Mastering Perl/Tk

Tracing Perl/Tk Variables

This is something of an oddball topic for this Anatomy lesson, but it introduces background information we’ll use later. Plus, it lets us do some neat things.

The Perl way to trace (or set watchpoints upon) a variable is by using the built-in tie function or the CPAN module Tie::Watch. Tcl has three commands associated with variable tracing: trace variable, trace vdelete, and trace vinfo. We’ll examine sample code that uses three similar Perl subroutines, then briefly illustrate how our new Trace module is implemented.

First we need to define three new commands, the analogs of the Tcl/Tk Trace commands. They are traceVariable (start tracing a variable), traceVinfo (show trace information), and traceVdelete (stop tracing a variable). Using these commands, we can write a program that animates an analog dial via a Scale widget (see Figure 15-5).

Figure 15-5. Animating a meter

The dial is actually a fat Canvas line item with an arrow on one end. The Scale goes from 0 to 100, with the dial pointing straight up when it reads 50. The Scale’s value is updated in the variable $v.

my $c = $mw->Canvas(qw/-width 200 -height 110 -bd 2 -relief sunken/)->grid;
$c->createLine(qw/ 100 100 10 100  -tag meter -arrow last -width 5/);
my $s = $mw->Scale(qw/-orient h -from 0 -to 100 -variable/ => \my $v)->grid;
$mw->Label(-text => 'Slide Me for > 5 Seconds')->grid;

The ...