OperationQueue

Downloader.swift

func downloadImage(with completion: @escaping (Bool) -> ()) {
        let queue = OperationQueue()
        queue.name = "Image Download queue"
        queue.maxConcurrentOperationCount = 2
        queue.qualityOfService = .userInteractive
        self.downloadImage.state = .downloaded
        queue.addOperation { () -> Void in
            let downloader = DownloadOp(downloadImage: self.downloadImage)
            downloader.downloadImage(url:URL(string:"http://i.imgur.com/5ac1apZ.jpg")! , handler: { image in
                OperationQueue.main.addOperation({
                    print("done")
                    self.imageView.image = image
                })
            })
        }
        completion(true)
    }

func downloadButtonTapped() {
        switch (downloadImage.state) {
        case .preparing:
            downloadImage(with: { result in
                print(result)
                dump(self.downloadImage.image)
            })
        case .downloading:
            print("nope")
        case .downloaded:
            print("already downloaded")
        }
    }

DownloadImage.swift

import UIKit

class DownloadImage {
    var image: UIImage?
    var state = ImageState.preparing
}

enum ImageState {
    case preparing, downloading, downloaded
}

DownloadOp.swift

import Foundation

class DownloadOp: Operation {  
    let downloadImage: DownloadImage
    let session = URLSession(configuration: URLSessionConfiguration.default)
     private var _ready = false {
        willSet {
            willChangeValue(forKey: "isReady")
        }
        didSet {
            didChangeValue(forKey: "isReady")
        }
     }
    
    override var isReady: Bool {
        return _ready
    }
    
    private var _executing = false {
        willSet {
            willChangeValue(forKey: "isExecuting")
        }
        didSet {
            didChangeValue(forKey: "isExecuting")
        }
    }
    
    override var isExecuting: Bool {
        return _executing
    }
    
    private var _finished = false {
        willSet {
            willChangeValue(forKey: "isFinished")
        }
        
        didSet {
            didChangeValue(forKey: "isFinished")
        }
    }
    
    override var isFinished: Bool {
        return _finished
    }
    
    override func start() {
        _ready = true
        super.start()
        _executing = true
        execute()
    }

    func execute() {
        _ready = false
        downloadImage(url: URL(string:"http://i.imgur.com/5ac1apZ.jpg")!, handler: { image in
            OperationQueue.main.addOperation({
                self.downloadImage.image = image
            })
        })
    }
    
    func finish() {
        _executing = false
        _finished = true
        _ready = true
    }
    
    init(downloadImage: DownloadImage) {
        _isReady = true
        _isExecuting = false
        _isFinished = false
        _isCancelled = false
        self.downloadImage = downloadImage
        super.init() 
    }
    
    override func main() {
        downloadImage(url: URL(string:"http://i.imgur.com/5ac1apZ.jpg")!, handler: { image in
            OperationQueue.main.addOperation({
                self.downloadImage.image = image
            })
        })
    }
    
    func getDataFromUrl(url: URL, completion: @escaping (_ data: Data?, _  response: URLResponse?, _ error: Error?) -> Void) {
        let urlRequest = URLRequest(url:url)
        session.dataTask(with: urlRequest, completionHandler: { data, response, error in
            completion(data, response, error)
        }).resume()
    }
    
    func downloadImage(url: URL, handler: @escaping (UIImage) -> Void) {
        print("Download Started")
        getDataFromUrl(url: url) { (data, response, error)  in
            guard let data = data, error == nil else { return }
            OperationQueue.main.addOperation({
                handler(UIImage(data:data)!)
            })
        }
    }
}