class ViewController: UIViewController {

    var img1: UIImage!

    override func viewDidLoad() {
        super.viewDidLoad()

        guard let i1 = UIImage(named: "p3x3"),
              let i2 = UIImage(named: "p2x2"),
              let i3 = UIImage(named: "p1x1")
        else {
            fatalError("failed to load images")
        }

        i1.mergeWithTopmage(topImage: i2, completion: { img in
            img.mergeWithTopmage(topImage: i3, completion: { img in
                self.img1 = img
            })
        })
    }

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)

        let imgView = UIImageView(image: img1)
        view.addSubview(imgView)
        imgView.center = view.center

    }

}