private function getGridClass() {
        $class = [];
        foreach($this->grid as $grid){
            if(!isset($grid['type']) || !isset($grid['size'])){
                throw new InvalidConfigException(get_called_class() . ' must have type and size.');
            }
            if(!isset($grid['positiontype']))
                $class[] = $grid['type'].$grid['size'];
            else
                $class[] = $grid['type'].$grid['positiontype'].'-'.$grid['size'];            
        }
        return implode(' ',$class);
    }