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('node.add', [
                '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);
    }

}