FOS\UserBundle\Model\UserManagerInterface::findUserByUsername PHP Method

    function findUserByUsername($username);

 public function __invoke(Request $request)
 {
     if ($this->container->hasParameter('partkeepr.auth.allow_password_change') && $this->container->getParameter('partkeepr.auth.allow_password_change') === false) {
         throw new PasswordChangeNotAllowedException();
     }
     $user = $this->userService->getUser();
     if (!$request->request->has('oldpassword') && !$request->request->has('newpassword')) {
         throw new \Exception('old password and new password need to be specified');
     }
     $FOSUser = $this->userManager->findUserByUsername($user->getUsername());
     if ($FOSUser !== null) {
         $encoder = $this->encoderFactory->getEncoder($FOSUser);
         $encoded_pass = $encoder->encodePassword($request->request->get('oldpassword'), $FOSUser->getSalt());
         if ($FOSUser->getPassword() != $encoded_pass) {
             throw new OldPasswordWrongException();
         }
         $this->userManipulator->changePassword($user->getUsername(), $request->request->get('newpassword'));
     } else {
         if ($user->isLegacy()) {
             if ($user->getPassword() !== md5($request->request->get('oldpassword'))) {
                 throw new OldPasswordWrongException();
             }
             $user->setNewPassword($request->request->get('newpassword'));
             $this->userService->syncData($user);
         } else {
             throw new \Exception('Cannot change password for LDAP users');
         }
     }
     $user->setPassword('');
     $user->setNewPassword('');
     return $user;
 }