Find the Download Progress of a file in swift

You can simply observe progress property of the URLSessionDataTask object. And you don't need to calculate the progress as other answers suggest here. There is a fractionCompleted property on the Progress.

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()

Assuming you are downloading a file, there is a subclass of NSURLSessionTask for the just that, called NSURLSessionDownloadTask. Below is an excerpt from the NSURLSession documentation on a specific function:

Periodically informs the delegate about the download’s progress.

func URLSession(_ session: NSURLSession,
    downloadTask downloadTask: NSURLSessionDownloadTask,
    didWriteData bytesWritten: Int64,
    totalBytesWritten totalBytesWritten: Int64,
    totalBytesExpectedToWrite totalBytesExpectedToWrite: Int64
)

For example, you could output the progress to the console by doing:

println("\(totalBytesWritten) / \(totalBytesExpectedToWrite)")

The progress status can be calculated in

URLSession(_:downloadTask:didWriteData:totalBytesWritten:totalBytesExpectedToWrite:)

This is a one of three required methods of protocol NSURLSessionDownloadDelegate. In my case the code of the method looks like this:

func URLSession(session: NSURLSession, downloadTask: NSURLSessionDownloadTask, didWriteData bytesWritten: Int64, totalBytesWritten: Int64, totalBytesExpectedToWrite: Int64) {
    // println("download task did write data")

    let progress = Float(totalBytesWritten) / Float(totalBytesExpectedToWrite)

    dispatch_async(dispatch_get_main_queue()) {
        self.progressDownloadIndicator.progress = progress
    }
}

I've created a small project, which implements three different approaches:

• download synchronously
• download asynchronously
• download with progress

Check it out: http://goo.gl/veRkA7