Flutter model 用于嵌套 json object

Question

我是 Flutter 的新手。 我曾尝试在 dart 中开发 Model class。 但总是显示错误信息

Exception:type '_InternalLinkedHashMap<String, dynamic>' is not a subtype of type 'String', tt:#0      new User.fromJson

我不明白，我的代码会在哪里出错。 我想要解决方案。

我想在 dart 中转换 model class。

[
    {
        "route": {
            "routeID": "aaaaaaaa",
            "routeTitle": "bbbbbbbb"
        },
        "distributor": {
            "distributorID": "cccccccccccc",
            "distributorName": "ddddddddddd"
        },
        "visitDate": null
    }
]

我已经尝试过我的源代码。 像下面的代码

import 'dart:convert';

List<User> userFromJson(String str) =>
    List<User>.from(json.decode(str).map((x) => User.fromJson(x)));

String userToJson(List<User> data) =>
    json.encode(List<dynamic>.from(data.map((x) => x.toJson())));

class User {
  User({
    this.visitDate,
    this.route,
    this.distributor,
  });

  String visitDate;
  String route;
  String distributor;

  factory User.fromJson(Map<String, dynamic> json) => User(
    visitDate: json["visitDate"],
    route: json["route"],
    distributor: json["distributor"],
  );

  Map<String, dynamic> toJson() => {
    "visitDate": visitDate,
    "route": route,
    "distributor": distributor,
  };
}

Route RouteFromJson(String str) => Route.fromJson(json.decode(str));

String RouteToJson(Route data) => json.encode(data.toJson());

class Route {
  Route({
    this.routeID,
    this.routeTitle,
  });

  String routeID;
  String routeTitle;

  factory Route.fromJson(Map<String, dynamic> json) => Route(
    routeID: json["routeID"],
    routeTitle: json["routeTitle"],
  );

  Map<String, dynamic> toJson() => {
    "routeID": routeID,
    "routeTitle": routeTitle,
  };
}
Distributor DistributorFromJson(String str) => Distributor.fromJson(json.decode(str));

String DistributorToJson(Distributor data) => json.encode(data.toJson());
class Distributor {
  Distributor({
    this.distributorID,
    this.distributorName,
  });

  String distributorID;
  String distributorName;

  factory Distributor.fromJson(Map<String, dynamic> json) => Distributor(
    distributorID: json["distributorID"],
    distributorName: json["distributorName"],
  );

  Map<String, dynamic> toJson() => {
    "distributorID": distributorID,
    "distributorName": distributorName,
  };
}

如何纠正我的 model class。 请帮我。 谢谢

Answer 1

像这样更改您的用户 class ：

 class User {
      User({
        this.visitDate,
        this.route,
        this.distributor,
      });
    
      String visitDate;
      Route route;
      Distributor distributor;
    
      factory User.fromJson(Map<String, dynamic> json) => User(
        visitDate: json["visitDate"],
        route = json['route'] != null ? Route.fromJson(json['route']) : null;
        distributor = json['distributor'] != null
            ? Distributor.fromJson(json['distributor'])
            : null;
      );
    
      Map<String, dynamic> toJson() => {
        "visitDate": visitDate,
        if (route != null) {
          data['route'] = this.route.toJson();
        }
        if (distributor != null) {
          data['distributor'] = this.distributor.toJson();
        }
      };
    }

Answer 2

您还可以为此查看json_serializable package。 这样，您不必自己编写此代码。

https://pub.dev/packages/json_serializable

Answer 3

这是重写 model 的方法：

import 'dart:convert';

List<User> usersFromJson(String str) =>
    List<User>.from(json.decode(str).map((x) => User.fromJson(x)));

String usersToJson(List<User> data) =>
    json.encode(List<dynamic>.from(data.map((x) => x.toJson())));

class User {
  User({
    required this.visitDate,
    required this.route,
    required this.distributor,
  });

  String? visitDate;
  Route route;
  Distributor distributor;

  User.fromJson(Map<String, dynamic> json)
      : visitDate = json["visitDate"],
        route = Route.fromJson(json["route"]),
        distributor = Distributor.fromJson(json["distributor"]);

  Map<String, dynamic> toJson() => {
        "visitDate": visitDate,
        "route": route.toJson(),
        "distributor": distributor.toJson(),
      };
}

class Route {
  Route({
    required this.routeID,
    required this.routeTitle,
  });

  String routeID;
  String routeTitle;

  Route.fromJson(Map<String, dynamic> json)
      : routeID = json["routeID"],
        routeTitle = json["routeTitle"];

  Map<String, dynamic> toJson() => {
        "routeID": routeID,
        "routeTitle": routeTitle,
      };
}

class Distributor {
  Distributor({
    required this.distributorID,
    required this.distributorName,
  });

  String distributorID;
  String distributorName;

  Distributor.fromJson(Map<String, dynamic> json)
      : distributorID = json["distributorID"],
        distributorName = json["distributorName"];

  Map<String, dynamic> toJson() => {
        "distributorID": distributorID,
        "distributorName": distributorName,
      };
}

void main() {
  const data = '''[
    {
        "route": {
            "routeID": "aaaaaaaa",
            "routeTitle": "bbbbbbbb"
        },
        "distributor": {
            "distributorID": "cccccccccccc",
            "distributorName": "ddddddddddd"
        },
        "visitDate": null
    }
  ]''';
  final decoded = usersFromJson(data);
  final encoded = usersToJson(decoded);
  print(decoded);
  print(encoded);
}

有些事情要注意。

您似乎使用的是过时版本的 dart。 我建议将您项目中的2.14.0的版本更新为 2.14.0，这是当前最新的主要版本。 您可能必须运行flutter upgrade才能执行此操作。 您可以在pubspec.yaml文件中更改 dart 的版本。 您的 dart 版本的设置将如下所示：

environment:
  sdk: ">=2.14.0 <3.0.0"

您在代码中遇到的主要问题是，在您的User class 中，您已将route和distributor定义为String类型，而实际上，您希望它们为Route和Distributor类型。 在您的User class 的构造函数中，表达式json["route"]将返回类似{ "routeID": "aaaaaaaa", "routeTitle": "bbbbbbbb" }的东西，它不是String而是Map<String, dynamic>这就是您收到上述错误消息的原因。

Answer 4

Flutter 盒子！

您只需要做简单的工作就是访问以下网站。 并且需要将您想要转换的响应或地图/列表放入 model class 和 rest 将由服务器完成。

顺便说一句，对于您的 List<Map<String, dynamic> 数据的 model class 是，

// To parse this JSON data, do
//
//     final routeModelClass = routeModelClassFromJson(jsonString);

import 'dart:convert';

List<RouteModelClass> routeModelClassFromJson(String str) => List<RouteModelClass>.from(json.decode(str).map((x) => RouteModelClass.fromJson(x)));

String routeModelClassToJson(List<RouteModelClass> data) => json.encode(List<dynamic>.from(data.map((x) => x.toJson())));

class RouteModelClass {
    RouteModelClass({
        this.route,
        this.distributor,
        this.visitDate,
    });

    Route route;
    Distributor distributor;
    dynamic visitDate;

    factory RouteModelClass.fromJson(Map<String, dynamic> json) => RouteModelClass(
        route: Route.fromJson(json["route"]),
        distributor: Distributor.fromJson(json["distributor"]),
        visitDate: json["visitDate"],
    );

    Map<String, dynamic> toJson() => {
        "route": route.toJson(),
        "distributor": distributor.toJson(),
        "visitDate": visitDate,
    };
}

class Distributor {
    Distributor({
        this.distributorId,
        this.distributorName,
    });

    String distributorId;
    String distributorName;

    factory Distributor.fromJson(Map<String, dynamic> json) => Distributor(
        distributorId: json["distributorID"],
        distributorName: json["distributorName"],
    );

    Map<String, dynamic> toJson() => {
        "distributorID": distributorId,
        "distributorName": distributorName,
    };
}

class Route {
    Route({
        this.routeId,
        this.routeTitle,
    });

    String routeId;
    String routeTitle;

    factory Route.fromJson(Map<String, dynamic> json) => Route(
        routeId: json["routeID"],
        routeTitle: json["routeTitle"],
    );

    Map<String, dynamic> toJson() => {
        "routeID": routeId,
        "routeTitle": routeTitle,
    };
}

列表/映射到 Model class