NSURLProtocol-example.swift

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        // by calling [NSURLProtocol registerClass:[MyURLProtocol class]]; in -application:didFinishLoadingWithOptions:, your protocol will have priority over any of the built-in protocols.
        URLProtocol.registerClass(ActivityURLProtocol.self)
        //URLProtocol.unregisterClass(ActivityURLProtocol.self)
        print(URLSession.shared.configuration.protocolClasses)
    }

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        let photoURL = URL(string: "https://jxccsa.bn1.livefilestore.com/y2m5XyfjXFW78kXvn3-McCXl5926JSzKH-n7hysppnCZxn_hnMPjwWA2dT11K_pInOZYJArvCzlh0_Aozw2ZK_4sBMspJx23DN1kNX8IfRW4_cBuasM9pEUvOTup6p9KCCm/AE1.jpg?psid=1")!

        URLSession.shared.dataTask(with: photoURL) { data, response, error in
            print(response)
        }.resume()
    }

}

class ActivityURLProtocol: URLProtocol {

    // References:
    //  - http://nshipster.com/nsurlprotocol/
    //  - https://www.raywenderlich.com/59982/nsurlprotocol-tutorial
    //  - https://developer.apple.com/reference/foundation/nsurlprotocol

    // NSURLProtocolClient protocol:
    //  - Each instance of a NSURLProtocol subclass has a client property, which is the object that is communicating with the URL Loading system.

    var connection: NSURLConnection?

    override class func canInit(with request: URLRequest) -> Bool {
        // If the method returns YES, then the loading system will rely on this NSURLProtocol subclass to handle the request, and ignore all other handlers. If none of the custom registered handlers can handle the request, then the URL Loading System will handle it by itself, using the system’s default behavior.

        // True means: it’s entirely your class’s responsibility to handle everything about that request.
        return false
    }

    override public func startLoading() {
        print("startLoading")
        UIApplication.shared.isNetworkActivityIndicatorVisible = true

        connection = NSURLConnection(request: request, delegate: self) //Depecrated!
    }

    override public func stopLoading() {
        print("stopLoading")
        UIApplication.shared.isNetworkActivityIndicatorVisible = false
    }

}