Shoes.app :width => 100, :height => 100 do
  def self.toggle_rectangle_color
    @is_blue = !@is_blue
    color = @is_blue ? blue : red

    clear do 
      fill  color
      rect  0, 0, 100, 100
    end
  end

  flow :width => 100, :height => 100 do
    every(0.02) do
      toggle_rectangle_color
    end
  end
end