#!/usr/bin/perl

# http://perlmonks.org/?node_id=1196818

use strict;
use warnings;
use Tk;

# golux:  flag to detect mouse button 1 pressed
my $b_mouse1 = 0;

sub xy { $_[0]->XEvent->x, $_[0]->XEvent->y }

my $mw = MainWindow->new;
$mw->overrideredirect(1);

$mw->Label(
  -text => 'Press 1, Move, then Release in green Canvas to move window',
  )->pack(-fill => 'x');

my $c = $mw->Canvas(-width => 500, -height => 400,
  -bg => 'green',
  )->pack;

$c->Tk::bind('<1>' => \&leftdown);
$c->Tk::bind('<Motion>' => \&motion);       # golux:  also detect Motion
$c->Tk::bind('<ButtonRelease-1>' => \&leftup);

$mw->Button(-text => 'Exit', -command => sub { $mw->destroy },
  )->pack(-fill => 'x');

MainLoop;

my ($startx, $starty);

sub leftup
  {
  # golux:  moved the "heavy lifting" into subroutine motion(); the
  # only thing we need do here is clear the mouse button 1 down flag.
  $b_mouse1 = 0;
  }

sub leftdown
  {
  $b_mouse1 = 1;
  ($startx, $starty) = &xy;
  }

sub motion {
  $b_mouse1 or return;
  my $oldposition = $mw->geometry;
  my ($endx, $endy) = &xy;
  my $deltax = $endx - $startx;
  my $deltay = $endy - $starty;
  #  golux:  Not enough to look for \d+, we need to look for [-\d]+, since
  #  the (X,Y) coordinates can also be negative
  my $newposition = $oldposition =~
    s/\+\K([-\d]+)\+([-\d]+)$/ ($1 + $deltax) . '+' . ($2 + $deltay) /er;
  $mw->geometry( $newposition );
  print "$newposition\n";
}