class EntityDisplayModeFormBase

abstract class EntityDisplayModeFormBase extends EntityForm {
    
    /**
     * The entity type definition.
     *
     * @var \Drupal\Core\Entity\EntityTypeInterface
     */
    protected $entityType;
    
    /**
     * {@inheritdoc}
     */
    protected function init(FormStateInterface $form_state) {
        parent::init($form_state);
        $this->entityType = $this->entityTypeManager
            ->getDefinition($this->entity
            ->getEntityTypeId());
    }
    
    /**
     * {@inheritdoc}
     */
    public function form(array $form, FormStateInterface $form_state) {
        $form['label'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Name'),
            '#maxlength' => 100,
            '#default_value' => $this->entity
                ->label(),
        ];
        $form['id'] = [
            '#type' => 'machine_name',
            '#description' => $this->t('A unique machine-readable name. Can only contain lowercase letters, numbers, and underscores.'),
            '#disabled' => !$this->entity
                ->isNew(),
            '#default_value' => $this->entity
                ->id(),
            '#field_prefix' => $this->entity
                ->isNew() ? $this->entity
                ->getTargetType() . '.' : '',
            '#machine_name' => [
                'exists' => [
                    $this,
                    'exists',
                ],
                'replace_pattern' => '[^a-z0-9_.]+',
            ],
        ];
        return $form;
    }
    
    /**
     * Determines if the display mode already exists.
     *
     * @param string|int $entity_id
     *   The entity ID.
     * @param array $element
     *   The form element.
     *
     * @return bool
     *   TRUE if the display mode exists, FALSE otherwise.
     */
    public function exists($entity_id, array $element) {
        // Do not allow to add internal 'default' view mode.
        if ($entity_id == 'default') {
            return TRUE;
        }
        return (bool) $this->entityTypeManager
            ->getStorage($this->entity
            ->getEntityTypeId())
            ->getQuery()
            ->condition('id', $element['#field_prefix'] . $entity_id)
            ->execute();
    }
    
    /**
     * {@inheritdoc}
     */
    public function save(array $form, FormStateInterface $form_state) {
        $this->messenger()
            ->addStatus($this->t('Saved the %label @entity-type.', [
            '%label' => $this->entity
                ->label(),
            '@entity-type' => $this->entityType
                ->getSingularLabel(),
        ]));
        $this->entity
            ->save();
        \Drupal::service('entity_field.manager')->clearCachedFieldDefinitions();
        $form_state->setRedirectUrl($this->entity
            ->toUrl('collection'));
    }

}