Drupal Commerce 2.x Onsite payment gateway token register method

Onsite.php

<?php
// Drupal Commerce 2.x Onsite payment method

// ...

  /**
   * {@inheritdoc}
   */
  public function createPaymentMethod(PaymentMethodInterface $payment_method, array $payment_details) {
    $required_keys = [
      'response$type', 'response$paypageRegistrationId', 'expiration',
    ];
    foreach ($required_keys as $required_key) {
      if (empty($payment_details[$required_key])) {
        throw new \InvalidArgumentException(sprintf('$payment_details must contain the %s key.', $required_key));
      }
    }

    $expires = CreditCard::calculateExpirationTimestamp($payment_details['expiration']['month'], $payment_details['expiration']['year']);
    $payment_method->card_type = Helper::getCommerceCreditCardType($payment_details['response$type']);
    $payment_method->card_number = $payment_details['response$lastFour'];
    $payment_method->card_exp_month = $payment_details['expiration']['month'];
    $payment_method->card_exp_year = $payment_details['expiration']['year'];
    $payment_method->setRemoteId($payment_details['response$paypageRegistrationId']);
    $payment_method->setExpiresTime($expires);
    $payment_method->save();

    $this->registerToken($payment_method);
  }

  /**
   * Registers a token with Vantiv from the AJAX provided registration id.
   *
   * @param \Drupal\commerce_payment\Entity\PaymentMethodInterface $payment_method
   *    The payment method.
   */
  private function registerToken(PaymentMethodInterface $payment_method) {
    $hash_in = Helper::getLitleHashIn($this->configuration);
    /** @var ProfileInterface $billing_profile */
    $billing_profile = $payment_method->getBillingProfile();
    $request_data = [
      'id' => $payment_method->getOriginalId(),
      'customerId' => $billing_profile->getOwnerId(),
      'paypageRegistrationId' => $payment_method->getRemoteId()
    ];

    $request = new LitleOnlineRequest();
    $response = $request->registerTokenRequest($hash_in + $request_data);
    $response_array = Helper::getResponseArray($response, 'registerTokenResponse');
    if (!Helper::isResponseSuccess($response_array['response'])) {
      throw new SoftDeclineException('An error occurred registering a token for your payment method.');
    }

    $payment_method->setRemoteId($response_array['litleToken']);
    $payment_method->save();
  }

// ...