ViewControllerDownload.swift

class ViewController: UIViewController {
    
  let imageViewOne = UIImageView()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        imageViewOne.frame = UIScreen.main.bounds
        view.addSubview(imageViewOne)
        setImage()
    }
}

extension ViewController: ImageDownloadProtocol {
    
    func setImage() {
        let url = URL(string: "http://cdn-www.dailypuppy.com/dog-images/colby-the-golden-retriever_77539_2016-09-16_w450.jpg")!
        downloadImage(from: url) { image in
            if Thread.isMainThread {
                print("Thread is main")
            }
            self.imageViewOne.image = image
            print("Done")
        }
    }
}