#!/usr/bin/perl 

use strict;
use warnings;

use Tk;
use POE;

# Create the session that will drive the user interface.

POE::Session->create
  ( inline_states =>
      { _start => \&ui_start,
        ev_count => \&ui_count,
        ev_clear => \&ui_clear,
      }
  );

# Run the program until it is exited.

$poe_kernel->run();
exit 0;

# Create the user interface when the session starts.  This assumes
# some familiarity with Tk.  ui_start() illustrates four important
# points.
#
# 1. Tk events require a main window.  POE creates one for internal
# use and exports it as $poe_main_window.  ui_start() uses that as the
# basis for its user interface.
#
# 2. Widgets we need to work with later, such as the counter display,
# must be stored somewhere.  The heap is a convenient place for them.
#
# 3. Tk widgets expect callbacks in the form of coderefs.  The
# session's postback() method provides coderefs that post events when
# called.  The Button created in ui_start() fires an "ev_clear" event
# when it is pressed.
#
# 4. POE::Kernel methods such as yield(), post(), delay(), signal(),
# and select() (among others) work the same as they would without Tk.
# This feature makes it possible to write back end sessions that
# support multiple GUIs with a single code base.

sub ui_start {
    my ( $kernel, $session, $heap ) = @_[ KERNEL, SESSION, HEAP ];

    $poe_main_window->Label( -text => "Counter" )->pack;

    $heap->{counter_widget} =
      $poe_main_window->Label( -textvariable => \$heap->{counter} )->pack;

    $poe_main_window->Button
      ( -text => "Clear",
        -command => $session->postback("ev_clear")
      )->pack;

    $kernel->yield("ev_count");
}

# Handle the "ev_count" event by increasing a counter and displaying
# its new value.

sub ui_count {
    $_[HEAP]->{counter}++;
    $_[KERNEL]->yield("ev_count");
}

# Handle the "ev_clear" event by clearing and redisplaying the
# counter.

sub ui_clear {
    $_[HEAP]->{counter} = 0;
}