解析API响应至Model时出现JSON解码错误：List非String类型

问题描述 #

能正常获取API响应，但后续JSON解码时触发错误，提示内容为“List不是String类型”。

相关代码 #

Dio请求代码 #

import 'package:dio/dio.dart';
import 'package:dio_try/country_model.dart';


class DioClient {
final Dio dio = Dio();
static const baseUrl = 'some Url';

  Future<CountryListModel> fetchPost() async {
   try {
   dio.options.headers['Content-Type'] = 'application/json';
   dio.options.headers['fcm_token'] = 'test';
   final response = await dio.post(baseUrl);
  print(response.toString());
  return CountryListModel.fromJson(response.data);
} on DioError catch (e) {
  print(e.message + e.error);
  throw Exception(e.message + e.error);
}
}
}

API返回的JSON响应 #

[ 
   { 
     "countryCode": "ALB", 
     "countryName": "ALBANIA", 
     "TypeId": "3", 
     "eligible": "Y" 
   }
] 

Model类代码 #

import 'dart:convert';

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

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

class CountryListModel {
  CountryListModel({
  required this.countryCode,
required this.countryName,
required this.TypeId,
required this.eligible,
 });

 String countryCode;
 String countryName;
 String TypeId;
String eligible;

factory CountryListModel.fromJson(Map<String, dynamic> json) =>
  CountryListModel(
    countryCode: json["countryCode"],
    countryName: json["countryName"],
    TypeId: json["TypeId"],
    eligible: json["eligible"],
  );

 Map<String, dynamic> toJson() => {
    "countryCode": countryCode,
    "countryName": countryName,
    "TypeId": TypeId,
    "eligible": eligible,
  };

static List<CountryListModel> countryListModelFromJson(List<dynamic> list) =>
  List<CountryListModel>.from(
      list.map((x) => CountryListModel.fromJson(x)));
  }

错误原因 #

1. API返回的是**数组（List）**结构，但Dio请求代码里直接调用CountryListModel.fromJson(response.data)解析，而该方法仅支持解析单个对象的Map，与返回数据的List类型不匹配。
2. Model类中已定义处理List的解析方法countryListModelFromJson(List<dynamic> list)，但请求代码未正确使用该方法。

修复方案 #

修改Dio请求方法的返回类型与解析逻辑 #

将fetchPost的返回类型改为Future<List<CountryListModel>>，并调用Model类中处理List的解析方法：

import 'package:dio/dio.dart';
import 'package:dio_try/country_model.dart';


class DioClient {
final Dio dio = Dio();
static const baseUrl = 'some Url';

  // 修改返回类型为List<CountryListModel>
  Future<List<CountryListModel>> fetchPost() async {
   try {
   dio.options.headers['Content-Type'] = 'application/json';
   dio.options.headers['fcm_token'] = 'test';
   final response = await dio.post(baseUrl);
  print(response.toString());
  // 调用Model中处理List的解析方法
  return CountryListModel.countryListModelFromJson(response.data);
} on DioError catch (e) {
  print(e.message + e.error);
  throw Exception(e.message + e.error);
}
}
}

优化Model类（可选，避免方法名冲突） #

Model类存在两个同名的countryListModelFromJson方法，易引发混淆，建议重命名处理字符串的方法：

import 'dart:convert';

// 重命名处理字符串的方法
 List<CountryListModel> countryListModelFromJsonString(String str) =>
List<CountryListModel>.from(
    json.decode(str).map((x) => CountryListModel.fromJson(x)));

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

class CountryListModel {
  CountryListModel({
  required this.countryCode,
required this.countryName,
required this.TypeId,
required this.eligible,
 });

 String countryCode;
 String countryName;
 String TypeId;
String eligible;

factory CountryListModel.fromJson(Map<String, dynamic> json) =>
  CountryListModel(
    countryCode: json["countryCode"],
    countryName: json["countryName"],
    TypeId: json["TypeId"],
    eligible: json["eligible"],
  );

 Map<String, dynamic> toJson() => {
    "countryCode": countryCode,
    "countryName": countryName,
    "TypeId": TypeId,
    "eligible": eligible,
  };

static List<CountryListModel> countryListModelFromJson(List<dynamic> list) =>
  List<CountryListModel>.from(
      list.map((x) => CountryListModel.fromJson(x)));
  }

验证说明 #

修改完成后，API返回的List类型数据会被正确解析为List<CountryListModel>，类型不匹配的错误将不再出现。

内容的提问来源于stack exchange，提问作者Sam Verma