X Window System Programming with Tcl and Tk

One short note about Tcl syntax: Braces ( { ... }) are used to group a set of Tcl commands together as a ”sub-script“. The commands contained within the braces are passed to the Td interpreter without performing variable substitution. This is an important concept to understand. Without the braces, the Tcl interpreter would have attempted to substitute the value of the variable fname while interpreting the bind command in the script. That is, when the bind command is first executed, the variable fname has no value. Without braces, the Tcl interpreter would complain that fname is an unknown variable. Using braces, however, we delay the interpretation of $fname until the event binding is actually executed; in this case, when Return is pressed in the entry widget.

Tk is a set of extensions to Tcl which implement commands... permitting you to write X applications as simple Tcl scripts.

Note that Tcl has several odd rules with respect to line breaks. Tcl expects each command to consist of a single line; the end of a line indicates the end of a command, unless the line ends with a backslash, the same as in shell scripts. However, if a line ends in an opening brace, Tcl understands that you are beginning a sub-script, to be contained within braces, and continues to read the script until a closing brace. For this reason, you can't say:

bind .e <Return>
{
exec xterm -e vi $fname
{

Tcl will think that the bind command ends after the first line, and complain that it needs a script to execute for the event binding. Therefore, when using braces to encapsulate a sub-script, be sure that the opening brace is at the end of the line beginning the script.

In Tk, widgets are named in a hierarchial fashion. The topmost ”shellH widget (that is, the main wish window) is named “ . ” (dot). All widgets which are direct childIen of . are given names beginning with ., such as .b,.entry,.leftscroll, and so forth. The widget name can be any alphanumeric string beginning with a dot; you choose the widget name when you create the widget, using commands such as button and label. Further subwidgets are given names such as .foo.bar.bar, where each level is separated with a dot.

Creating a Menu Bar Listing

For example, you might have a menu bar widget named .mbar. It is a child, of course, of ., the main window. Menu buttons contained within the menu bar might be named .mbar.file,.mbar.options, and so forth. That is, the menu bar is a child of the main application window, and the individual menu buttons are children of the menu bar. Arranging widgets into a hierarchy allows you to group them together for logical and visual purposes. I'll cover this in more detail later.

In order to demonstrate the power of Tcl/Tk, I'm going to present an actual application, written entirely as a Tcl/Tk script. As I go along I will describe the syntax used and the features available. You can use the Tcl/Tk man pages to fill in the gaps.

This program is a simple drawing application, utilizing the Tk canvas widget. The canvas is a simple graphics display widget which will display various kinds of objects: rectangles, lines, text, ovals, and so forth. What we're going to do is combine the canvas widget with the user interface capabilities of Tk to allow the user to draw objects using the mouse.

Figure 3

Figure 3 demonstrates what our application looks like when used. The Color menu has been pulled down so that you can see the various selections available.

The entire script, draw.tcl, is given here. Note that it's less than 200 lines long. This is amazingly short for such a complicated X application involving menus, colors, mouse input, and so forth. If you don't feel like entering this entire script, the code is available via ftp from sunsite.unc.edu, in the directory /pub/Linux/docs/LJ.

At first glance, this script might appear to be complicated. There are a few rough spots, but the overall concepts presented here are quite simple. Let's take a closer look at this program, but let's start near the middle of the script, where we create the frame widget:

frame .mbar -relief groove -bd
pack .mbar -side top -expand yes -fill x

The frame command creates a frame widget, which is used to group widgets together. The frame itself is usually invisible, unless you specify that a border should be drawn around it.

Here we create a frame named .mbar, and specify that it should use the groove relief type. The “relief” of a widget indicates what kind of 3D border should appear around the widget. (Many options, such as -relief, bd, -foreground, and -background are supported by all widget types.) The valid types for -relief are:

The -ted option specifies the border width to use for the widget (in this case, the width of the groove). Here, we set the border width to 3 pixels.

Next, we pack .mbar into the wish window. (By default, widgets are packed into their direct parent. In this case, the parent of .mbar is ., the topmost window). The -side argument to pack indicates which side of the parent we should pack .mbar into. The -expand yes option indicates that the widget should be given all of the extra space around it. Because we are packing the widget into the top edge of the window, the -expand option gives the widget any extra horizontal space to its left and right. The -fill x command causes the widget to grow until it fills this space. Using -expand yes without -fill would give the widget the extra horizontal space, but the widget wouldn't grow to fill that space. (If you're interested in how this works, experiment with the pack command in various forms. Also, see the pack man page or Ousterhout's book for more details.)