I started with vertexes. When I click with the left button, I want the following behaviour:

• if I click on an empty space, a new vertex should be created centered on the point;
• if I click on an already-present vertex, it should be "selected".

I basically solved the first issue, as you may see in the following, but the second is still not satisfactory.

I tried to simplify things down to this (comment Smart::Comments out if you don't have it):

#!/usr/bin/perl
use strict;
use warnings;
use Tk;
use Smart::Comments;

my $mw = MainWindow->new();

my $canvas = $mw->Canvas(-width => 300, -height => 300, -closeenough =
+> 5);
$mw->bind($canvas,     '<1>', \&add_point);
$canvas->bind('point', '<1>', \&toggle);
$canvas->pack();
MainLoop();

sub add_point {
   ### adding a point...
   my ($canvas) = @_;
   my $e = $canvas->XEvent;
   my ($x, $y) = ($e->x, $e->y);
   my $id = $canvas->createOval(
      $x - 10, $y - 10, $x + 10, $y + 10,
      -fill => 'blue',
      -tags => ['point']
   );
   ### new point created: "$id => [$x, $y]"
   return;
} ## end sub add_point

sub toggle {
   ### toggle point...
   my ($canvas) = @_;
   my $item_id = $canvas->find('withtag', 'current')->[0];
   ### item id: $item_id
   my $current_color = $canvas->itemcget($item_id, 'fill');
   my $next_color = $current_color eq 'red' ? 'blue' : 'red';
   $canvas->itemconfigure($item_id, -fill => $next_color);
   Tk->break(); # Try to break, but with little luck...
   return;
} ## end sub toggle
[download]

When I click on an already-present point, I succeed in changing its color (i.e. "selecting" it) but I can't break the event chain. This results in a new vertex being always added, overlapping the toggled one.

I managed to solve the problem using a global status variable to track if I toggled or not, but this is really ugly and I hope there's a better way to solve this.

Thank you in advance for your patience,