Refactor the event controller.

namespace Drupal\fullcalendar_view\Controller;

use Drupal\Component\Render\PlainTextOutput;

use Drupal\Core\Access\CsrfTokenGenerator;

use Drupal\Core\Controller\ControllerBase;

use Drupal\Core\Entity\FieldableEntityInterface;

use Drupal\Core\Logger\LoggerChannelFactoryInterface;

use Symfony\Component\DependencyInjection\ContainerInterface;

use Symfony\Component\HttpFoundation\Request;

 */

class CalendarEventController extends ControllerBase {

  /**

   * Field types this endpoint may write.

   *

   * The calendar only moves or resizes dates, so writes are limited to date

   * field types. The 'created' type is included so that calendars keyed on

   * an entity's authored date stay editable for users who may edit that

   * field.

   *

   * @var string[]

   */

  const ALLOWED_FIELD_TYPES = ['datetime', 'daterange', 'timestamp', 'created'];

  /**

   * CSRF Token.

   *

        if (!empty($entity) && $entity->access('update')) {

          if ($entity->hasField($start_field)) {

            // Check the field type and field level edit access before writing.

            if (!$this->isWritableDateField($entity, $start_field)) {

              return new Response($this->t('Access denied!'));

            }

            // Field definitions.

            $fields_def = $entity->getFieldDefinition($start_field);

            $start_type = $fields_def->getType();

            if (isset($entity->$end_field) && !empty($end_date)) {

              if (!$this->isWritableDateField($entity, $end_field)) {

                return new Response($this->t('Access denied!'));

              }

              $fields_def = $entity->getFieldDefinition($end_field);

              $end_type = $fields_def->getType();

            }

              }

            }

            // Validate the entity so that field constraints and workflow rules

            // are enforced before saving.

            $entity->setValidationRequired(TRUE);

            $violations = $entity->validate();

            // Ignore violations on fields the current user cannot edit and on

            // fields this request does not touch. The moderation state is

            // always treated as touched because every save re-asserts it.

            $violations->filterByFieldAccess();

            $touched = array_filter([$start_field, $end_field]);

            if ($entity->hasField('moderation_state')) {

              $touched[] = 'moderation_state';

            }

            $violations->filterByFields(array_diff(array_keys($entity->getFieldDefinitions()), $touched));

            if ($violations->count() > 0) {

              // Report the remaining violations so the calendar can show why

              // the change was refused.

              $messages = [];

              foreach ($violations as $violation) {

                $messages[] = PlainTextOutput::renderFromHtml($violation->getMessage());

              }

              return new Response(implode("\n", $messages));

            }

            $entity->save();

            // Log the content changed.

            $this->loggerFactory->get($entity_type)->notice('%entity_type: updated %title', [

    }

  }

  /**

   * Checks whether a field may be written by this endpoint.

   *

   * The field must exist on the entity, be one of the supported date field

   * types, and be editable by the current user.

   *

   * @param \Drupal\Core\Entity\FieldableEntityInterface $entity

   *   The entity being updated.

   * @param string $field_name

   *   The field name.

   *

   * @return bool

   *   TRUE when the field may be written by the current user.

   */

  protected function isWritableDateField(FieldableEntityInterface $entity, $field_name) {

    if (empty($field_name) || !$entity->hasField($field_name)) {

      return FALSE;

    }

    $field_definition = $entity->getFieldDefinition($field_name);

    if (!in_array($field_definition->getType(), self::ALLOWED_FIELD_TYPES, TRUE)) {

      return FALSE;

    }

    return $entity->get($field_name)->access('edit');

  }

  /**

   * New event handler function.

   *