public function onAuthenticate(Authenticate $event)
 {
     $result = $event->getResult();
     if ($result instanceof Result && $result->isValid()) {
         //If a previous adapter has already returned a valid result don't change that
         return null;
     }
     if ($this->adapter instanceof ValidatableAdapterInterface) {
         $this->adapter->setIdentity($event->getParam('identity'));
         $this->adapter->setCredential($event->getParam('credential'));
     }
     $result = $this->adapter->authenticate();
     $event->setResult($result);
     return $result;
 }