use strict;
use warnings;
use v5.14;
use Tk;



my $mw = new MainWindow( -title => 'Event demo');
my $drawarea = $mw->Frame()->pack( -side => 'top', -fill => 'both' );
$mw->bind('<<RowDone>>' => \&next_row);
my $p = $mw->Photo(-width=>500, -height=>500);

my $canvas = $drawarea->Canvas(
    -relief      => 'ridge',
    -width       => 500,
    -height      => 500,
    -borderwidth => 4
)->pack();

$canvas->bind('<<RowDone>>' => \&next_row);
$canvas->createImage(0,0, -anchor=>'nw', image=>$p);


my $plot = $mw->Button(-text=>'Plot', -command=>\&init)
    ->pack(-side=>'left');

MainLoop;

# Callbacks

sub init {
    our $y = 0;
    $mw->eventGenerate('<<RowDone>>');
    return;
}

sub next_row {
    our $y;
    for  my $x ( 0..499 ) {
        my $quality = long_computation($x, $y);
        $p->put(color($quality), '-to', $x, $y);
    }
    $canvas->update;
    if (++$y < 500) {
        $mw->eventGenerate('<<RowDone>>', -when => 'tail' );
    }
    return;
}

# stubs for demo only
sub color {
    return 'red'
}

sub long_computation {
    return 10;
}