class NodePreviewForm

class NodePreviewForm extends FormBase {
  
  /**
   * The entity display repository.
   *
   * @var \Drupal\Core\Entity\EntityDisplayRepositoryInterface
   */
  protected $entityDisplayRepository;
  
  /**
   * The config factory.
   *
   * @var \Drupal\Core\Config\ConfigFactoryInterface
   */
  protected $configFactory;
  
  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    return new static($container->get('entity_display.repository'), $container->get('config.factory'));
  }
  
  /**
   * Constructs a new NodePreviewForm.
   *
   * @param \Drupal\Core\Entity\EntityDisplayRepositoryInterface $entity_display_repository
   *   The entity display repository.
   * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
   *   The configuration factory.
   */
  public function __construct(EntityDisplayRepositoryInterface $entity_display_repository, ConfigFactoryInterface $config_factory) {
    $this->entityDisplayRepository = $entity_display_repository;
    $this->configFactory = $config_factory;
  }
  
  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'node_preview_form_select';
  }
  
  /**
   * Form constructor.
   *
   * @param array $form
   *   An associative array containing the structure of the form.
   * @param \Drupal\Core\Form\FormStateInterface $form_state
   *   The current state of the form.
   * @param \Drupal\Core\Entity\EntityInterface $node
   *   The node being previews.
   *
   * @return array
   *   The form structure.
   */
  public function buildForm(array $form, FormStateInterface $form_state, ?EntityInterface $node = NULL) {
    $view_mode = $node->preview_view_mode;
    $query_options = [
      'query' => [
        'uuid' => $node->uuid(),
      ],
    ];
    $query = $this->getRequest()->query;
    if ($query->has('destination')) {
      $query_options['query']['destination'] = $query->get('destination');
    }
    $form['backlink'] = [
      '#type' => 'link',
      '#title' => $this->t('Back to content editing'),
      '#url' => $node->isNew() ? Url::fromRoute('entity.node.add_form', [
        'node_type' => $node->bundle(),
      ]) : $node->toUrl('edit-form'),
      '#options' => [
        'attributes' => [
          'class' => [
            'node-preview-backlink',
          ],
        ],
      ] + $query_options,
    ];
    // Always show full as an option, even if the display is not enabled.
    $view_mode_options = [
      'full' => $this->t('Full'),
    ] + $this->entityDisplayRepository
      ->getViewModeOptionsByBundle('node', $node->bundle());
    // Unset view modes that are not used in the front end.
    unset($view_mode_options['default']);
    unset($view_mode_options['rss']);
    unset($view_mode_options['search_index']);
    $form['uuid'] = [
      '#type' => 'value',
      '#value' => $node->uuid(),
    ];
    $form['view_mode'] = [
      '#type' => 'select',
      '#title' => $this->t('View mode'),
      '#options' => $view_mode_options,
      '#default_value' => $view_mode,
      '#attributes' => [
        'data-drupal-autosubmit' => TRUE,
      ],
    ];
    $form['submit'] = [
      '#type' => 'submit',
      '#value' => $this->t('Switch'),
      '#attributes' => [
        'class' => [
          'js-hide',
        ],
      ],
    ];
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    $route_parameters = [
      'node_preview' => $form_state->getValue('uuid'),
      'view_mode_id' => $form_state->getValue('view_mode'),
    ];
    $options = [];
    $query = $this->getRequest()->query;
    if ($query->has('destination')) {
      $options['query']['destination'] = $query->get('destination');
      $query->remove('destination');
    }
    $form_state->setRedirect('entity.node.preview', $route_parameters, $options);
  }

}