class ViewsForm

class ViewsForm implements FormInterface, ContainerInjectionInterface {
  use DependencySerializationTrait;

  /**
   * The class resolver to get the subform form objects.
   *
   * @var \Drupal\Core\DependencyInjection\ClassResolverInterface
   */
  protected $classResolver;

  /**
   * The request stack.
   *
   * @var \Symfony\Component\HttpFoundation\RequestStack
   */
  protected $requestStack;

  /**
   * The URL generator to generate the form action.
   *
   * @var \Drupal\Core\Routing\UrlGeneratorInterface
   */
  protected $urlGenerator;

  /**
   * The ID of the view.
   *
   * @var string
   */
  protected $viewId;

  /**
   * The ID of the active view's display.
   *
   * @var string
   */
  protected $viewDisplayId;

  /**
   * The arguments passed to the active view.
   *
   * @var string[]
   */
  protected $viewArguments;

  /**
   * Constructs a ViewsForm object.
   *
   * @param \Drupal\Core\DependencyInjection\ClassResolverInterface $class_resolver
   *   The class resolver to get the subform form objects.
   * @param \Drupal\Core\Routing\UrlGeneratorInterface $url_generator
   *   The URL generator to generate the form action.
   * @param \Symfony\Component\HttpFoundation\RequestStack $requestStack
   *   The request stack.
   * @param string $view_id
   *   The ID of the view.
   * @param string $view_display_id
   *   The ID of the active view's display.
   * @param string[] $view_args
   *   The arguments passed to the active view.
   */
  public function __construct(ClassResolverInterface $class_resolver, UrlGeneratorInterface $url_generator, RequestStack $requestStack, $view_id, $view_display_id, array $view_args) {
    $this->classResolver = $class_resolver;
    $this->urlGenerator = $url_generator;
    $this->requestStack = $requestStack;
    $this->viewId = $view_id;
    $this->viewDisplayId = $view_display_id;
    $this->viewArguments = $view_args;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, $view_id = NULL, $view_display_id = NULL, array $view_args = NULL) {
    return new static($container
      ->get('class_resolver'), $container
      ->get('url_generator'), $container
      ->get('request_stack'), $view_id, $view_display_id, $view_args);
  }

  /**
   * Returns a string for the form's base ID.
   *
   * @return string
   *   The string identifying the form's base ID.
   */
  public function getBaseFormId() {
    $parts = [
      'views_form',
      $this->viewId,
      $this->viewDisplayId,
    ];
    return implode('_', $parts);
  }

  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    $parts = [
      $this
        ->getBaseFormId(),
    ];
    if (!empty($this->viewArguments)) {

      // Append the passed arguments to ensure form uniqueness.
      $parts = array_merge($parts, $this->viewArguments);
    }
    return implode('_', $parts);
  }

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state, ViewExecutable $view = NULL, $output = []) {
    if (!($step = $form_state
      ->get('step'))) {
      $step = 'views_form_views_form';
      $form_state
        ->set('step', $step);
    }
    $form_state
      ->set([
      'step_controller',
      'views_form_views_form',
    ], 'Drupal\\views\\Form\\ViewsFormMainForm');

    // Views forms without view arguments return the same Base Form ID and
    // Form ID. Base form ID should only be added when different.
    if ($this
      ->getBaseFormId() !== $this
      ->getFormId()) {
      $form_state
        ->addBuildInfo('base_form_id', $this
        ->getBaseFormId());
    }
    $form = [];
    $query = $this->requestStack
      ->getCurrentRequest()->query
      ->all();
    $query = UrlHelper::filterQueryParameters($query, [
      '_wrapper_format',
    ], '');
    $options = [
      'query' => $query,
    ];
    $form['#action'] = $view
      ->hasUrl() ? $view
      ->getUrl()
      ->setOptions($options)
      ->toString() : Url::fromRoute('<current>')
      ->setOptions($options)
      ->toString();

    // Tell the preprocessor whether it should hide the header, footer, pager,
    // etc.
    $form['show_view_elements'] = [
      '#type' => 'value',
      '#value' => $step == 'views_form_views_form' ? TRUE : FALSE,
    ];
    $form_object = $this
      ->getFormObject($form_state);
    $form += $form_object
      ->buildForm($form, $form_state, $view, $output);
    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {
    $form_object = $this
      ->getFormObject($form_state);
    $form_object
      ->validateForm($form, $form_state);
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    $form_object = $this
      ->getFormObject($form_state);
    $form_object
      ->submitForm($form, $form_state);
  }

  /**
   * Returns the object used to build the step form.
   *
   * @param \Drupal\Core\Form\FormStateInterface $form_state
   *   The form_state of the current form.
   *
   * @return \Drupal\Core\Form\FormInterface
   *   The form object to use.
   */
  protected function getFormObject(FormStateInterface $form_state) {

    // If this is a class, instantiate it.
    $form_step_class = $form_state
      ->get([
      'step_controller',
      $form_state
        ->get('step'),
    ]) ?: 'Drupal\\views\\Form\\ViewsFormMainForm';
    return $this->classResolver
      ->getInstanceFromDefinition($form_step_class);
  }

}