PreAuth

    func preAuthOperation() {
        guard let ref = Reference(consumerRef: "payment reference") else { return }
        let amount: Amount = "2 GBP"
        try! self.judoKitSession.invokePreAuth(judoId, amount: amount, reference: ref, completion: { (response, error) -> () in
            self.dismissViewControllerAnimated(true, completion: nil)
            if let error = error {
                if error.code == .UserDidCancel {
                    self.dismissViewControllerAnimated(true, completion: nil)
                    return
                }
                var errorTitle = "Error"
                if let errorCategory = error.category {
                    errorTitle = errorCategory.stringValue()
                }
                self.alertController = UIAlertController(title: errorTitle, message: error.message, preferredStyle: .Alert)
                self.alertController!.addAction(UIAlertAction(title: "OK", style: .Cancel, handler: nil))
                self.dismissViewControllerAnimated(true, completion:nil)
                return // BAIL
            }
            if let resp = response, transactionData = resp.items.first {
                self.cardDetails = transactionData.cardDetails
                self.paymentToken = transactionData.paymentToken()
            }
            let sb = UIStoryboard(name: "Main", bundle: nil)
            let viewController = sb.instantiateViewControllerWithIdentifier("detailviewcontroller") as! DetailViewController
            viewController.response = response
            self.navigationController?.pushViewController(viewController, animated: true)
            })
    }