##
# see also a demonstration by zentara, particularly -textvariable
# http://www.perlmonks.org/?node_id=1087787
#
#    my $msg : shared = "Start";
#    $btn1 = $mw->Button(-textvariable => \$msg, ...);
#    later : $msg = "Step Two"; $btn1->update;
#
# thank you, zentara++
##

use strict;
use warnings;

use threads;
use threads::shared;
use Thread::Queue;

use Tk;

my $que = Thread::Queue->new();
my $msg : shared = "Start";

# detach the thread so not to get thread warnings
# about running threads when exiting the app

(my $thr = threads->create('bg_task'))->detach();

my $mw = MainWindow->new();

$mw->protocol( WM_DELETE_WINDOW => \&quit );
$mw->geometry("+150+100");

$mw->Label( -text => "Tk + threads Demo", -height => 2, -width => 22 )->pack;

my $btn1 = $mw->Button( -text => $msg, -command => \&fun, -width => 10 );
   $btn1->pack();

my $btn2 = $mw->Button( -text => "Quit", -command => \&quit, -width => 10 );
   $btn2->pack();

my $timer = $mw->repeat( 100, sub {
   $btn1->configure( -text => $msg );
});

MainLoop();

sub fun {
   $que->enqueue("some event");
   return;
}

sub quit {
   $timer->cancel;
   # for threads, is helpful to signal the app to
   # terminate so not to segfault on some platforms
   kill('TERM', -$$);
   exit;
}

sub bg_task {
   while ( my $event = $que->dequeue ) {
      $msg = "Step One";
      sleep 1;
      $msg = "Step Two";
      sleep 1;
      $msg = "Step Three";
   }
}