public function toArray($flag = self::EXTR_DATA)
    {
        switch ($flag) {
            case self::EXTR_BOTH:
                return $this->items;
            case self::EXTR_PRIORITY:
                return array_map(function ($item) {
                    return $item['priority'];
                }, $this->items);
            case self::EXTR_DATA:
            default:
                return array_map(function ($item) {
                    return $item['data'];
                }, $this->items);
        }
    }