Swift cancellation of NSOperationQueue does not work - ios

Question

I'm trying to cancel NSOperationQueue process using method cancelAllOperations() using swift but it does not work.
I am downloading a file from url when i cancel the download it doesn't work.
here is the code i am using:
var s = NSOperationQueue.mainQueue()
lazy var session : NSURLSession = {
let config = NSURLSessionConfiguration.ephemeralSessionConfiguration()
config.allowsCellularAccess = false
let session = NSURLSession(configuration: config, delegate: self, delegateQueue: self.s)
return session
}
override func viewDidLoad() {
progressBar.setProgress(0.0, animated: true) //set progressBar to 0 at start
}
#IBAction func cancel(sender: AnyObject) {
s.cancelAllOperations()
}
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)")
if(error != nil){
}
}
// this is the only required NSURLSessionDownloadDelegate method
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("Music.mp3"))
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")
}
}
}