iOS-Swift How to update progressView - ios

Question

I am developing an app where i am downloading data from web.
here i am using cancel button to cancel the download progress,this works fine if i am in the same view controller and if i navigate to new view controller and return back to download view controller progress view is not updating but the download is happening.
This is the code i am using to download data
func URLSession(session: NSURLSession,
downloadTask: NSURLSessionDownloadTask,
didFinishDownloadingToURL location: NSURL){
let path = NSSearchPathForDirectoriesInDomains(NSSearchPathDirectory.DocumentDirectory, NSSearchPathDomainMask.UserDomainMask, true)
let documentDirectoryPath:String = path[0]
let fileManager = NSFileManager()
let destinationURLForFile = NSURL(fileURLWithPath: documentDirectoryPath.stringByAppendingString( Variables.name ))
if fileManager.fileExistsAtPath(destinationURLForFile.path!){
print("File Already Exists")
} else {
do {
try fileManager.moveItemAtURL(location, toURL: destinationURLForFile)
print("File doesn't exists")
}catch{
print("An error occurred while moving file to destination url")
}
}
}
// 2
func URLSession(session: NSURLSession,
downloadTask: NSURLSessionDownloadTask,
didWriteData bytesWritten: Int64,
totalBytesWritten writ: Int64,
totalBytesExpectedToWrite exp: Int64){
//progressView.setProgress(Float(totalBytesWritten)/Float(totalBytesExpectedToWrite), animated: true)
print("downloaded \(100*writ/exp)")
dispatch_async(dispatch_get_main_queue(), {
self.counter = Float(100*writ/exp)
return
})
}
func URLSession(session: NSURLSession,
task: NSURLSessionTask,
didCompleteWithError error: NSError?){
downloadTask = nil
// progressView.setProgress(0.0, animated: true)
if (error != nil) {
print(error?.description)
} else {
print("The task finished transferring data successfully")
}
}
func documentInteractionControllerViewControllerForPreview(controller: UIDocumentInteractionController) -> UIViewController{
return self
}