require "tkclass"

$tkline_init = FALSE
def start_random
  return if $tkline_init
  $tkline_init = TRUE
  if defined? Thread
    Thread.start do
      loop do
        sleep 2
        Line.new($c, rand(400), rand(200), rand(400), rand(200))
      end
    end
  end
end

Label.new('text'=>'Please press or drag button-1').pack

$c = Canvas.new
$c.pack
$start_x = start_y = 0

def do_press(x, y)
  $start_x = x
  $start_y = y
  $current_line = Line.new($c, x, y, x, y)
  start_random
end
def do_motion(x, y)
  if $current_line
    $current_line.coords $start_x, $start_y, x, y
  end
end

def do_release(x, y)
  if $current_line
    $current_line.coords $start_x, $start_y, x, y
    $current_line.fill 'black'
    $current_line = nil
  end
end

$c.bind("1", proc{|e| do_press e.x, e.y})
$c.bind("B1-Motion", proc{|x, y| do_motion x, y}, "%x %y")
$c.bind("ButtonRelease-1", proc{|x, y| do_release x, y}, "%x %y")
Tk.mainloop