Flutter 错误：期望一个类型为“Map<String, dynamic>”的值，但出现了一个类型为“List<dynamic>”的值

Posted 2023-02-23

【中文标题】Flutter 错误：期望一个类型为“Map<String, dynamic>”的值，但出现了一个类型为“List<dynamic>”的值

【英文标题】：Flutter Error: Expected a value of type ‘Map<String, dynamic>', but got one of type ‘List<dynamic>' occured

【发布时间】：2021-07-29 02:44:05

【问题描述】：

我正在使用http：^0.13.3， 这是我的简单代码，当我运行此代码时出现错误：“预期类型为 'Map'，但出现类型为 'List' 的值之一”，我不知道为什么收到此错误。如果有任何其他方法可以做到这一点，请告诉我。

class SubCategoryObject 
  final int id;
  final String category;

  SubCategoryObject(@required this.id, @required this.category);

  factory SubCategoryObject.fromJson(Map<String, dynamic> json) 
    return SubCategoryObject(
      id: json['id'],
      category: json['title'],
    );
  

class SubCategoryListScreen extends StatelessWidget 
  static const route = 'sub-category-list';
  SubCategoryListScreen(Key key) : super(key: key);

  Future<SubCategoryObject> getData() async 
    final res =
        await http.get(Uri.https("jsonplaceholder.typicode.com", "todos"));

    if (res.statusCode == 200) 
      return SubCategoryObject.fromJson(jsonDecode(res.body));
     else 
      // If that call was not successful, throw an error.
      throw Exception('Failed to load ');
    
  

  @override
  Widget build(BuildContext context) 
    return Scaffold(
      appBar: AppBar(
        title: Text("martman"),
      ),
      body: FutureBuilder<SubCategoryObject>(
        future: getData(),
        builder: (ctx, snapshot) 
          // Checking if future is resolved or not
          if (snapshot.connectionState == ConnectionState.done) 
            // If we got an error
            if (snapshot.hasError) 
              return Center(
                child: Text(
                  '$snapshot.error occured',
                  style: TextStyle(fontSize: 18),
                ),
              );

              // if we got our data
             else if (snapshot.hasData) 
              return Text(snapshot.data.category);
            
          

          // Displaying LoadingSpinner to indicate waiting state
          return Center(
            child: CircularProgressIndicator(),
          );
        ,

        
      ),
    );
  

【问题讨论】：

打印并分享您从 api 获取的数据的屏幕截图。我相信这是一个列表而不是地图。

【参考方案1】：

请检查以下链接。它返回列表，而不是地图数据。 https://jsonplaceholder.typicode.com/todos

Future<List<SubCategoryObject>> getData() async 
    final res =
        await http.get(Uri.https("jsonplaceholder.typicode.com", "todos"));

    if (res.statusCode == 200) 
      dynamic result = jsonDecode(res.body);
      return (result as List).map((e) => SubCategoryObject.fromJson(e)).toList();
     else 
      // If that call was not successful, throw an error.
      throw Exception('Failed to load ');