Parallax scrolling with Sprite Kit and RubyMotion

my_scene.rb

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

class MyScene < SKScene
  def add_skyline
    texture = SKTexture.textureWithImageNamed "skyline.png"
    x = CGRectGetMidX(self.frame)

    2.times do |i|
      skyline = SKSpriteNode.spriteNodeWithTexture texture
      skyline.scale = 1.12
      skyline.position = CGPointMake(x + (i * x * 2), CGRectGetMidY(self.frame))
      skyline.zPosition = -20

      skyline.runAction scroll_action(x)

      addChild skyline
    end
  end
  
  def add_ground
    texture = SKTexture.textureWithImageNamed "ground.png"
    x = CGRectGetMidX(self.frame) + 7

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

      addChild ground
    end
  end

  def scroll_action(x)
    width = (x * 2)
    move = SKAction.moveByX(-width, y: 0, duration: 0.02 * width)
    reset = SKAction.moveByX(width, y: 0, duration: 0)

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