Flutter/dart 将数据格式化为实例无效

Question

因此，我可以接收数据并将其打印出来，但它无法再正确格式化为列表。我附上了下面用于获取数据和模型的所有代码。加上打印输出。有人可以帮帮我吗

获取数据

import 'package:http/http.dart' as http;
import 'dart:convert';
import 'package:world_time/services/model/article_model.dart';
class News {


  String? city;
  List? articles;

  News({this.city});
  Future<void> getNews() async {
    try {

      Response response = await get(Uri.parse('https://newsapi.org/v2/everything?q=$city&from=2021-12-05&sortBy=popularity&apiKey=MY API KEY'));
      Map<String, dynamic> json = jsonDecode(response.body);
      print(json);
      List<dynamic> body = json['articles'];
      articles = body.map((dynamic item) => Article.fromJson(item)).toList();
      print(articles);
    }
    catch (e){
      print("Caught error: $e");
    }
  }
}

打印(json)

{status: ok, totalResults: 520, articles: [{source: {id: bbc-news, name: BBC News}, author: null, title: Unstoppable Salah closes in on more records - his year in numbers, description: As Liverpool's Mohamed Salah closes in on another record, BBC Sport takes a statistical look at his superb 2021., url: https://www.bbc.co.uk/sport/football/59646200, urlToImage: https://ichef.bbci.co.uk/live-experience/cps/624/cpsprodpb/1269A/production/_122081457_mohamedsalah.jpg, publishedAt: 2021-12-16T08:49:23Z, content: Mohamed Salah has scored 36 goals in all competitions for Liverpool in 2021
I/flutter ( 2307): "He is phenomenal. For sure, at the moment, he is the best player in the world."
I/flutter ( 2307): Liverpool manager Jurgen Klopp may well … [+5278 chars]}, {source: {id: reuters, name: Reuters}, author: null, title: Man Utd face Villa, Leicester host Watford in FA Cup third round - Reuters, description: Manchester United entertain Aston Villa in the FA Cup third round, while holders Leicester City welcome Watford and West Ham United are at home

打印（文章） 由于 articles 为空，请捕获 returns this

I/flutter ( 2307): Caught error: type 'Null' is not a subtype of type 'String' in type cast

article_model

class Article {
  Source? source;
  String? author;
  String? title;
  String? description;
  String? url;
  String? urlToImage;
  String? publishedAt;
  String? content;
  Article(
      {this.source,
        this.author,
        this.title,
        this.description,
        this.url,
        this.urlToImage,
        this.publishedAt,
        this.content});
  factory Article.fromJson(Map<String, dynamic> json) {
    return Article(
      source: Source.fromJson(json['source']),
      author: json['author'] as String,
      title: json['title'] as String,
      description: json['description'] as String,
      url: json['url'] as String,
      urlToImage: json['urlToImage'] as String,
      publishedAt: json['publishedAt'] as String,
      content: json['content'] as String,
    );
  }
}

source_model

  String? id;
  String? name;

  Source({this.id, this.name});

  factory Source.fromJson(Map<String, dynamic> json) {
    return Source(id: json['id'], name: json['name']);
  }
}

Answer 1

在您的模型上，所有变量都是 String? 类型，也就是说，变量将是 String 或 null。但是当你制作模型时，你像这样分配所有变量：variable: json['variable'] as String，这意味着如果它们是 null 他们会抛出错误，因为你做的是 as String，而不是 as String?，这就是错误的来源。

要解决此问题，我相信应该可以完全删除 as String，如下所示：

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

上面的方法可能不起作用，如果不起作用，则应该：

factory Article.fromJson(Map<String, dynamic> json) {
    return Article(
      source: Source.fromJson(json['source']),
      author: json['author'] as String?,
      title: json['title'] as String?,
      description: json['description'] as String?,
      url: json['url'] as String?,
      urlToImage: json['urlToImage'] as String?,
      publishedAt: json['publishedAt'] as String?,
      content: json['content'] as String?,
    );
  }

Flutter/dart 将数据格式化为实例无效

Flutter/dart formatting data to instances not working

api

formatting

http

dart

flutter