import UIKit

class ViewController: UIViewController
{

    var one: [UIImage] = []
    var two: [UIImage] = []



override func viewDidLoad()
{
    super.viewDidLoad()

    one = createImageArray(total: 20, imagePrefix: "images1")
    two = createImageArray(total: 20, imagePrefix: "images2")

    UIView.animateKeyframes(withDuration: 3.0, delay: 0.0, options: [], animations: {
        UIView.addKeyframe(withRelativeStartTime: 0.0, relativeDuration: 3, animations: {
            self.animatingImage.animationImages = self.one
            self.animatingImage.animationDuration = 2
            self.animatingImage.startAnimating()
        })

        UIView.addKeyframe(withRelativeStartTime: 0.0, relativeDuration: 3, animations: {
            self.animatingImage.animationImages = self.two
            self.animatingImage.animationDuration = 2
            self.animatingImage.startAnimating()
        })

    }, completion:{ _ in
        print("end")
    })
}

override func didReceiveMemoryWarning()
{
    super.didReceiveMemoryWarning()
}

func createImageArray(total: Int, imagePrefix: String) -> [UIImage]
{
    var imageArray: [UIImage] = []

    for imageCount in 1...total
    {
        let imageName = "(imagePrefix)(imageCount).jpg"
        let image = UIImage(named: "(imageName)")!
        imageArray.append(image)
    }

    return imageArray
}


@IBOutlet weak var animatingImage: UIImageView!

}

self.animatingImage = UIImage.animatedImage(with: one, duration: 1)

      // animate after 1 sec
DispatchQueue.main.asyncAfter(deadline: .now() + 1.0) {
     let animatedImage = UIImage.animatedImage(with: two, duration: 1)
}