#!/usr/local/bin/perl -w

use Tk;

{
 package Packer;
use base  qw(Tk::Frame);
 Construct Tk::Widget 'Packer';

 sub new
  {my $class  = shift;
   my $parent = shift;
   my $obj = $parent->Frame('-class' => Packer);
   $obj->{Slaves} = [];
   $obj->{LayoutPending} = 0;
   bless $obj,$class;
  }

 sub Layout
 {
  my $m = shift;
  $m->{LayoutPending} = 0;
  my $w = 0;
  my $h = 0;
  my $x = 0; 
  my $y = 0; 
  my $s;
  # find size of largest slave
  foreach $s (@{$m->{Slaves}})
   {
    my $sw = $s->ReqWidth;
    my $sh = $s->ReqHeight;
    $w = $sw if ($sw > $w);
    $h = $sh if ($sh > $h);
   }
  # Set size and position of slaves
  foreach $s (@{$m->{Slaves}})
   {
    $s->MoveResizeWindow($x,$y,$w,$h);
    $s->MapWindow;
    $y += $h;
   }
  # Now ask for enough space
  $m->GeometryRequest($w,$y);
 }

 sub QueueLayout
 {
  my $m = shift;
  $m->afterIdle(['Layout',$m]) unless ($m->{LayoutPending}++);
 }

 sub SlaveGeometryRequest
 {
  shift->QueueLayout;
 }

 sub LostSlave
 {
  my ($m,$s) = @_;
  @{$m->{Slaves}} = grep($_ != $s,@{$m->{Slaves}});
  $m->QueueLayout;
 }

 sub Manage
 {
  my ($m,$s) = @_;
  $m->ManageGeometry($s);
  push(@{$m->{Slaves}},$s);
  $m->QueueLayout;
 }

}

$top = MainWindow->new();

$packer = $top->Packer();

$var = "This can grow";

$one  = $top->Label('-text' => 'Label One');

$packer->Manage($one);
$packer->Manage($packer->Label('-textvariable' => \$var));
$packer->Manage($packer->Label('-text' => 'A Very Long Label'));

$packer->pack;

$top->Button('-text' => 'Grow', '-command' => sub { $var .= " more" })->pack;
$top->Button('-text' => 'Loose', '-command' => [ 'pack' , $one])->pack;
$top->Button('-text' => 'Add', '-command' => [ 
   sub 
   {
    my $packer = shift; 
     $packer->Manage($packer->Label('-text' => 'Added thing'));
   }, $packer ])->pack;
  $top->Button('-text' => 'Quit', '-command' => [ 'destroy' , $top ])->pack;

MainLoop;