Parallax scrolling with Sprite Kit and RubyMotion

my_scene.rb

# Video of it in action: http://cl.ly/VSIF

class MyScene < SKScene
  def scroll_action(x, duration)
    width = (x * 2)
    move = SKAction.moveByX(-width, y: 0, duration: duration * width)
    reset = SKAction.moveByX(width, y: 0, duration: 0)

    SKAction.repeatActionForever(SKAction.sequence([move, reset]))
  end

  def add_ground
    x = CGRectGetMidX(self.frame) + 7

    2.times do |i|
      ground = SKSpriteNode.spriteNodeWithTexture ground_texture
      ground.position = CGPointMake(x + (i * x * 2), 56)
      ground.runAction scroll_action(x, 0.02)

      addChild ground
    end
  end

  def mid_x
    CGRectGetMidX(self.frame)
  end

  def mid_y
    CGRectGetMidY(self.frame)
  end

  def sky_texture
    SKTexture.textureWithImageNamed "skyline.png"
  end

  def ground_texture
    SKTexture.textureWithImageNamed "ground.png"
  end
  
  def add_skyline
    2.times do |i|
      x_position = mid_x + (i * mid_x * 2)
      
      skyline = SKSpriteNode.spriteNodeWithTexture sky_texture
      skyline.position = CGPointMake(x_position, mid_y)
      skyline.zPosition = -20
      skyline.scale = 1.12
      skyline.runAction scroll_action(mid_x, 0.1)

      addChild skyline
    end
  end
end