Show loading percentage of image download from url with swift

Question

So I have an image downloading from a url like so

        let request = NSMutableURLRequest(URL: NSURL(string: "\(self.ip)")!)
        request.HTTPMethod = "POST"
        let postString = "userID=\(userID)"
        request.HTTPBody = postString.dataUsingEncoding(NSUTF8StringEncoding)

        let configuration = NSURLSessionConfiguration.defaultSessionConfiguration()
        let session = NSURLSession(configuration: configuration, delegate: self, delegateQueue: NSOperationQueue.mainQueue())

        let task = session.dataTaskWithRequest(request) {
            data, response, error in
                   .........
        }

and my delegate functions look like

func URLSession(session: NSURLSession, task: NSURLSessionTask, didSendBodyData bytesSent: Int64, totalBytesSent: Int64, totalBytesExpectedToSend: Int64) {

    let uploadProgress:Float = Float(totalBytesSent) / Float(totalBytesExpectedToSend)
    let progressPercent = Int(uploadProgress*100)
    print(progressPercent)
}

func URLSession(session: NSURLSession, dataTask: NSURLSessionDataTask, didBecomeDownloadTask downloadTask: NSURLSessionDownloadTask) {
    let downloadProgress:Float = Float(downloadTask.countOfBytesReceived) / Float(downloadTask.countOfBytesExpectedToReceive)
    print(downloadProgress)
}

The upload progress works just fine for a different function, but when downloading an image, the second URLSession function does not get called. What am I doing wrong?

Answer 1

URLSession has 5 types of delegate: URLSessionDelegate, URLSessionTaskDelegate, URLSessionDataDelegate, URLSessionDownloadDelegate and URLSessionStreamDelegate. You can read about them in the documentation.

The ones you need to care about are URLSessionDelegate and URLSessionDownloadDelegate:

class ViewController: UIViewController, URLSessionDelegate, URLSessionDownloadDelegate {

    @IBOutlet weak var imageView: UIImageView!
    @IBOutlet weak var progressView: UIProgressView!

    private let byteFormatter: ByteCountFormatter = {
        let formatter = ByteCountFormatter()
        formatter.allowedUnits = [.useKB, .useMB]
        return formatter
    }()

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

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

    @IBAction func downloadImage(sender : AnyObject) {
        // A 10MB image from NASA
        let url = URL(string: "https://photojournal.jpl.nasa.gov/jpeg/PIA08506.jpg")!

        let config = URLSessionConfiguration.default
        let session = URLSession(configuration: config, delegate: self, delegateQueue: nil)

        // Don't specify a completion handler here or the delegate won't be called
        session.downloadTask(with: url).resume()
    }

    func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didWriteData bytesWritten: Int64, totalBytesWritten: Int64, totalBytesExpectedToWrite: Int64) {
        let written = byteFormatter.string(fromByteCount: totalBytesWritten)
        let expected = byteFormatter.string(fromByteCount: totalBytesExpectedToWrite)
        print("Downloaded \(written) / \(expected)")

        DispatchQueue.main.async {
            self.progressView.progress = Float(totalBytesWritten) / Float(totalBytesExpectedToWrite)
        }
    }

    func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) {
        // The location is only temporary. You need to read it or copy it to your container before
        // exiting this function. UIImage(contentsOfFile: ) seems to load the image lazily. NSData
        // does it right away.
        if let data = try? Data(contentsOf: location), let image = UIImage(data: data) {
            DispatchQueue.main.async {
                self.imageView.contentMode = .scaleAspectFit
                self.imageView.clipsToBounds = true
                self.imageView.image = image
            }
        } else {
            fatalError("Cannot load the image")
        }

    }
}