错误：List<dynamic> 不是 Map<String, dynamic> 类型的子类型

Question

我目前正在构建一个应用程序来通过 api 读取数据，并且我正在尝试从 JSON Placeholder 解析 JSON api。

我为用户创建了一个模型类（users_future_model.dart）：

class Users {
  final int userID;
  final String name;
  final String username;
  final String email;
  final Address address;

  Users({this.userID, this.name, this.username, this.email, this.address});

  factory Users.fromJson(Map<String, dynamic> usersjson)=> Users(
      userID: usersjson["id"],
      name: usersjson["name"],
      username: usersjson["username"],
      email: usersjson["email"],
      address: Address.fromJson(usersjson["address"])
  );
}

class Address{

  final String street;
  final String suite;
  final String city;
  final String zipcode;

  Address({this.street, this.suite, this.city, this.zipcode});

  factory Address.fromJson(Map<String, dynamic> addjson){

    return Address(
      street: addjson["street"],
      suite:  addjson["suite"],
      city: addjson["city"],
      zipcode: addjson["zipcode"]
    );
  }
}

这是将 json 读入小部件的 main.dart：

import 'package:flutter/material.dart';
import 'model/users_future_model.dart';
import 'dart:convert';
import 'dart:async';
import 'package:http/http.dart' as http;

final String jsonplaceholder = "http://jsonplaceholder.typicode.com/users/";

//Future method to read the URL
Future<Users> fetchInfo() async{
  final response = await http.get(jsonplaceholder);
  final jsonresponse = json.decode(response.body);

  return Users.fromJson(jsonresponse);
}

void main() {
  runApp(new MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      home: new MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text("Parse JSON"),
      ),
      body: new Center(
        child: new Column(
          children: <Widget>[
            new FutureBuilder(
                future: fetchInfo(),
                builder: (context, snapshot){
                  if(snapshot.hasData){
                    return new Text(snapshot.data.email);
                  }else if(snapshot.hasError){
                    return new Text("Error ${snapshot.error}");
                  }
                })
          ],
        ),
      )
    );
  }
}

这是我试图读取的 JSON 信息，尽管它只是其中的一小部分：

{
id: 1,
name: "Leanne Graham",
username: "Bret",
email: "Sincere@april.biz",
address: {
 street: "Kulas Light",
 suite: "Apt. 556",
 city: "Gwenborough",
 zipcode: "92998-3874",
 geo: {
  lat: "-37.3159",
  lng: "81.1496"
 }
},
phone: "1-770-736-8031 x56442",
website: "hildegard.org",
company: {
 name: "Romaguera-Crona",
 catchPhrase: "Multi-layered client-server neural-net",
 bs: "harness real-time e-markets"
}
}

我目前遇到的错误是：

Error type 'List<dynamic>' is not a subtype of type 'Map<String, dynamic>'

我该如何纠正这个错误？

Answer 1

API 返回 JSON 数组而不是 json 对象，因此是 List 而不是 Map。

即用户 json 是 Json Array 的第一个元素。

所以要获取第一个元素，请使用第一个索引。 内部获取信息更新

return Users.fromJson(jsonresponse[0]);

Answer 2

var streetsFromJson = parsedJson['streets'];
List<String> streetsList = new List<String>.from(streetsFromJson);

感谢https://medium.com/flutter-community/parsing-complex-json-in-flutter-747c46655f51

Answer 3

看看这个答案

 Future<List<ProductList>> getProductList() async {
  print("comes");
  String productURl= mainURL+'api/store/product-with-category/';

  final response = await http.get(productURl,headers:{"Content-Type": 
 "application/json"});
  List jsonResponse = json.decode(response.body);
  return jsonResponse.map((job) => new ProductList.fromJson(job)).toList();
  
}

获取函数

    FutureBuilder<List<ProductList>>(
    future: getProductList(),
    builder: (context, snapshot) {
      print("snapshot");
      print(snapshot.data);
      if (snapshot.hasData) {
        return Padding(
          padding: const EdgeInsets.all(8.0),
          child: GridView.builder(
          itemCount: snapshot.data.length,
            gridDelegate:SliverGridDelegateWithFixedCrossAxisCount( crossAxisCount: 2,),
            itemBuilder:  (BuildContext context, int i){
              return Card(
                child: Container(
                  decoration: BoxDecoration(
                      border: Border.all(width: 0.5,color: Colors.grey)
                  ),
                  child: Padding(
                    padding: const EdgeInsets.all(8.0),
                    child: Column(
                      children: <Widget>[
                         text18(snapshot.data[i].title, Colors.black, FontWeight.bold)
                      ],
                    ),
                  ),
                ),
              );
            }
        )
        );
      } else if (snapshot.hasError) {
        return Text("${snapshot.error}");
      }


      // By default, show a loading spinner.
      return CircularProgressIndicator();
    },
  ),

只需更改 URL 并创建模型类

Answer 4

注意错误

List<dynamic> is not a subtype of type Map<String, dynamic>

基本上这意味着您在访问 Web 服务后收到的数据是 List 形式，但您使用的内部数据类是 Map 类型。因此，由于 Map 的结构不匹配，它无法解析数据在数据类中定义而不是 List。

重新检查您的 Json 响应和相应的 PODO 类

Answer 5

另一种轻松做到这一点的方法：

import 'package:json_helpers/json_helpers.dart';

void main() {
  // If request returns list of users
  var body = '[{"id": 1}, {"id": 2} ]';
  final users = body.jsonList((e) => User.fromJson(e));
  assert(users[1].id == 2);
  print(users[1].id);

  // If request returns the user
  body = '{"id": 1}';
  final user = body.json((e) => User.fromJson(e));
  assert(user.id == 1);
  print(user.id);
}

Answer 6

使用

List<dynamic> output = jsonDecode(yourJsonSource);

而不是

Map<String, dynamic> output = jsonDecode(yourJsonSource);