firebase - 多提供者无效成员为null:提供者构造上的 'collection'

Question

收到的错误:

NoSuchMethodError: invalid member on null: 'collection'

我正在使用 Flutter Provider Package 4.3.2+2进行复杂的状态管理。
我从Firebase获取数据，这是数据结构:

这是提供程序类:

class ClientsProvider extends ChangeNotifier {

  FirebaseFirestore _fs;
  StreamSubscription<QuerySnapshot> _stream;
  List<QueryDocumentSnapshot> clients = [];
  ClientsProvider() {
    _stream = _fs.collection("clients").snapshots().listen((event) {//THIS IS WHERE THE ERROR POINTS 
      clients = event.docs;
      notifyListeners();
    });
  }

  @override
  void dispose() {
    super.dispose();
    _stream.cancel();
  }
}

这是父窗口小部件:

class _ParentState extends State<Parent> {
  
  @override
  Widget build(BuildContext context) {
        return MultiProvider(
          providers: [
            Provider<ClientsProvider>(
                create: (context) => ClientsProvider()),
            ChangeNotifierProvider<MyProvider>(
                create: (context) => MyProvider()),
          ],
          child: Dashboard(),//This widget contains the Child() widget two widgets down. 
        );
  }
}

这是Firebase快照更新时需要更新的子窗口小部件:

class _ChildState extends State<Child> {

  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        _buildAddNew(),
        Consumer<ClientsProvider>(
          builder: (context, clientProvider, child) {
            return Container(child: Text(clientProvider.clients.toString()));
          },
        ),
      ],
    );
  }
}

再次收到错误:

NoSuchMethodError: invalid member on null: 'collection'

数据不为空，为什么我会收到此错误？

Answer 1

_fs为空，因为您从未分配它。也许您打算这样做，如documentation所示:

FirebaseFirestore _fs = FirebaseFirestore.instance;