Downloading multiple files as one batch in swift 2

Question

I am trying to download multiple images and what happens is that it downloads one by one, Is there a way to download multiple files as one? What I want to happen is that, instead of the progress view tracking each file's progress, I want it to track all of the files as one.

import UIKit

class ViewController: UIViewController, NSURLSessionDownloadDelegate {


@IBOutlet weak var progressBar: UIProgressView!
@IBOutlet weak var progressCount: UILabel!

var task : NSURLSessionTask!

var pics = ["http://pillar.foundationu.com/wp-content/uploads/2016/03/banner.jpg", "http://pillar.foundationu.com/wp-content/uploads/2016/03/Abad-Edmerl.jpg", "http://pillar.foundationu.com/wp-content/uploads/2016/03/Abellana-Deniz-Dawn-1.jpg", "http://pillar.foundationu.com/wp-content/uploads/2016/03/Abequibel-Arneth.jpg", "http://pillar.foundationu.com/wp-content/uploads/2016/03/Abilla-Harriette.jpg"]

var counter:Float = 0.0 {

    didSet {
        let fractionalProgress = Float(counter) / 100.0
        let animated = counter != 0

        progressBar.setProgress(fractionalProgress, animated: animated)
        progressCount.text = ("\(counter)%")
    }
    //The didSet is called immediately after the new value is stored. The fractionalProgress constant keeps track of the progress.
}

lazy var session : NSURLSession = {
    let config = NSURLSessionConfiguration.ephemeralSessionConfiguration()
    config.allowsCellularAccess = false
    let session = NSURLSession(configuration: config, delegate: self, delegateQueue: NSOperationQueue.mainQueue())
    return session
    }()

override func viewDidLoad() {
    progressBar.setProgress(0.0, animated: true)  //set progressBar to 0 at start
}

@IBAction func doElaborateHTTP (sender:AnyObject!) {

    progressCount.text = "0%"
    if self.task != nil {
        return
    }
   dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), {

        let taskID = self.beginBackgroundUpdateTask()

        var url = NSURL!()
        var req = NSMutableURLRequest()

        for s in self.pics {
            url = NSURL(string: s)
            req = NSMutableURLRequest(URL:url)
            let task = self.session.downloadTaskWithRequest(req)
            self.task = task
            task.resume()
        }

        // Do something with the result

   self.endBackgroundUpdateTask(taskID)

    })

}

func URLSession(session: NSURLSession, downloadTask: NSURLSessionDownloadTask, didWriteData bytesWritten: Int64, totalBytesWritten writ: Int64, totalBytesExpectedToWrite exp: Int64) {
    print("downloaded \(100*writ/exp)")

    dispatch_async(dispatch_get_main_queue(), {
        self.counter = Float(100*writ/exp)
        return
    })
}

func URLSession(session: NSURLSession, downloadTask: NSURLSessionDownloadTask, didResumeAtOffset fileOffset: Int64, expectedTotalBytes: Int64) {
    // unused in this example
}

func URLSession(session: NSURLSession, task: NSURLSessionTask, didCompleteWithError error: NSError?) {
    print("completed: error: \(error)")
}

// this is the only required NSURLSessionDownloadDelegate method

func URLSession(session: NSURLSession, downloadTask: NSURLSessionDownloadTask, didFinishDownloadingToURL location: NSURL) {

    print(downloadTask.response!.suggestedFilename!)

  }
}

Answer 1

You keep converting back and forth between the fractional amount (for example: 0.15) and the percentage (15). Don't do that. Use the percentage for display. Keep all your calculations as fractional numbers.

Try this:

var taskProgress = [NSURL: (written: Int64, expected: Int64)]()    

func URLSession(session: NSURLSession, downloadTask: NSURLSessionDownloadTask, didWriteData bytesWritten: Int64, totalBytesWritten writ: Int64, totalBytesExpectedToWrite exp: Int64) {
    dispatch_async(dispatch_get_main_queue()) {
        self.taskProgress[downloadTask.originalRequest!.URL!] = (writ, exp)

        // Update your views, you may separate it into a different function
        let totalWritten = self.taskProgress.reduce(0) { $0 + $1.1.written }
        let totalExpected = self.taskProgress.reduce(0) { $0 + $1.1.expected }
        let progress = Float(totalWritten) / Float(totalExpected)

        self.progressBar.setProgress(progress, animated: true)
        self.progressCount.text = "\(progress * 100)%"
    }
}