animationSky.swift

class ViewController: UIViewController {
  let background = UIImage(named: "clouds.png")
  var animationDuration = 12.0

  override func viewDidLoad() {
    ///...
    backgroundView = initialBackground()
    backgroundViewDelayed = initialBackground()

    view.addSubview(backgroundView)
    view.addSubview(backgroundViewDelayed)

    /// First background view starts immediately
    startAnimation(on: backgroundView, delay: 0.0)
    /// Second view starts after a delay
    startAnimation(on: backgroundViewDelayed, delay: animationDuration / 2)
  }

  func initialBackground() -> UIImageView {
    let shiftedLeft = CGRect(origin: CGPoint(x: -view.bounds.width, y: view.bounds.minY),
                             size: view.bounds.size)
    let view = UIImageView(frame: shiftedLeft)
    view.contentMode = .scaleAspectFill
    view.image = background
    return view
  }
  
  /// Create sliding animation that repeats on a delay
  func startAnimation(on imageView: UIImageView, delay: Double) {
    UIView.animate(withDuration: animationDuration, delay: delay, options: [.repeat, .curveLinear], animations: {
      imageView.frame = CGRect(
        origin: CGPoint(x: self.view.bounds.width, y: self.view.bounds.minY),
        size: self.view.bounds.size
      )
    })
  }

  ///...
}