Tk::Trace(3)

NAME
Tk::Trace - emulate Tcl/Tk trace functions.

SYNOPSIS
 use Tk::Trace
 $mw->traceVariable(\$v, 'wru' => [\&update_meter, $scale]);
 %vinfo = $mw->traceVinfo(\$v);
 print "Trace info  :\n  ", join("\n  ", @{$vinfo{-legible}}), "\n";
 $mw->traceVdelete(\$v);

DESCRIPTION
This class module emulates the Tcl/Tk trace family of commands by binding subroutines of your devising to Perl variables using simple Tie::Watch features.

Callback format is patterned after the Perl/Tk scheme: supply either a code reference, or, supply an array reference and pass the callback code reference in the first element of the array, followed by callback arguments.

User callbacks are passed these arguments:

 $_[0]        = undef for a scalar, index/key for array/hash
 $_[1]        = variable's current (read), new (write), final (undef) value
 $_[2]        = operation (r, w, or u)
 $_[3 .. $#_] = optional user callback arguments
As a Trace user, you have an important responsibility when writing your callback, since you control the final value assigned to the variable. A typical callback might look like:

 sub callback {
    my($index, $value, $op, @args) = @_;
    return if $op eq 'u';
    # .... code which uses $value ...
    return $value;     # variable's final value
 }
Note that the callback's return value becomes the variable's final value, for either read or write traces.

For write operations, the variable is updated with its new value before the callback is invoked.

Only one callback can be attached to a variable, but read, write and undef operations can be traced simultaneously.

METHODS
varRef is a reference to the scalar, array or hash variable you wish to trace. op is the trace operation, and can be any combination of r for read, w for write, and u for undef. callback is a standard Perl/Tk callback, and is invoked, depending upon the value of op, whenever the variable is read, written, or destroyed. Returns a hash detailing the internals of the Trace object, with these keys:

 %vinfo = (
     -variable =>  varRef
     -debug    =>  '0'
     -shadow   =>  '1'
     -value    =>  'HELLO SCALAR'
     -destroy  =>  callback
     -fetch    =>  callback
     -store    =>  callback
     -legible  =>  above data formatted as a list of string, for printing
 );
For array and hash Trace objects, the -value key is replaced with a -ptr key which is a reference to the parallel array or hash. Additionally, for an array or hash, there are key/value pairs for all the variable specific callbacks. Stop tracing the variable.

EXAMPLE
 use Tk;
 # Trace a Scale's variable and move a meter in unison.
 $pi = 3.1415926;
 $mw = MainWindow->new;
 $c = $mw->Canvas(qw/-width 200 -height 110 -bd 2 -relief sunken/)->grid;
 $c->createLine(qw/100 100 10 100 -tag meter/);
 $s = $mw->Scale(qw/-orient h -from 0 -to 100 -variable/ => \$v)->grid;
 $mw->Label(-text => 'Slide Me for 5 Seconds')->grid;
 $mw->traceVariable(\$v, 'w' => [\&update_meter, $s]);
 $mw->after(5000 => sub {
     print "Untrace time ...\n";
     %vinfo = $s->traceVinfo(\$v);
     print "Watch info  :\n  ", join("\n  ", @{$vinfo{-legible}}), "\n";
     $c->traceVdelete(\$v);
 });
 MainLoop;
 sub update_meter {
     my($index, $value, $op, @args) = @_;
     return if $op eq 'u';
     $min = $s->cget(-from);
     $max = $s->cget(-to);
     $pos = $value / abs($max - $min);
     $x = 100.0 - 90.0 * (cos( $pos * $pi ));
     $y = 100.0 - 90.0 * (sin( $pos * $pi ));
     $c->coords(qw/meter 100 100/, $x, $y);
     return $value;
 }

HISTORY
 Stephen.O.Lidie@Lehigh.EDU, Lehigh University Computing Center, 2000/08/01
 . Version 1.0, for Tk800.022.

COPYRIGHT
Copyright (C) 2000 - 2000 Stephen O. Lidie. All rights reserved.

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.