FOS\UserBundle\Model\UserInterface::setUsername PHP Method

    function setUsername($username);

 /**
  * {@inheritdoc}
  */
 public function updateUser(UserInterface $user, $andFlush = true)
 {
     // Extract email part before the `@` character to use it as username is username not set
     if (null === $user->getUsername()) {
         $user->setUsername(substr($user->getEmail(), 0, strpos($user->getEmail(), '@')));
     }
     // Call parent after as does not override parent and parent do the flush
     parent::updateUser($user, $andFlush);
 }