类型'_InternalLinkedHashMap <String，dynamic>'不是类型转换中类型'TodoModel'的子类型

Question

嗨，我是Flutter的新手，遇到了问题。 我想将从Firestore提取的数据添加到List类型的变量中，但出现错误。我花了几个小时来解决这个问题，但效果不佳。

有人知道我怎么得到这个错误吗？

toto_model.dart

class TodoModel {
  String _title = '';
  bool _done = false;
  int _id = 1;
  String _docId = '';

  TodoModel(String title, bool done, int id, String docId) {
    this._title = title;
    this._done = done;
    this._id = id;
    this._docId = docId;
  }

  String get title => _title;
  bool get done => _done;
  int get id => _id;
  String get docId => _docId;
}

todo_provider.dart

class TodoProvider {
  List<TodoModel> _todoList = [];

  Future<List<TodoModel>> fetchToDo() async {
    final result = await Firestore.instance.collection('todos').getDocuments();
    final List<DocumentSnapshot> documents = result.documents;
    documents.forEach((document) {
      var data = {
        'title': document.data['title'],
        'done': document.data['done'],
        'id': document.data['id'],
        'docId': document.documentID,
      };

      _todoList.add(data as TodoModel);
    });

    return _todoList;
  }
}

错误

E/flutter ( 5859): [ERROR:flutter/shell/common/shell.cc(199)] Dart Error: Unhandled exception:
E/flutter ( 5859): type '_InternalLinkedHashMap<String, dynamic>' is not a subtype of type 'TodoModel' in type cast
E/flutter ( 5859): #0 TodoProvider.fetchToDo.<anonymous closure> (package:flutterbloc/resources/todo_provider.dart:21:26)
E/flutter ( 5859): #1 List.forEach (dart:core-patch/growable_array.dart:285:8)
E/flutter ( 5859): #2 TodoProvider.fetchToDo (package:flutterbloc/resources/todo_provider.dart:13:15)
E/flutter ( 5859): <asynchronous suspension>

Answer 1

您不能在Dart中创建一个Map，然后将其投射到给定的对象中。您需要调用要从中获取给定对象的类的构造函数。因此，在您情况下应该是：

      _todoList.add(TodoModel(
          document.data['title'] as String,
          document.data['done'] as bool,
          document.data['id'] as int,
          document.documentID as String));

我还想补充一点，您当前的TodoModel类可以简化为：

class TodoModel {
  final String title;
  final bool done;
  final int id;
  final String docId;

  TodoModel(this.title, this.done, this.id, this.docId);
}

如果您只想授予对变量的读取权限，并且它永远不会改变，则可以将其标记为最终值，并让公众看到。就像只为该变量做一个吸气剂一样。

Dart还支持构造函数参数，该参数直接指向应初始化的类的变量。同样，我们可以这样做使其更简单。