How to resume download in swift?

Question

I'm a freshman in Swift and working on an app which would resume download after being closed. I found some similar questions from Stack overflow but they are not very helpful to my work.

let dataTask = self.session.downloadTaskWithURL(self.bigFile!)
dataTask.resume()

I'm trying to resume download by this way, but it didn't work. I also want to ask how to update the progressView after the download is stopped and resumed

func URLSession(session: NSURLSession, task: NSURLSessionTask,
    didCompleteWithError sessionError: NSError?) {
    if let error = sessionError {
        print("error : \(error)")

        if let resumedData = error.userInfo[NSURLSessionDownloadTaskResumeData] 
    as? NSData {
            let resumeData = session.downloadTaskWithResumeData(resumedData)
            resumeData.resume()
        }
    }
}
func URLSession(session: NSURLSession, downloadTask: NSURLSessionDownloadTask, didWriteData bytesWritten: Int64, totalBytesWritten: Int64, totalBytesExpectedToWrite: Int64) {
    let progress = Float(totalBytesWritten)/Float(totalBytesExpectedToWrite)
    self.progressView.progress = progress
}

And do I need to generate a local path instead of the temporary one for storing the downloaded data (I don't need to publish the app, just show to my tutor.) ? Thanks in advance