get progress from dataTaskWithURL in swift

You can simply observe progress property of the URLSessionDataTask object.

Example:

import UIKit

class SomeViewController: UIViewController {

  private var observation: NSKeyValueObservation?

  deinit {
    observation?.invalidate()
  }

  override func viewDidLoad() {
    super.viewDidLoad()

    let url = URL(string: "https://source.unsplash.com/random/4000x4000")!
    let task = URLSession.shared.dataTask(with: url)

    observation = task.progress.observe(\.fractionCompleted) { progress, _ in
      print("progress: ", progress.fractionCompleted)
    }

    task.resume()
  }
}

Playground example:

import Foundation
import PlaygroundSupport

let page = PlaygroundPage.current
page.needsIndefiniteExecution = true

let url = URL(string: "https://source.unsplash.com/random/4000x4000")!
let task = URLSession.shared.dataTask(with: url) { _, _, _ in
  page.finishExecution()
}

// Don't forget to invalidate the observation when you don't need it anymore.
let observation = task.progress.observe(\.fractionCompleted) { progress, _ in
  print(progress.fractionCompleted)
}

task.resume()

you can use this code for showing download process with progress bar with its delegate functions.

import UIKit

class ViewController: UIViewController,NSURLSessionDelegate,NSURLSessionDataDelegate{

    @IBOutlet weak var progress: UIProgressView!

    var buffer:NSMutableData = NSMutableData()
    var session:NSURLSession?
    var dataTask:NSURLSessionDataTask?
    let url = NSURL(string:"http://i.stack.imgur.com/b8zkg.png" )!
    var expectedContentLength = 0


    override func viewDidLoad() {
        super.viewDidLoad()
        progress.progress = 0.0
        let configuration = NSURLSessionConfiguration.defaultSessionConfiguration()
        let manqueue = NSOperationQueue.mainQueue()
        session = NSURLSession(configuration: configuration, delegate:self, delegateQueue: manqueue)
        dataTask = session?.dataTaskWithRequest(NSURLRequest(URL: url))
        dataTask?.resume()

        // Do any additional setup after loading the view, typically from a nib.
    }
    func URLSession(session: NSURLSession, dataTask: NSURLSessionDataTask, didReceiveResponse response: NSURLResponse, completionHandler: (NSURLSessionResponseDisposition) -> Void) {

        //here you can get full lenth of your content
        expectedContentLength = Int(response.expectedContentLength)
        println(expectedContentLength)
        completionHandler(NSURLSessionResponseDisposition.Allow)
    }
    func URLSession(session: NSURLSession, dataTask: NSURLSessionDataTask, didReceiveData data: NSData) {


        buffer.appendData(data)

        let percentageDownloaded = Float(buffer.length) / Float(expectedContentLength)
        progress.progress =  percentageDownloaded
    }
    func URLSession(session: NSURLSession, task: NSURLSessionTask, didCompleteWithError error: NSError?) {
        //use buffer here.Download is done
        progress.progress = 1.0   // download 100% complete
    }
}