public function checkUsername(EventInterface $event)
 {
     $userEntity = $event->getUserEntity();
     if ($userEntity->getUsername() === 'admin') {
         $form = $event->getForm();
         $username = $form->get('username');
         $username->setMessages(array_merge($username->getMessages(), ['username 不能为 admin']));
         return false;
     }
 }