Flutter'未处理的异常：类型'List<dynamic>'不是'List<class>'类型的子类型

Question

我正在从 API 获取数据，并尝试将这些数据推送到两个变量中。我可以访问返回的部分数据，但不能访问全部，我可以打印所有数据，但是当我尝试访问其中的特定部分时，它不起作用。

例如： res = 来自 API 的响应

print(res.data); // This works
print(res.feeds); // This doesn't work
print(res.feeds.id); // This doesn't work

我得到的错误是：

'Unhandled Exception: type 'List<dynamic>' is not a subtype of type 'List<FeedItem>'

这就是我转换和读取数据的方式：

class FeedData {
  String data;
  List<FeedItem> feeds;

  FeedData({this.data, this.feeds});

  factory FeedData.fromJson(Map<String, dynamic> json) {
    return FeedData(
      data: json['data'],
      feeds: json['feeds'],
    );
  }
}

我可以读取和访问“数据”，但不能读取和访问“提要”，这是我的问题，我需要能够访问“提要”中的数据 'FeedItem' 类的构建方式与 'FeedData' 类相同，所以我确信它是正确构建的

这就是我调用数据的方式：

getUserFeed() async {
    FeedData res = FeedData.fromJson(
        await requests.get(url: 'feeds/u?u=jxchumber&PageNumber=1'));
    print(res.data);
    // res.data works
    print(res.feeds);
    // res.feeds doesn't work
  }

这是从 API 返回的数据：

{
    "data": "default",
    "feeds": [
        {
            "id": 2,
            "photoUrl": "https://cartalkio-image-storage-dev.s3.eu-west-2.amazonaws.com/o85DvOaTXXE.jpg",
            "description": "Magna duis consectetur sit ut commodo non eiusmod.",
            "dateAdded": "0001-01-01T00:00:00",
            "isMain": false,
            "publicId": null,
            "isImage": true,
            "mainImage": "https://randomuser.me/api/portraits/men/76.jpg",
            "userId": 9,
            "likers": null,
            "username": "Larsen",
            "thumbnail": null,
            "likes": 0
        },
        {
            "id": 4,
            "photoUrl": "https://cartalkio-image-storage-dev.s3.eu-west-2.amazonaws.com/lXR833PRh3g.jpg",
            "description": "Magna duis consectetur sit ut commodo non eiusmod.",
            "dateAdded": "0001-01-01T00:00:00",
            "isMain": false,
            "publicId": null,
            "isImage": true,
            "mainImage": "https://randomuser.me/api/portraits/men/4.jpg",
            "userId": 8,
            "likers": null,
            "username": "Lloyd",
            "thumbnail": null,
            "likes": 0
        },
}

Answer 1

错误是因为您没有将 json['feeds'] 作为列表传递。您的 FeedData 类必须更改为以下内容，这是假设您的 FeedItem 类也有 fromJson 方法：

class FeedData {
  String data;
  List<FeedItem> feeds;

  FeedData({this.data, this.feeds});

  factory FeedData.fromJson(Map<String, dynamic> json) {
    return FeedData(
      data: json['data'],
      feeds: List.from(json['feeds']).map((item)=>FeedItem.fromJson(item)).toList(),
    );
  }
}

请根据您的数据查看工作代码。也可以在DartPadhttps://dartpad.dev/24694a911ddd0619277f5780573086f1上加载如下代码

void main() {

  Map<String, dynamic> json = {
    "data": "default",
    "feeds": [
        {
            "id": 2,
            "photoUrl": "https://cartalkio-image-storage-dev.s3.eu-west-2.amazonaws.com/o85DvOaTXXE.jpg",
            "description": "Magna duis consectetur sit ut commodo non eiusmod.",
            "dateAdded": "0001-01-01T00:00:00",
            "isMain": false,
            "publicId": null,
            "isImage": true,
            "mainImage": "https://randomuser.me/api/portraits/men/76.jpg",
            "userId": 9,
            "likers": null,
            "username": "Larsen",
            "thumbnail": null,
            "likes": 0
        },
        {
            "id": 4,
            "photoUrl": "https://cartalkio-image-storage-dev.s3.eu-west-2.amazonaws.com/lXR833PRh3g.jpg",
            "description": "Magna duis consectetur sit ut commodo non eiusmod.",
            "dateAdded": "0001-01-01T00:00:00",
            "isMain": false,
            "publicId": null,
            "isImage": true,
            "mainImage": "https://randomuser.me/api/portraits/men/4.jpg",
            "userId": 8,
            "likers": null,
            "username": "Lloyd",
            "thumbnail": null,
            "likes": 0
        },
      ]
};
  
  
  FeedData feedData = FeedData.fromJson(json);
  print(feedData.feeds.length);
}


class FeedData {
  String data;
  List<FeedItem> feeds;

  FeedData({this.data, this.feeds});

  factory FeedData.fromJson(Map<String, dynamic> json) {
    return FeedData(
      data: json['data'],
      feeds: List.from(json['feeds']).map((item)=>FeedItem.fromJson(item)).toList(),
    );
  }
}


class FeedItem {
  int id;
  String photoUrl;
  String description;
  String dateAdded;
  bool isMain;
  Null publicId;
  bool isImage;
  String mainImage;
  int userId;
  Null likers;
  String username;
  Null thumbnail;
  int likes;

  FeedItem(
      {this.id,
      this.photoUrl,
      this.description,
      this.dateAdded,
      this.isMain,
      this.publicId,
      this.isImage,
      this.mainImage,
      this.userId,
      this.likers,
      this.username,
      this.thumbnail,
      this.likes});

  FeedItem.fromJson(Map<String, dynamic> json) {
    id = json['id'];
    photoUrl = json['photoUrl'];
    description = json['description'];
    dateAdded = json['dateAdded'];
    isMain = json['isMain'];
    publicId = json['publicId'];
    isImage = json['isImage'];
    mainImage = json['mainImage'];
    userId = json['userId'];
    likers = json['likers'];
    username = json['username'];
    thumbnail = json['thumbnail'];
    likes = json['likes'];
  }
}

Answer 2

当你在做feeds: json['feeds']时，你基本上是在尝试将Map<String,dynamic>分配给一个类FeedItem，而且你想将一个完整的列表分配给List<Map<String,dynamic>>到List<FeedItem，问题是颤动就是不能自动解析数据并像这样赋值，需要逐级深入，将Map<String,dynamic>赋值给FeedItem，最后转化为List

我将根据我的知识分享一段代码，我是如何解决这类问题的，我相信这会让你更好地了解你的问题（如果没有解决它）

Future<void> getData() async {
http.Response response =
    await http.get('Example address');
if (response.statusCode == HttpStatus.ok) {
//if the status code is 200 or 'OK' parse the data and convert Outer layer as List
//after that , Map each element as a FeedItem and finally convert the result to List
  feeds = (json.decode(response.body) as List)
      .map<FeedItem>(
          (_feed) => FeedItem.fromJSON(_feed))
      .toList();
} else {
  throw response;
}

}

Answer 3

您没有将 feedItems 转换为对象

  factory FeedData.fromJson(Map<String, dynamic> json) {
    return FeedData(
      data: json['data'],
      feeds: json['feeds'],  # <- here is the problem
    );

应该是这样的：

feeds: (json['feeds'] as List)
        ?.map((e) =>
            e == null ? null : FeedItem.fromJson(e as Map<String, dynamic>))
        ?.toList(),

Answer 4

请更改此代码

print(res.feeds);    -->  print(res.feeds[0]);
print(res.feeds.id); --> print(res.feeds[0].id);