Api 调用抛出错误类型“Null”不是“List”类型的子类型<Quotes> &#39;

Question

这是我的 api 调用。

    class Httpservice {
    final Uri postURL = Uri.parse(
    "https://quotes.rest/qod?category=inspire",
  );
 static dynamic quoteOfTheDay;

 Future<List<Quotes>> getQuotes() async {
 Response res = await get(postURL);

final jsonItems = json.decode(res.body)["contents"]["quotes"];
List<Quotes> quote = jsonItems.map<Quotes>((json) {
  return Quotes.fromJson(json);
}).toList();
quoteOfTheDay = quote;
return quote;
 }
}
 

这是显示页面

    class PostsPage extends StatelessWidget {
    final List<Quotes> quote = Httpservice.quoteOfTheDay;

    PostsPage({Key? key}) : super(key: key);

    @override
   Widget build(BuildContext context) {
   return Scaffold(
    appBar: AppBar(
    title: const Text("quotes"),
   ),
   body: FutureBuilder(
    //future: httpService.getQuotes(),
    builder: (BuildContext context, AsyncSnapshot<List<Quotes>> 
    snapshot) {
      if (snapshot.hasData) {
        //Future.delayed(const Duration(seconds: 5));
        // Quote? quotes = snapshot.data;
        if (kDebugMode) {
          print('" ${quote[0].quote} ?? '' "');
        }
        return Text('" ${quote[0].quote} ?? '' "');
      } else {
        return Scaffold(
          body: Column(children: const [
            SizedBox(height: 400.0),
            Center(
              child: CircularProgressIndicator(
                color: Colors.black,
              ),
            ),
            SizedBox(
              height: 10.0,
            ),
            Text('loading')
          ]),
         );
       }
      },
     ),
   );
  }
 }

这是模型数据

       class Quotes {
       String? quote;
       String? length;
       String? author;
       List<String>? tags;
       String? category;
      String? language;
      String? date;
      String? permalink;
      String? id;
      String? background;
      String? title;

     Quotes(
    {this.quote,
    this.length,
    this.author,
    this.tags,
    this.category,
    this.language,
    this.date,
    this.permalink,
    this.id,
    this.background,
    this.title});

   Quotes.fromJson(Map<String, dynamic> json) {
   quote = json['quote'];
   length = json['length'];
   author = json['author'];
   tags = json['tags'].cast<String>();
   category = json['category'];
   language = json['language'];
   date = json['date'];
   permalink = json['permalink'];
   id = json['id'];
   background = json['background'];
   title = json['title'];
   }

  Map<String, dynamic> toJson() {
  final Map<String, dynamic> data = <String, dynamic>{};
  data['quote'] = quote;
  data['length'] = length;
  data['author'] = author;
  data['tags'] = tags;
 data['category'] = category;
 data['language'] = language;
 data['date'] = date;
 data['permalink'] = permalink;
 data['id'] = id;
 data['background'] = background;
 data['title'] = title;
 return data;
 }
}

模型数据2

 import 'quotes.dart';

 class Contents {
 List<Quotes>? quotes;

 Contents({this.quotes});

Contents.fromJson(Map<String, dynamic> json) {
if (json['quotes']) {
  quotes = <Quotes>[];
  json['quotes'].forEach((v) {
    quotes!.add(Quotes.fromJson(v));
  });
  }
 }

Map<String, dynamic> toJson() {
final Map<String, dynamic> data = <String, dynamic>{};
if (quotes != null) {
  data['quotes'] = quotes!.map((v) => v.toJson()).toList();
  }
 return data;
   }
  }

这是所有主要页面的代码。 当我尝试显示 Quote 类中的报价时，没有任何显示。 当我尝试导航到显示页面时，我不断收到此错误。引发了另一个异常：类型'Null'不是'List'类型的子类型我真的不知道我做错了什么并且我陷入了困境。

Answer 1

我认为问题出在这行final List<Quotes> quote = Httpservice.quoteOfTheDay; . 由于变量quote期望List<Quotes>并且您正在调用返回空值的Httpservice.quoteOfTheDay 。

相反，您应该调用Httpservice.getQuotes() ，因为此方法返回预期值。

final List<Quotes> quote = Httpservice.getQuotes();

Answer 2

您正在使用 FutureBuilder 获取数据。 返回的快照确实具有列表，但是您在 FutureBuilder 之前获得了 quoteOfTheDay，它为空。 最好的方法是获取数据，当它被加载时，是通过快照。 这是工作代码：

帖子页面：

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';

import 'http_service.dart';
import 'model_quote.dart';

class PostsPage extends StatelessWidget {
  PostsPage({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text("quotes"),
      ),
      body: FutureBuilder(
        future: Httpservice().getQuotes(),
        builder: (BuildContext context, AsyncSnapshot<List<Quotes>> snapshot) {
          if (snapshot.hasData) {
            //Future.delayed(const Duration(seconds: 5));
            // Quote? quotes = snapshot.data;
            final List<Quotes> quotes = snapshot.data!;
            return ListView.builder(
                itemCount: quotes.length,
                itemBuilder: (ctx, i) {
                  return Container(
                      child: Text('" ${quotes[0].quote} ?? ' ' "'));
                });
          } else {
            return Column(children: const [
              SizedBox(height: 400.0),
              Center(
                child: CircularProgressIndicator(
                  color: Colors.black,
                ),
              ),
              SizedBox(
                height: 10.0,
              ),
              Text('loading')
            ]);
          }
        },
      ),
    );
  }
}

HTTP 服务

import 'dart:convert';
import 'package:http/http.dart';
import 'model_quote.dart';

class Httpservice {
  final Uri postURL = Uri.parse(
    "https://quotes.rest/qod?category=inspire",
  );

  Future<List<Quotes>> getQuotes() async {
    Response res = await get(postURL);
    final jsonItems = json.decode(res.body)["contents"]["quotes"];
    List<Quotes> quote = jsonItems.map<Quotes>((json) {
      print(json);
      return Quotes.fromJson(json);
    }).toList();
    return quote;
  }
}