Kode KDE Kindly, Kan You?
With a little programming experience, you'll have everything you need to build applications for the Linux desktop when you're done reading this.
by Jason Mott
There are many toolkits to choose from for building Linux desktop applications. Some say this is Linux's downfall; others say it is its greatest feature. I'll stand somewhere in the middle and say choice is good if you choose what meets your needs. Most graphical user interfaces (GUIs) on Linux are based on X, a client/server architecture that allows for networked computers to share GUI applications. With X, the application is the client that sends its graphical output to an X server. The X server accepts applications' output on behalf of its local hardware (or sometimes virtual hardware, but I won't go there in this article). In most cases, the X server and the X client are on the same machine, but use the client/server architecture nonetheless.
The base-level toolkit for building an X client is called Xlib. Xlib is much too low-level and difficult to use by itself to build an application from scratch. As a result, many toolkits have been built on top of Xlib to make it easier to write GUI applications for X. Subsequently, when writing GUI applications using one of the high-level toolkits, you never even know it's a networked application (that it's sending its graphical output to a server).
The two most popular open-source toolkits that layer on top of Xlib are Qt and GTK+, upon which KDE and GNOME are respectively built. Motif is another popular toolkit (not open source, but there is an open-source clone called Lesstif). Figure 1 shows a diagram of these (and more) and their relationship with one another. The farther down on the diagram, the lower the level of API it is. I prefer KDE/Qt for many reasons, but mostly because it's focused on a good user interface and a clean and well-designed API.
I actually built an application from scratch for this article (a calculator) to show how quick and easy it can be. To follow along you'll need some tools—most importantly, KDevelop. I used version 2.0.2 running on KDE 2.2.2. When using KDevelop's setup wizard you'll be told what dependencies you're missing. Visit the KDevelop web site (www.kdevelop.org) or check with your distribution's included software, and get KDevelop installed. If you have KDE as part of your distribution, then you'll have KDevelop.
Most tutorials on application development try to be IDE-agnostic. I've decided against this for a few reasons: KDevelop is free and comes with KDE; KDevelop makes an already easy API even easier, and KDevelop's wizards help the developer conform to KDE user-interface standards. For those who don't like to deal with Makefiles or creating configure files (or even dealing with the easier automake and autoconf), KDevelop will handle these things for you. And for geeks who do like to mess around with Makefiles, KDevelop allows you to tweak them. In fact, your KDevelop application is ready to be built with the traditional ./configure, make, make install. In other words, you're free to break free from KDevelop anytime you want.
The first time you fire up KDevelop it will run you through a setup wizard. The most important piece of the wizard is when it checks for dependencies (Figure 2). Take the time to look through this output; if there are failures listed, do yourself a favor and try to find the libraries needed. The easiest way to do this is to use www.rpmfind.com (if your distribution is RPM-based). For each library that failed, do a search for it on the rpmfind web site. From the search results, select the RPM appropriate for your distribution and install it. Once you've done that, rerun the KDevelop wizard (type kdevelop --setup on the command line or find an entry in your K menu that says KDevelop Setup). If there are still missing items, repeat as necessary.
The calculator I built is simple and only does basic arithmetic. The whole thing took about two hours to complete (okay, three hours, but I wasn't in a hurry). I was going to name it Kalculator, but there is an obscure application out there that already uses that name. So I chose Kalculate. I did not use nor view any code from KDE's stock calculator called KCalc, which has more mathematical features than mine. I've always felt KDE should have a basic calculator anyway, so I'd like to see user-friendly features added to Kalculate rather than more mathematical features. Therefore, when you're done reading this you can be a part of a living experiment in open-source development—come join the team and add a feature! You'll get special preference if you say this article sent you (sourceforge.net/projects/kalculate).