class HtmlEntityFormController

class HtmlEntityFormController extends FormController {

  /**
   * The entity type manager service.
   *
   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
   */
  protected $entityTypeManager;

  /**
   * Constructs a new \Drupal\Core\Routing\Enhancer\FormEnhancer object.
   *
   * @param \Symfony\Component\HttpKernel\Controller\ArgumentResolverInterface $argument_resolver
   *   The argument resolver.
   * @param \Drupal\Core\Form\FormBuilderInterface $form_builder
   *   The form builder.
   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
   *   The entity type manager service.
   */
  public function __construct(ArgumentResolverInterface $argument_resolver, FormBuilderInterface $form_builder, EntityTypeManagerInterface $entity_type_manager) {
    parent::__construct($argument_resolver, $form_builder);
    $this->entityTypeManager = $entity_type_manager;
  }

  /**
   * {@inheritdoc}
   */
  protected function getFormArgument(RouteMatchInterface $route_match) {
    return $route_match
      ->getRouteObject()
      ->getDefault('_entity_form');
  }

  /**
   * {@inheritdoc}
   *
   * Instead of a class name or service ID, $form_arg will be a string
   * representing the entity and operation being performed.
   * Consider the following route:
   * @code
   *   path: '/foo/{node}/bar'
   *   defaults:
   *     _entity_form: 'node.edit'
   * @endcode
   * This means that the edit form for the node entity will used.
   * If the entity type has a default form, only the name of the
   * entity {param} needs to be passed:
   * @code
   *   path: '/foo/{node}/baz'
   *   defaults:
   *     _entity_form: 'node'
   * @endcode
   */
  protected function getFormObject(RouteMatchInterface $route_match, $form_arg) {

    // If no operation is provided, use 'default'.
    $form_arg .= '.default';
    [
      $entity_type_id,
      $operation,
    ] = explode('.', $form_arg);
    $form_object = $this->entityTypeManager
      ->getFormObject($entity_type_id, $operation);

    // Allow the entity form to determine the entity object from a given route
    // match.
    $entity = $form_object
      ->getEntityFromRouteMatch($route_match, $entity_type_id);
    $form_object
      ->setEntity($entity);
    return $form_object;
  }

}