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;
    }

}