get progress from dataTaskWithURL in swift

Question

Is there any way to get progress from dataTaskWithURL in swift while the data is downloading?

NSURLSession.sharedSession().dataTaskWithURL(...)         


        

Answer 1

Update for Swift4:
Supports execution of multiple simultaneous operations.

File: DownloadService.swift. Keeps reference to URLSession and tracks executing tasks.

final class DownloadService: NSObject {

   private var session: URLSession!
   private var downloadTasks = [GenericDownloadTask]()

   public static let shared = DownloadService()

   private override init() {
      super.init()
      let configuration = URLSessionConfiguration.default
      session = URLSession(configuration: configuration,
                           delegate: self, delegateQueue: nil)
   }

   func download(request: URLRequest) -> DownloadTask {
      let task = session.dataTask(with: request)
      let downloadTask = GenericDownloadTask(task: task)
      downloadTasks.append(downloadTask)
      return downloadTask
   }
}


extension DownloadService: URLSessionDataDelegate {

   func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive response: URLResponse,
                   completionHandler: @escaping (URLSession.ResponseDisposition) -> Void) {

      guard let task = downloadTasks.first(where: { $0.task == dataTask }) else {
         completionHandler(.cancel)
         return
      }
      task.expectedContentLength = response.expectedContentLength
      completionHandler(.allow)
   }

   func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive data: Data) {
      guard let task = downloadTasks.first(where: { $0.task == dataTask }) else {
         return
      }
      task.buffer.append(data)
      let percentageDownloaded = Double(task.buffer.count) / Double(task.expectedContentLength)
      DispatchQueue.main.async {
         task.progressHandler?(percentageDownloaded)
      }
   }

   func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) {
      guard let index = downloadTasks.index(where: { $0.task == task }) else {
         return
      }
      let task = downloadTasks.remove(at: index)
      DispatchQueue.main.async {
         if let e = error {
            task.completionHandler?(.failure(e))
         } else {
            task.completionHandler?(.success(task.buffer))
         }
      }
   }
}

File: DownloadTask.swift. Lightweight interface just to hide concrete implementation.

protocol DownloadTask {

   var completionHandler: ResultType.Completion? { get set }
   var progressHandler: ((Double) -> Void)? { get set }

   func resume()
   func suspend()
   func cancel()
}

File: GenericDownloadTask.swift. Concrete implementation of DownloadTask interface.

class GenericDownloadTask {

   var completionHandler: ResultType.Completion?
   var progressHandler: ((Double) -> Void)?

   private(set) var task: URLSessionDataTask
   var expectedContentLength: Int64 = 0
   var buffer = Data()

   init(task: URLSessionDataTask) {
      self.task = task
   }

   deinit {
      print("Deinit: \(task.originalRequest?.url?.absoluteString ?? "")")
   }

}

extension GenericDownloadTask: DownloadTask {

   func resume() {
      task.resume()
   }

   func suspend() {
      task.suspend()
   }

   func cancel() {
      task.cancel()
   }
}

File: ResultType.swift. Reusable type to keep result or error.

public enum ResultType {

   public typealias Completion = (ResultType) -> Void

   case success(T)
   case failure(Swift.Error)

}

Usage: Example how to run two download tasks in parallel (macOS App):

class ViewController: NSViewController {

   @IBOutlet fileprivate weak var loadImageButton1: NSButton!
   @IBOutlet fileprivate weak var loadProgressIndicator1: NSProgressIndicator!
   @IBOutlet fileprivate weak var imageView1: NSImageView!

   @IBOutlet fileprivate weak var loadImageButton2: NSButton!
   @IBOutlet fileprivate weak var loadProgressIndicator2: NSProgressIndicator!
   @IBOutlet fileprivate weak var imageView2: NSImageView!

   fileprivate var downloadTask1:  DownloadTask?
   fileprivate var downloadTask2:  DownloadTask?

   override func viewDidLoad() {
      super.viewDidLoad()
      loadImageButton1.target = self
      loadImageButton1.action = #selector(startDownload1(_:))
      loadImageButton2.target = self
      loadImageButton2.action = #selector(startDownload2(_:))
   }

}


extension ViewController {

   @objc fileprivate func startDownload1(_ button: NSButton) {
      let url = URL(string: "http://localhost:8001/?imageID=01&tilestamp=\(Date.timeIntervalSinceReferenceDate)")!
      let request = URLRequest(url: url, cachePolicy: .reloadIgnoringLocalCacheData, timeoutInterval: 30)
      downloadTask1 = DownloadService.shared.download(request: request)
      downloadTask1?.completionHandler = { [weak self] in
         switch $0 {
         case .failure(let error):
            print(error)
         case .success(let data):
            print("Number of bytes: \(data.count)")
            self?.imageView1.image = NSImage(data: data)
         }
         self?.downloadTask1 = nil
         self?.loadImageButton1.isEnabled = true
      }
      downloadTask1?.progressHandler = { [weak self] in
         print("Task1: \($0)")
         self?.loadProgressIndicator1.doubleValue = $0
      }

      loadImageButton1.isEnabled = false
      imageView1.image = nil
      loadProgressIndicator1.doubleValue = 0
      downloadTask1?.resume()
   }

   @objc fileprivate func startDownload2(_ button: NSButton) {
      let url = URL(string: "http://localhost:8002/?imageID=02&tilestamp=\(Date.timeIntervalSinceReferenceDate)")!
      let request = URLRequest(url: url, cachePolicy: .reloadIgnoringLocalCacheData, timeoutInterval: 30)
      downloadTask2 = DownloadService.shared.download(request: request)
      downloadTask2?.completionHandler = { [weak self] in
         switch $0 {
         case .failure(let error):
            print(error)
         case .success(let data):
            print("Number of bytes: \(data.count)")
            self?.imageView2.image = NSImage(data: data)
         }
         self?.downloadTask2 = nil
         self?.loadImageButton2.isEnabled = true
      }
      downloadTask2?.progressHandler = { [weak self] in
         print("Task2: \($0)")
         self?.loadProgressIndicator2.doubleValue = $0
      }

      loadImageButton2.isEnabled = false
      imageView2.image = nil
      loadProgressIndicator2.doubleValue = 0
      downloadTask2?.resume()
   }
}

---

Bonus 1. File StartPHPWebServer.command. Example script to run 2 Build-in PHP servers to simulate simultaneous downloads.

#!/bin/bash

AWLScriptDirPath=$(cd "$(dirname "$0")"; pwd)
cd "$AWLScriptDirPath"

php -S localhost:8001 &
php -S localhost:8002 &

ps -afx | grep php
echo "Press ENTER to exit."
read
killall php

Bonus 2. File index.php. Example PHP script to implement slow download.

 20,5 kb/s)
if (file_exists($local_file) && is_file($local_file)) {
    header('Cache-control: private');
    header('Content-Type: image/jpeg');
    header('Content-Length: '.filesize($local_file));

    flush();
    $file = fopen($local_file, "r");
    while(!feof($file)) {
        // send the current file part to the browser
        print fread($file, round($download_rate * 1024));
        flush(); // flush the content to the browser
        usleep(0.25 * 1000000);
    }
    fclose($file);}
else {
    die('Error: The file '.$local_file.' does not exist!');
}

?>

Misc: Contents of directory which simulates 2 PHP servers.

Image-01.jpg
Image-02.jpg
StartPHPWebServer.command
index.php