private function _last(callable $callback = null)
    {
        $values = $this->getValues();
        if ($callback){
            foreach(array_reverse($values) as $key => $value){
                if ($callback($value, $key)){
                    return $value;
                }
            }
        }
        else{
            return empty($values) ? null : $values[count($values)-1];
        }
        return null;
    }