     private func buildDefaultImageView() -> UIImageView {
         let imageView = UIImageView(frame: bounds)
         imageView.autoresizingMask = [.flexibleHeight, .flexibleWidth]
-        imageView.contentMode = UIViewContentMode.center
+        imageView.contentMode = UIView.ContentMode.center
         self.addSubview(imageView)
 
         return imageView

    private func animateTransitionWithDuration(duration: Double, imageView: UIImageView, nextImageView: UIImageView, completion: @escaping () -> ()) {
        UIView.animate(withDuration: duration,
                                   delay: 0.0,
                                   options: UIViewAnimationOptions.curveEaseInOut,
                                   animations: {
                                       imageView.alpha = 0.0
                                       nextImageView.alpha = 1.0

        guard superview == nil else {
            notificationCenter.addObserver(self,
                                           selector: #selector(applicationWillResignActive),
                                           name: UIApplication.willResignActiveNotification,
                                           object: nil)
            notificationCenter.addObserver(self,
                                           selector: #selector(applicationDidBecomeActive),
                                           name: UIApplication.didBecomeActiveNotification,
                                           object: nil)
            return
        }
