错误：获取JSON时，列表不是Map类型的子类型

Question

我正在使用flatter构建一个事件应用程序，它从Rails api获取数据并显示其细节。我犯了与这个问题相同的错误，并且已经尝试了“kapsid”的答案，但还不能解决。

我的模型类（evento.dart）。基本上，它代表一个较大的事件，其中包含多个小事件。

import 'package:flutter/material.dart';    

class Evento {
  int id;
  String titulo, descricao, inicio, fim, dataFimInscricao;
  List<Evento> children;

  Evento(
      {this.id,
      this.titulo,
      this.descricao,
      this.inicio,
      this.fim,
      this.dataFimInscricao,
      this.children});

  factory Evento.fromJson(Map<String, dynamic> json) {
    var list = json['children'] as List;
    List<Evento> childrenList = list.map((c) => Evento.fromJson(c)).toList();

    return new Evento(
      id: json['id'],
      titulo: json['titulo'],
      descricao: json['descricao'],
      inicio: json['inicio'],
      fim: json['fim'],
      dataFimInscricao: json['data_fim_inscricao'],
      children: childrenList
    );
  }

main.dart列举各种类似的大型活动

import 'dart:convert';

import 'package:eventos_app/detalhesEvento.dart';
import 'package:eventos_app/model/evento.dart';
import 'package:eventos_app/utils/utils.dart';
import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;

void main() => runApp(new MyApp());

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  Future<Evento> futureEvento;

  @override
  void initState() {
    super.initState();
    futureEvento = fetchEvento();
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: FutureBuilder(
          future: futureEvento,
          builder: (context, snapshot) {
            if (!snapshot.hasData) {
              print(snapshot.error);
              return Padding(
                padding: EdgeInsets.all(16),
                child: Center(
                  child: Text(snapshot.error.toString()),
                ),
              );
            } else {
              var eventos = json.decode(snapshot.data.toString());

              return ListView.builder(
                itemCount: eventos.length,
                itemBuilder: (BuildContext context, int index) {
                  return Card(
                    margin: EdgeInsets.all(8),
                    elevation: 5,
                    child: ListTile(
                      contentPadding: EdgeInsets.all(8),
                      leading: Icon(
                        Icons.local_florist,
                        size: 35,
                      ),
                      title: Text(eventos[index]['titulo']),
                      subtitle: Text(
                          'Data: ${parseDate(eventos[index]['inicio'])}\n${eventos[index]['endereco']}'),
                      onTap: () {
                        var evento = Evento.fromJson(eventos[index]);
                        print(evento.titulo);
                        Navigator.push(
                            context,
                            MaterialPageRoute(
                                builder: (context) =>
                                    DetalhesEvento(evento: evento)));

                        // Scaffold.of(context).showSnackBar(SnackBar(
                        //   content: Text('${event.title} tapped!'),
                        //   duration: new Duration(seconds: 1)
                        // ));
                      },
                    ),
                  );
                },
              );
            } // else
          },
        ),
      ),
    );
  }
}

Future<Evento> fetchEvento() async {
  final response =
      await http.get('https://api.myjson.com/bins/1d8142');

  print('----------------------------------------\n');
  print(response.body);
  print('\n----------------------------------------');

  if (response.statusCode == 200) {
    // If the server did return a 200 OK response,
    // then parse the JSON.
//    return Evento.fromJson2(json.decode(response.body));
    return Evento.fromJson(json.decode(response.body));
  } else {
    // If the server did not return a 200 OK response,
    // then throw an exception.
    throw Exception('Failed to load events');
  }
}

这是示例JSON

[
  {
    "id": 1,
    "titulo": "MAIN EVENT",
    "descricao": "Lorem ipsum dolor sit amet, consectetur adipiscing elit. In elementum metus erat. Morbi sit amet pellentesque elit. Sed at gravida nisi. In in diam in mi gravida iaculis. Phasellus ac leo vel arcu faucibus porta at nec elit. Proin imperdiet leo congue, mattis nunc non, laoreet turpis. Morbi sed faucibus lorem.",
    "inicio": "2020-03-13T10:00:00.000-03:00",
    "fim": "2020-03-31T23:59:00.000-03:00",
    "data_fim_inscricao": "2020-03-31T23:59:00.000-03:00",
    "children": [
      {
        "id": 4,
        "titulo": "SECONDARY EVENT AAA",
        "descricao": "AAAAA",
        "inicio": "2020-03-24T08:00:00.000-03:00",
        "fim": "2020-03-25T18:00:00.000-03:00",
        "data_fim_inscricao": "2020-03-23T08:00:00.000-03:00",
        "children": [

        ]
      },
      {
        "id": 3,
        "titulo": "SECONDARY EVENT BBB",
        "descricao": "BBBBB",
        "inicio": "2020-03-13T23:59:00.000-03:00",
        "fim": "2020-03-25T23:59:00.000-03:00",
        "data_fim_inscricao": "2020-03-13T23:59:00.000-03:00",
        "children": [

        ]
      },
      {
        "id": 2,
        "titulo": "SECONDARY EVENT CCC",
        "descricao": "CCCCC",
        "inicio": "2020-03-14T10:00:00.000-03:00",
        "fim": "2020-03-16T23:59:00.000-03:00",
        "data_fim_inscricao": "2020-03-16T23:59:00.000-03:00",
        "children": [

        ]
      }
    ]
  }
]

我做错了什么？？

Answer 1

问题是您从预期的数据接收方式接收到的数据之间不匹配。具体来说，您正试图传递响应。具有的正文类型为<代码>列表

我不能运行你的代码，但看起来问题发生在FetchEvento的这一行：

return Evento.fromJson(json.decode(response.body)); // <-- you're passing List<dynamic>

因为你在传递回应。类型为<代码>列表的正文

factory Evento.fromJson(Map<String, dynamic> json) {  //<-- you defined json as Map<String,dynamic>

因为dart是一种强类型语言，所以必须将它们定义为相同的类型。

最简单的解决方案要么是三件事之一： 1。最简单的方法是将收到的变量类型更改为：

factory Evento.fromJson(List<dynamic> json) {

2.然而，这可能会破坏其他东西。（为什么要将其定义为Map

var map1 = Map.fromIterable(response.body, key: (e) => e.name, value: (e) => e.value); 

（您可以在此处了解更多信息：https://bezkoder.com/dart-convert-list-map/)

我希望这是选项1！