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.

       Multiple read, write and undef callbacks can be attached to a variable, which are invoked
       in reverse order of creation.

METHODS

       $mw->traceVariable(varRef, op => callback);
           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.

       %vinfo = $mw->traceVinfo(varRef);
           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.

       $mw->traceVdelete(\$v);
           Stop tracing the variable.

EXAMPLES

        # Trace a Scale's variable and move a meter in unison.

        use Tk;
        use Tk::widgets qw/Trace/;

        $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 -arrow last -width 5/ );
        $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;
        }

        # Predictive text entry.

        use Tk;
        use Tk::widgets qw/ LabEntry Trace /;
        use strict;

        my @words =  qw/radio television telephone turntable microphone/;

        my $mw = MainWindow->new;

        my $e = $mw->LabEntry(
            qw/ -label Thing -width 40 /,
            -labelPack    => [ qw/ -side left / ],
            -textvariable => \my $thing,
        );
        my $t = $mw->Text( qw/ -height 10 -width 50 / );;

        $t->pack( $e, qw/ -side top / );

        $e->focus;
        $e->traceVariable( \$thing, 'w', [ \&trace_thing, $e, $t ] );

        foreach my $k ( 1 .. 12 ) {
            $e->bind( "<F${k}>" => [ \&ins, $t, Ev('K') ] );
        }
        $e->bind( '<Return>' =>
                  sub {
                      print "$thing\n";
                      $_[0]->delete( 0, 'end' );
                  }
        );

        MainLoop;

        sub trace_thing {

            my( $index, $value, $op, $e, $t ) = @_;

            return unless $value;

            $t->delete( qw/ 1.0 end / );
            foreach my $w ( @words ) {
                if ( $w =~ /^$value/ ) {
                    $t->insert( 'end', "$w\n" );
                }
            }

            return $value;

        } # end trace_thing

        sub ins {

            my( $e, $t, $K ) = @_;

            my( $index ) = $K =~ /^F(\d+)$/;

            $e->delete( 0, 'end' );
            $e->insert( 'end', $t->get( "$index.0", "$index.0 lineend" ) );
            $t->delete( qw/ 1.0 end / );

        } # end ins

HISTORY

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

        sol0@Lehigh.EDU, Lehigh University Computing Center, 2003/09/22
        . Version 1.1, for Tk804.025, add support for multiple traces of the same
          type on the same variable.

COPYRIGHT

       Copyright (C) 2000 - 2003 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.