/**
  * Creates a sorting protobuffer message from filter list
  * @param  \App_ListFilter          $filterList
  * @return DrSlump\ProtoBuf\Message type of $_sortingClass, defined above.
  */
 public function mapSortingFieldToMessage(\App_ListFilter $filterList)
 {
     if ($filterList->getSortingField() === null || !$this->_sortingClass) {
         return;
     }
     $protoClass = $this->_sortingClass;
     if ($protoClass) {
         $proto = new $protoClass();
         $proto->setColumn($this->_sortingFieldNameMap[$filterList->getSortingField()]);
         $proto->setType($this->_sortTypeMap[$filterList->getSortType()]);
         return $proto;
     }
 }