List <dynamic>类型不是Map <String，dynamic>的子类型

问题描述

我已经能够遵循Flutter上有关基本UI设计的一些教程，并且是API调用的新手。我认为我需要在进行API调用后转换类型，但是我不知道如何。

注意：我知道一个文件中有很多代码，但是我会将API调用文件和自定义类文件与我的主文件分开

import 'dart:async';
import 'dart:convert';
import 'package:Flutter/foundation.dart';
import 'package:Flutter/material.dart';
import 'package:http/http.dart' as http;

Future<Country> fetchAlbum() async {
  final response = await http.get('https://restcountries.eu/rest/v2/all');

  if (response.statusCode == 200) {
    // If the server did return a 200 OK response,// then parse the JSON.
    return Country.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 album');
  }
}

class Country {
  final String name;
  final List<String> topLevelDomain;
  final String alpha2Code;
  final String alpha3Code;
  final String callingCodes;
  final String capital;
  final String region;
  final String subregion;
  final int population;
  final List<int> latlng;
  final String demonym;
  final int area;
  final int gini;
  final List<String> timezones;
  final List<String> borders;
  final String nativeName;
  final int numericCode;
  final List<String> currencies;
  final List<String> translations;
  final String flag;
  final String cioc;

  Country({
    @required this.name,@required this.topLevelDomain,@required this.alpha2Code,@required this.alpha3Code,@required this.callingCodes,@required this.capital,@required this.region,@required this.subregion,@required this.population,@required this.latlng,@required this.demonym,@required this.area,@required this.gini,@required this.timezones,@required this.borders,@required this.nativeName,@required this.numericCode,@required this.currencies,@required this.translations,@required this.flag,@required this.cioc,});

  factory Country.fromJson(Map<String,dynamic> json) {
    return Country(
      name: json['name'],topLevelDomain: json['topLevelDomain'],alpha2Code: json['alpha2Code'],alpha3Code: json['alpha3Code'],callingCodes: json['callingCodes'],capital: json['capital'],region: json['region'],subregion: json['subregion'],population: json['population'],latlng: json['latlng'],demonym: json['demonym'],area: json['area'],gini: json['gini'],timezones: json['timezones'],borders: json['borders'],nativeName: json['nativeName'],numericCode: json['numericCode'],currencies: json['currencies'],translations: json['translations'],flag: json['flag'],cioc: json['cioc'],);
  }
}

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

class MyApp extends StatefulWidget {
  MyApp({Key key}) : super(key: key);

  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  Future<Country> futureAlbum;

  @override
  void initState() {
    super.initState();
    futureAlbum = fetchAlbum();
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Fetch Data Example',theme: ThemeData(
        primarySwatch: Colors.blue,),home: Scaffold(
        appBar: AppBar(
          title: Text('Fetch Data Example'),body: Center(
          child: FutureBuilder<Country>(
            future: futureAlbum,builder: (context,snapshot) {
              if (snapshot.hasData) {
                return Text(snapshot.data.name);
              } else if (snapshot.hasError) {
                return Text("${snapshot.error}");
              }

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

这是我的模拟器屏幕的样子：http://i1.imgrr.com/18Z/7512_Screenshot1599026660.png

解决方法

第12行“返回Country.fromJson（json.decode（response.body））;'

我认为“ json.decode（response.body）”的类型是“列表”，但此处的参数类型是Map。也许您的响应是一个Country对象列表，但它返回了1个Country对象。这就是为什么出现此错误

,

您的API返回List而不是Map。

您需要将每个元素映射为Country。

Future<List<Country>> fetchAlbum() async {
  final response = await http.get('https://restcountries.eu/rest/v2/all');

  if (response.statusCode == 200) {
     List<Country> countryList = ((json.decode(response.body) as List).map((i) => Country.fromJson(i)).toList();
    return countryList;
  } else {
    throw Exception('Failed to load album');
  }
}