class RedirectDestination

class RedirectDestination implements RedirectDestinationInterface {
  
  /**
   * The request stack.
   *
   * @var \Symfony\Component\HttpFoundation\RequestStack
   */
  protected $requestStack;
  
  /**
   * The URL generator.
   *
   * @var \Drupal\Core\Routing\UrlGeneratorInterface
   */
  protected $urlGenerator;
  
  /**
   * The destination used by the current request.
   *
   * @var string
   */
  protected $destination;
  
  /**
   * Constructs a new RedirectDestination instance.
   *
   * @param \Symfony\Component\HttpFoundation\RequestStack $request_stack
   *   The request stack.
   * @param \Drupal\Core\Routing\UrlGeneratorInterface $url_generator
   *   The URL generator.
   */
  public function __construct(RequestStack $request_stack, UrlGeneratorInterface $url_generator) {
    $this->requestStack = $request_stack;
    $this->urlGenerator = $url_generator;
  }
  
  /**
   * {@inheritdoc}
   */
  public function getAsArray() {
    return [
      'destination' => $this->get(),
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  public function get() {
    if (!isset($this->destination)) {
      $query = $this->requestStack
        ->getCurrentRequest()->query;
      if ($query->has('destination')) {
        $this->destination = $query->get('destination');
        if (UrlHelper::isExternal($this->destination)) {
          // See https://www.drupal.org/node/2454955 for external redirects.
          $this->destination = '/';
        }
      }
      else {
        $this->destination = $this->urlGenerator
          ->generateFromRoute('<current>', [], [
          'query' => UrlHelper::filterQueryParameters($query->all()),
        ]);
      }
    }
    return $this->destination;
  }
  
  /**
   * {@inheritdoc}
   */
  public function set($new_destination) {
    $this->destination = $new_destination;
    return $this;
  }

}