public function getRequiredHandlePHIDs()
 {
     $old = $this->getOldValue();
     $new = $this->getNewValue();
     $type = $this->getTransactionType();
     $phids = parent::getRequiredHandlePHIDs();
     switch ($type) {
         case self::TYPE_DEFAULT_QUEUE:
             if ($old) {
                 $phids[] = $old;
             }
             if ($new) {
                 $phids[] = $new;
             }
             break;
     }
     return $phids;
 }