public function getDataCellValue($model, $key, $index)
    {
        if ($this->value !== null) {
            if (is_string($this->value)) {
                return ArrayHelper::getValue($model, $this->value);
            } else {
                return call_user_func($this->value, $model, $key, $index, $this);
            }
        } elseif ($this->attribute !== null) {
            return ArrayHelper::getValue($model, $this->attribute);
        }
        return null;
    }