 2.X and its underlying text rendering library Pango, wxGTK fully supports the Unicode character set and it can render text in any language and script, that is supported by Pango.
Design Principles
The three main design goals of the wxWidgets library are portability across the supported platforms, complete integration with the supported platforms and a broad range of functionality covering most aspects of GUI and non-GUI application programming. Sometimes, various aspects of these design goals contradict each other and this holds true especially for the Linux platform which – from the point of view of the desktop environment integration – is lagging behind the other two major desktops (Windows and OS X) mostly because of the schism between the GTK+ based GNOME desktop and the Qt based KDE desktop. So far, the typical wxWidgets user targeted Windows, maybe OS X and Linux in general, so the aim was to make wxGTK applications run as well as possible on as many versions of Linux as possible, including those using the KDE environment. Luckily, most of these distributions included the GTK+ library (for running applications like the GIMP, GAIM, Evolution or Mozilla) whereas the GNOME libraries were not always installed by default. Also, the GNOME libraries didn't really offer substantial value so that the hassle of installing them was hardly justified. Therefore, much effort was spent on making wxGTK fully functional without relying on the GNOME libraries, mostly by reimplementing as much as sensible of the missing functionality. This included a usable file selection dialog, a printing system for PostScript output, code for querying MIME-types and file-icon associations, classes for storing application preferences and configurations, the possibility to display mini-apps in the taskbar, a full-featured HTML based help system etc. With all that in place you can write a pretty fully featured wxWidgets application on an old Linux system with little more installed than X11 and GTK+.
Recent Developments
Recently, several key issues have been addressed by the GNOME project. Sometimes integrated into the newest GTK+ releases (such as the file selecter), sometimes as part of the GNOME libraries (such as the new printing system with Pango integration or the mime-types handling in gnome-vfs), sometimes as outside projects (such as the media/video backend based on the Gstreamer project). Also, care has been taken to unify the look and feel of GNOME applications by writing down a number of rules (modestly called "Human Interface Guidelines") and more and more decisions are taken in a desktop neutral way (for both GNOME and KDE), mostly as part of the FreeDesktop initiative. This development together with the rising number of OpenSource projects using wxWidgets mainly for the Linux and more specifically GNOME desktop has led to a change of direction within the wxWidgets project, now working on making more use of GNOME features when present. The general idea is to call the various GNOME libraries if they are present and to offer a reasonable fallback if not. I'll detail on the various methods chosen below:
Printing System
The old printing system ....
MIME-type Handling
The old mime-type system used to simply query some files stored in "typical" locations for the respective desktop environment. Since both the format and the location of these files changed rather frequently, this system was never fully working as desired for reading the MIME-types and it never worked at all for writing MIME-types or icon/file associations. ...
File Dialogs
Previously, wxGTK application made use of a file dialog written in wxWidgets itself, since the default GTK+ file dialog was simplistic to say the least. This has changed with version GTK+ 2.4, where a nice and powerful dialog has been added. wxGTK is using it now.
File Configuration and Preferences
The usual Unix way of saving file configuration and preferences is to write and read a so called "dot-file", basically a text file in a user's home directory starting with a dot. This was deemed insufficient by the GNOME desktop project and therefore they introduced the so called GConf system, for storing and retrieving application and sessions information....
Results and Discussion
One of wxWidgets' greatest merits is the ability to write an application that not only runs on different operating systems but especially under Linux even on rather old systems with only a minimal set of libraries installed – using a single application binary. This was possible since most of the relevant functionality was either located in the only required library (GTK+) or was implemented within wxWidgets. Recent development outside the actual GTK+ project has made it necessary to rethink this design and make use of other projects' features in order to stay up-to-date with current techological trends. Therefore, a system was implemented within wxWidgets that queries the system at runtime about various libraries and makes use of their features whenever possible, but falls back to a reasonable solution if not. The result is that you can create and distribute application binaries that run on old Linux systems and integrate fully with modern desktops, if they are available. This is not currently possible with any other software.
Copyright 2004 © Robert Roebling, MD. No reprint permitted without written prior authorisation. Last modified 14/11/04.
About the Author
Robert Roebling works as a medical doctor in the Department of Neurology at the University clinic of Ulm in Germany. He has studied Computer Sciences for a few semesters and is involved in the wxWidgets projects since about 1996. He has started and written most of wxGTK port (beginning with GTK+ around 0.9) and has contributed to quite a number projects within wxWidgets, ranging from the image classes to Unicode support to making both the Windows and the GTK+ ports work on embedded platform (mostly PDAs). He is happily married, has two children and never has time.