Download on Thread

ImageDownloadProtocol.swift

import UIKit 

protocol ImageDownloadProtocol {
    func downloadImage(from url: URL, completion: @escaping (UIImage?) -> Void)
}

extension ImageDownloadProtocol {
    func downloadImage(from url: URL, completion: @escaping (UIImage?) -> Void) {
        let session = URLSession(configuration: .default)
        DispatchQueue.global(qos: .background).async {
            print("In background")
            session.dataTask(with: URLRequest(url: url)) { data, response, error in
                if error != nil {
                    print(error?.localizedDescription ?? "Unknown error")
                }
                if let data = data, let image = UIImage(data: data) {
                    print("Downloaded image data")
                    DispatchQueue.main.sync {
                        print("Dispatched to main")
                        completion(image)
                    }
                }
                }.resume()
        }
    }
}

ViewController.swift

import UIKit

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

extension ViewControllerOne: 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")
        }
        
    }
}