NoSuchMethodError：类 '_InternalLinkedHashMap<String, dynamic>' 没有具有匹配参数的实例方法 'cast'

Posted 2023-02-23

【中文标题】NoSuchMethodError：类 \'_InternalLinkedHashMap<String, dynamic>\' 没有具有匹配参数的实例方法 \'cast\'

【英文标题】：NoSuchMethodError: Class'_InternalLinkedHashMap<String, dynamic>'has no instance method 'cast' with matching argumentsNoSuchMethodError：类 '_InternalLinkedHashMap<String, dynamic>' 没有具有匹配参数的实例方法 'cast'

【发布时间】：2021-11-07 00:46:42

【问题描述】：

我正在尝试从 newsApi 获取数据，但我似乎收到了上述错误。 包括的是我的 main.dart 代码

import 'dart:async';
import 'dart:convert';

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

Future<List<Article>> fetchArticles(http.Client client) async 
  //final response = await client.get(Uri.parse('https://jsonplaceholder.typicode.com/Articles'));
  String link = "https://newsapi.org/v2/top-headlines?country=us&category=business&apiKey=MykEYhere";
final response = await client.get(Uri.parse(link));
  // Use the compute function to run parseArticles in a separate isolate.
  return compute(parseArticles, response.body);


// A function that converts a response body into a List<Article>.
List<Article> parseArticles(String responseBody) 
  final parsed = jsonDecode(responseBody).cast<Map<String, dynamic>>();

  return parsed.map<Article>((json) => Article.fromJson(json)).toList();


class Article 
  final String author;
  final String title;
  final String description;
  final String url;
  final String urlToImage;
  final String content;

  const Article(
    required this.author,
    required this.title,
    required this.description,
    required this.url,
    required this.urlToImage,
    required this.content
  );

  factory Article.fromJson(Map<String, dynamic> json) 
    return Article(
      author: json['author'],
      title: json['title'],
      description: json['description'],
      url: json['url'],
      urlToImage: json['urlToImage'],
      content: json['content'],


    );
  


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

class MyApp extends StatelessWidget 
  const MyApp(Key? key) : super(key: key);

  @override
  Widget build(BuildContext context) 
    const appTitle = 'NewApi Trial';

    return const MaterialApp(
      title: appTitle,
      home: MyHomePage(title: appTitle),
    );
  


class MyHomePage extends StatelessWidget 
  const MyHomePage(Key? key, required this.title) : super(key: key);

  final String title;

  @override
  Widget build(BuildContext context) 
    return Scaffold(
      appBar: AppBar(
        title: Text(title),
      ),
      body: FutureBuilder<List<Article>>(
        future: fetchArticles(http.Client()),
        builder: (context, snapshot) 
          if (snapshot.hasError) 
            return Center(
              child: Text('An error has occurred! $snapshot.error'),
            );
           else if (snapshot.hasData) 
            return ArticlesList(articles: snapshot.data!);
           else 
            return const Center(
              child: CircularProgressIndicator(),
            );
          
        ,
      ),
    );
  


class ArticlesList extends StatelessWidget 
  const ArticlesList(Key? key, required this.articles) : super(key: key);

  final List<Article> articles;

  @override
  Widget build(BuildContext context) 
    return ListView.builder(
      itemCount: articles.length,
      itemBuilder: (context, index) 
        return ClipRect(
          child: Column(
            children: [
              Image.network(articles[index].urlToImage),
              Text(articles[index].title)
            ],
          ),
        );
          //Image.network(Articles[index].thumbnailUrl);
      ,
    );
  

下面是运行代码后出现的错误

NoSuchMethodError: Class '_InternalLinkedHashMap'没有具有匹配参数的实例方法'cast'。接收者：_LinkedHashMap len:3 尝试调用：cast

我们将不胜感激您的帮助

【参考方案1】：

main.dart

import 'dart:convert';
import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;
import 'news_model.dart';

void main() 
  runApp(MaterialApp(
    debugShowCheckedModeBanner: false,
    home: MyApp(),
  ));


// ignore: use_key_in_widget_constructors
class MyApp extends StatefulWidget 
  @override
  _MyAppState createState() => _MyAppState();


class _MyAppState extends State<MyApp> 
  NewsModel? newsmodel;
  List<Articles>? articles;

  Future<List<Articles>?> fetchNews() async 
    final response = await http.get(Uri.parse(
        "https://newsapi.org/v2/top-headlines?country=us&category=business&apiKey=YourAPIKEYHere"));
    if (response.statusCode == 200) 
      newsmodel = NewsModel.fromJson(jsonDecode(response.body));
      setState(() 
        articles = newsmodel!.articles;
      );
      return articles;
     else 
      articles = null;
    
  

  @override
  void initState() 
    fetchNews();
    super.initState();
  

  @override
  Widget build(BuildContext context) 
    return Scaffold(
      body: FutureBuilder<List<Articles>?>(
        future: fetchNews(),
        builder: (context, snapshot) 
          if (snapshot.hasError) 
            return Center(
              child: Text('An error has occurred! $snapshot.error'),
            );
           else if (snapshot.hasData) 
            return ArticlesList(articles: articles);
           else 
            return const Center(
              child: CircularProgressIndicator(),
            );
          
        ,
      ),
    );
  


class ArticlesList extends StatelessWidget 
  const ArticlesList(Key? key, this.articles) : super(key: key);
  final List<Articles>? articles;

  @override
  Widget build(BuildContext context) 
    return ListView.builder(
      itemCount: articles!.length,
      itemBuilder: (context, index) 
        return ClipRect(
          child: Column(
            children: [
              Image.network(articles![index].urlToImage.toString()),
              Text(articles![index].title.toString())
            ],
          ),
        );
        //Image.network(Articles[index].thumbnailUrl);
      ,
    );
  

news_model.dart

class NewsModel 
  String? status;
  int? totalResults;
  List<Articles>? articles;

  NewsModel(this.status,this.totalResults,this.articles);

  NewsModel.fromJson(Map<String, dynamic> json) 
    status = json['status'];
    totalResults = json['totalResults'];
    if (json['articles'] != null) 
      articles = <Articles>[];
      json['articles'].forEach((v) 
        articles!.add(Articles.fromJson(v));
      );
    
  

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


class Articles 
  Source? source;
  String? author;
  String? title;
  String? description;
  String? url;
  String? urlToImage;
  String? publishedAt;
  String? content;

  Articles(
      this.source,
      this.author,
      this.title,
      this.description,
      this.url,
      this.urlToImage,
      this.publishedAt,
      this.content);

  Articles.fromJson(Map<String, dynamic> json) 
    source =
        json['source'] != null ? Source.fromJson(json['source']) : null;
    author = json['author'] ?? "Anonymous";
    title = json['title'] ?? "Information Not Available";
    description = json['description'] ?? "Description Not Available";
    url = json['url'] ?? "https://i.imgur.com/8UdKNS4.jpg";
    urlToImage = json['urlToImage'] ?? "https://i.imgur.com/8UdKNS4.jpg";
    publishedAt = json['publishedAt'] ?? "Date Unavailable";
    content = json['content'] ?? "Content Unavailable";
  

  Map<String, dynamic> toJson() 
    final Map<String, dynamic> data = <String, dynamic>;
    if (source != null) 
      data['source'] = source!.toJson();
    
    data['author'] = author;
    data['title'] = title;
    data['description'] = description;
    data['url'] = url;
    data['urlToImage'] = urlToImage;
    data['publishedAt'] = publishedAt;
    data['content'] = content;
    return data;
  


class Source 
  String? id;
  String? name;

  Source(this.id, this.name);

  Source.fromJson(Map<String, dynamic> json) 
    id = json['id'];
    name = json['name'];
  

  Map<String, dynamic> toJson() 
    final Map<String, dynamic> data = <String, dynamic> ;
    data['id'] = id;
    data['name'] = name;
    return data;
  

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