public function setColumnsSummary(array $columns, $rowCallback = null)
	{
		if ($this->hasSomeAggregationFunction()) {
			throw new DataGridException('You can use either ColumnsSummary or AggregationFunctions');
		}

		if (!empty($rowCallback)) {
			if (!is_callable($rowCallback)) {
				throw new \InvalidArgumentException('Row summary callback must be callable');
			}
		}

		$this->columnsSummary = new ColumnsSummary($this, $columns, $rowCallback);

		return $this->columnsSummary;
	}