Class as a delegate placeholder for a struct - swift

Question

I have a simple struct where I use NSURLSession to get the data from the web. I would like to extend it to provide progress of downloading. I cannot use struct as a NSURLSession delegate because it needs to be a NSObject so I have created simple class where I pass to NSURLSession delegate. This is short version of my code:
struct Resource {
private var progressDelegate: ProgressSessionDelegate?
private var session: NSURLSession!
init() {
self.progressDelegate = ProgressSessionDelegate()
let configuration = NSURLSessionConfiguration.defaultSessionConfiguration()
session = NSURLSession(configuration: configuration, delegate: self.progressDelegate!, delegateQueue: nil)
}
func loadAsynchronous(callback: A? -> ()) {
session.dataTaskWithURL(resourceURL) {
data, response, error in
let json = data.flatMap {
try? NSJSONSerialization.JSONObjectWithData($0, options: NSJSONReadingOptions())
}
callback(mycalback)
}.resume()
}
}
and thats my cass which should handle the delegate methods:
class ProgressSessionDelegate: NSObject, NSURLSessionDelegate, NSURLSessionDataDelegate {
var expectedContentLength = 0
var currentLength = 0
override init() {
super.init()
}
func URLSession(session: NSURLSession, dataTask: NSURLSessionDataTask, didReceiveResponse response: NSURLResponse, completionHandler: (NSURLSessionResponseDisposition) -> Void) {
expectedContentLength = Int(response.expectedContentLength)
print("0: \(expectedContentLength)")
completionHandler(NSURLSessionResponseDisposition.Allow)
}
func URLSession(session: NSURLSession, dataTask: NSURLSessionDataTask, didReceiveData data: NSData) {
self.currentLength += data.length
let percentageDownloaded = Float(self.currentLength) / Float(self.expectedContentLength)
print("1: \(percentageDownloaded)")
}
func URLSession(session: NSURLSession, task: NSURLSessionTask, didCompleteWithError error: NSError?) {
print("Downloaded")
}
}
The problem is that the delegate methods are not called.
When I move the code from Resource and ProgressSessionDelegate struct to my ViewController and set up ViewController as a NSURLSession delegate all is working fine. I think the problem is in a way how the class is stored is struct.
Any help would be most appreciated.