class TkMove
  def initialize( canvas, tag )
    @lastx, @lasty = 0, 0
    bind( canvas, tag )
  end
  def bind( canvas, tag )
    canvas.itembind(tag,'1',proc{|x,y| self.plotDown canvas, x, y }, "%x %y")
    canvas.itembind(tag, 'B1-Motion',proc{|x,y| self.plotMove canvas, x, y }, "%x %y")
  end
  def unbind( canvas, tag )
    canvas.itembind(tag, '1', proc{})
    canvas.itembind(tag, 'B1-Motion', proc{})
  end
  def plotDown (w, x, y)
    w.dtag 'selected'
    w.addtag_withtag 'selected', 'current'
    w.raise 'current'
    @lastx, @lasty = x, y
  end

  def plotMove (w, x, y)
    w.move 'selected', x - @lastx, y - @lasty
    @lastx, @lasty = x, y
  end
end