XForms: Review and Tutorial

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Exploring XForms, a graphical user interface toolkit for X.

In this article I would like to introduce you to XForms, a graphical user interface (GUI) toolkit for C and C++ I discovered a few months ago. I've been writing programs for several platforms (including, of course, Linux) for some time now and lately have been snooping around for a good GUI toolkit for X: one that works, can be obtained free of charge, and (most importantly) one that you can use right away, without any knowledge of X intrinsics. Then I heard about XForms, downloaded it, and tried it out. I wrote my first X program in less than a day. Impressive!

If you have ever looked into one of the programmer's books about the bare X library, you understand the difficulties I'm talking about. Writing a “hello world” program using only X11 itself is no small task. Even the smallest program will require several hundreds of lines of C code.

This is where toolkits come in. A GUI toolkit is a library of functions that make the creation and maintenance of a user interface more programmer-friendly. One of the better-known packages is, of course, Motif. If you're lucky, the Motif package you buy comes with a code generator which enables you to define a simplified GUI, which is then converted into C code. If you're very lucky, your package has an interactive GUI builder, where you can see how your interface will look once coded. Many GUI toolkits are, however, expensive; and many GUI toolkits aren't at all friendly to a novice X-programmer.

The XForms GUI package is not only free (for non-commercial applications), but it also makes good-looking programs, has an excellent interactive design builder, comes with good documentation, and is very easy for novices to use.

Obtaining XForms

XForms is written by T.C. Zhao and Mark Overmars.

You can obtain XForms via anonymous ftp at:

ftp://bloch.phys.uwm.edu/pub/xforms andftp://ftp.cs.ruu.nl/pub/XFORMS

You should download the documentation and the XForms toolkit archive itself. The documentation is contained in a subdirectory DOC and is available in single or double sided postscript and dvi format. Since July 1995, the Linux ELF executable format is supported, as well as the a.out format. To download the toolkit in a.out format, get the file bxform-075.tgz from the directory linux. The ELF-based library is in linux/elf. The package is also available for other platforms; see the documentation.

Related WWW pages are at http://bragg.phys.uwm.edu/xforms/ and www.uwm.edu/~zhao

Finally, you can join a mailing list. Send a message to listserv@imageek.york.cuny.edu and put in the body of the mail message:

subscribe xforms Your Name

The authors can be reached through this list.

XForms Basics

Every program that uses XForms for its user interface manages one or more “forms”. A form is simply a window under X: with or without a border, with or without resizing capabilities, with some window title, etc. The form is a box, into which you can put “objects”: buttons, dials, input fields and lots more. Objects are what many other toolboxes call “widgets”. Each object is one of a given “class”. For instance, XForms supports the object class “button”, of which many flavors exist (a simple button, with a light-bulb on it, round ones, check buttons, radio buttons etc.). Most programs can be written using XForms' built-in classes, though XForms supports a free object class to handle special situations.

The concepts of XForms, such as classes and objects, resemble object-oriented programming (and C++ in particular). However, the XForms code is strictly C, which means that you can use it for both C and C++. What's more, XForms is built on top of the bare X11 library. Linking an XForms program requires only the presence of the XForms library, the X11 library and the math library; no other libraries are necessary.

One Input Source vs. Many

The programming paradigm you have to use with XForms (and in fact, with any X application) is somewhat different from what you might be used to. When writing an interactive program that reads its input from only one source (say, the keyboard), you are likely to use the following scheme:

/* forever do: */
while (1)
{
    /* fetch user's input */
    input = get_input ();
    /* determine what it is and
     * take appropriate action
     */
    if (input == one_thing)
        do_one_thing ();
    else if (input == other_thing)
        do_other_thing ();
    .
    .
    .
    /* if `quit` signaled: we're
     * all done here
     */
    else if (input == all_done)
        break;
}

Conversely, when you build a user interface for X, you are likely to create several input sources. For example, you can have several buttons which, when pressed, activate some part of your program's code. Input sources can even be objects that are not selectable by a user, such as a timer that runs out. The programming paradigm that represents this is implemented in X with callbacks. You create several buttons, each with its own specific callback function. The main program loop then consists of polling the events at the various input sources and activating the appropriate callback when something happens. This is the approach taken by XForms, and we will see a typical program with callbacks later on.

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