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, [], '');
        $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);
    }

}