completionHandler Alamofire

// http://stackoverflow.com/q/32371792/5552518

func authenticateUser(completionHandler: (responseObject: String?, error: NSError?) -> ()) {
        makeAuthenticateUserCall(completionHandler)
    }

    func makeAuthenticateUserCall(completionHandler: (responseObject: String?, error: NSError?) -> ()) {
        Alamofire.request(.GET, loginUrlString)
            .authenticate(user: "a", password: "b")
            .responseString { request, response, responseString, responseError in
                completionHandler(responseObject: responseString as String!, error: responseError)
        }
    }