Flutter/Dart如何筛选指定月份的cardBillsTotal数据并保存

原始数据结构 #

{
    "results": [
        {
            "firstName": "Thomas",
            "lastName": "Papioanou",
            "userID": 238,
            "month": 5,
            "year": 2022,
            "cardBillsTotal": [
                {
                    "serial": "884221337251",
                    "value": 1.0450,
                    "vat": 0.2508,
                    "total": 1.2958,
                    "energy": 0
                }
            ]
        },
        {
            "firstName": "Thomas",
            "lastName": "Papioanou",
            "userID": 238,
            "month": 6,
            "year": 2022,
            "cardBillsTotal": [
                {
                    "serial": "884221337251",
                    "value": 3.4034,
                    "vat": 0.8168,
                    "total": 4.2202,
                    "energy": 0
                }
            ]
        },
        {
            "firstName": "Thomas",
            "lastName": "Papioanou",
            "userID": 238,
            "month": 7,
            "year": 2022,
            "cardBillsTotal": [
                {
                    "serial": "884221337251",
                    "value": 2.0900,
                    "vat": 0.5016,
                    "total": 2.5916,
                    "energy": 0
                }
            ]
        },
        {
            "firstName": "Thomas",
            "lastName": "Papioanou",
            "userID": 238,
            "month": 5,
            "year": 2022,
            "cardBillsTotal": [
                {
                    "serial": "941368618045",
                    "value": 2.2884,
                    "vat": 0.5492,
                    "total": 2.8376,
                    "energy": 0
                }
            ]
        },
        {
            "firstName": "Thomas",
            "lastName": "Papioanou",
            "userID": 238,
            "month": 6,
            "year": 2022,
            "cardBillsTotal": [
                {
                    "serial": "941368618045",
                    "value": 1.1083,
                    "vat": 0.2660,
                    "total": 1.3743,
                    "energy": 0
                }
            ]
        }
    ],
    "count": 5
}

需求 #

筛选出month字段值等于5的条目，并提取其中的cardBillsTotal数组进行存储。

现有请求代码 #

Future<List<Evsebillall>> fetchall() async {
    String? token = await this.storage.read(key: "token");
    Map<String, String> headers = {
      "Content-Type": "application/json",
      "Accept": "application/json",
      "Authorization": "Bearer " + (token ?? ""),
    };
    final response = await http.get(Uri.parse(
        this.serverIP + ':' + this.serverPort +
            '/user/contractedChargeTransactionsList?page=1&limit=10'),
        headers: headers);
    if (response.statusCode == 200) {
      setState(() {
        cardBillsall = jsonDecode(response.body)["results"] as List;
        print(cardBillsall);
        var result = cardBillsall.map((e) => Evsebillall.fromJson(e)).toList();
        print (data);
      });
      return cardBillsall.map((e) => Evsebillall.fromJson(e)).toList();
    }
    else{
      throw Exception('Failed to load Bills');
    }
  }

无效尝试代码 #

var data = cardBillsall.map((e) => e["month"]=5).toList();

正确实现方案 #

你的无效代码问题在于使用了赋值运算符=，这会将所有条目的month强制改为5，而非筛选符合条件的条目。正确逻辑是先筛选、再提取，分两种场景实现：

1. 直接处理原始JSON List（不转模型） #

如果不需要将数据转为Evsebillall模型，直接在setState中处理：

if (response.statusCode == 200) {
  setState(() {
    cardBillsall = jsonDecode(response.body)["results"] as List;
    // 第一步：筛选month等于5的条目
    var filteredEntries = cardBillsall.where((entry) => entry["month"] == 5).toList();
    // 第二步：提取所有符合条件的cardBillsTotal，合并为一个大列表
    List<dynamic> allCardBillsTotal = filteredEntries.expand((entry) => entry["cardBillsTotal"] as List).toList();
    // 若需保留每个条目对应的cardBillsTotal数组，改用以下代码：
    // List<List<dynamic>> cardBillsTotalList = filteredEntries.map((entry) => entry["cardBillsTotal"] as List).toList();
    
    print(allCardBillsTotal); // 打印验证结果
  });
  // 其他逻辑...
}

2. 转为Evsebillall模型后处理 #

如果需要基于模型操作，先将JSON转为模型列表，再筛选提取：

if (response.statusCode == 200) {
  setState(() {
    cardBillsall = jsonDecode(response.body)["results"] as List;
    var result = cardBillsall.map((e) => Evsebillall.fromJson(e)).toList();
    // 筛选month=5的模型实例
    var filteredModels = result.where((model) => model.month == 5).toList();
    // 提取cardBillsTotal（需确保Evsebillall类中定义了cardBillsTotal字段）
    List<CardBillTotal> allCardBills = filteredModels.expand((model) => model.cardBillsTotal).toList();
    
    print(allCardBills);
  });
  return result;
}

关键要点 #

• 使用where()方法完成筛选，判断条件为entry["month"] == 5（原始JSON）或model.month ==5（模型实例）
• 使用expand()可将多个子数组合并为一个大列表；若需保留原条目与子数组的对应关系，改用map()即可
• 注意类型转换，确保cardBillsTotal被正确识别为List类型

内容的提问来源于stack exchange，提问作者Bill Tertis