_MapStream<QuerySnapshot<Map<String,dynamic>>,List<UserModel>?>引發例外

我正在嘗試從 firestore 資料庫中列出一些資料。在提取資料時，我得到了這個例外：

======== Exception caught by provider ==============================================================
The following assertion was thrown:
An exception was throw by _MapStream<QuerySnapshot<Map<String, dynamic>>, List<UserModel>?> listened by

StreamProvider<List<UserModel>?>, but no `catchError` was provided.

Exception:
type 'int' is not a subtype of type 'double?'

====================================================================================================

這些是我的資料庫功能：

  // users list from snapshot
  List<UserModel>? _usersListFromSnapshot(QuerySnapshot snapshot) {
    return snapshot.docs.map((doc) {
      return UserModel(
        uid: doc.get('uid'),
        username: doc.get('username') ?? '',
        email: doc.get('email') ?? '',
        type: doc.get('type') ?? '',
        balance: doc.get('balance'),
        usertag: doc.get('usertag') ?? '',
        connected: doc.get('connected') ?? '',
      );
    }).toList();
  }

  // get users stream
  Stream<List<UserModel>?> get usersList {
    User? user = _auth.currentUser;
    UserModel? userModel = _userFromFirebaseUser(user);
    final CollectionReference userRef = FirebaseFirestore.instance.collection("users");
    final List? connectedList = userModel?.connected;
    final Query inList = userRef.where('uid', whereIn: connectedList);
    return inList.snapshots().map(_usersListFromSnapshot);
  }

這是我的 user.dart：

class UserModel {
  String? uid;
  String? username;
  String? email;
  String? type;
  double? balance;
  String? usertag;
  List<dynamic>? connected;

  UserModel({this.uid, this.username, this.email, this.type, this.balance, this.usertag, this.connected});

  // receive data from the server
  factory UserModel.fromMap(map) {
    return UserModel(
      uid: map['uid'],
      username: map['username'],
      email: map['email'],
      type: map['type'],
      balance: map['balance'],
      usertag: map['usertag'],
      connected: map['connected']
    );
  }

  // send data to the server
  Map<String, dynamic> toMap() {
    return {
      'uid': uid,
      'username': username,
      'email': email,
      'type': type,
      'balance': balance,
      'usertag': usertag,
      'connected': connected,
    };
  }
}

這是我的串列生成器：

class ConnectedScreen extends StatefulWidget {
  const ConnectedScreen({Key? key}) : super(key: key);

  @override
  _ConnectedScreenState createState() => _ConnectedScreenState();
}

class _ConnectedScreenState extends State<ConnectedScreen> {

  User? user = FirebaseAuth.instance.currentUser;
  UserModel userModel = UserModel();

  @override
  void initState() {
    super.initState();
    FirebaseFirestore.instance
        .collection('users')
        .doc(user!.uid)
        .get()
        .then((value) {
      userModel = UserModel.fromMap(value.data());
      setState(() {});
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: customAppBar("Connect Users", "back", ""),
      body: Padding(
        padding: EdgeInsets.all(8.0),
        child: SingleChildScrollView(
          child: StreamProvider<List<UserModel>?>.value(
            value: AuthService().usersList,
            initialData: [],
            child: ConnectedListBuilder(),
          ),
        ),
      ),
    );
  }
}


class ConnectedListBuilder extends StatefulWidget {
  const ConnectedListBuilder({Key? key}) : super(key: key);

  @override
  _ConnectedListBuilderState createState() => _ConnectedListBuilderState();
}

class _ConnectedListBuilderState extends State<ConnectedListBuilder> {
  @override
  Widget build(BuildContext context) {

    final connectedUsers = Provider.of<List<UserModel>?>(context);

    if (connectedUsers!.isNotEmpty) {
      return Padding(
        padding: EdgeInsets.only(top: 8.0),
        child: ListView.builder(
          itemCount: connectedUsers.length,
          itemBuilder: (context, index) {
            return ConnectedUserTile(userModel: connectedUsers[index]);
          },
        ),
      );
    } else if (connectedUsers.isEmpty) {
      return Text("List is empty");
    }
    return Center(
      child: CircularProgressIndicator(),
    );
  }
}


class ConnectedUserTile extends StatelessWidget {

  final UserModel? userModel;

  const ConnectedUserTile({Key? key, required this.userModel}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: EdgeInsets.all(8.0),
      child: Card(
        margin: EdgeInsets.fromLTRB(20, 6, 20, 0),
        child: ListTile(
          leading: CircleAvatar(
            backgroundImage: NetworkImage('https://picsum.photos/100'),
            radius: 25,
          ),
          title: Text(
            "${userModel!.username}",
            style: TextStyle(
              color: Colors.black54,
              fontSize: 18,
              fontWeight: FontWeight.bold,
            ),
          ),
          subtitle: Text(
            "${userModel!.usertag}",
            style: TextStyle(
              color: Colors.black38,
              fontSize: 14,
            ),
          ),
        ),
      ),
    );
  }
}

有趣的是，這個確切的代碼適用于另一個 ListView，所以我不認為這是資料庫函式的錯誤，但我不知道如何嘗試修復它。我也沒有在互聯網上找到太多有用的資訊，所以我會非常感謝任何幫助。

uj5u.com熱心網友回復：

看起來問題在于決議平衡欄位。在它作業的情況下，余額可能被格式化為雙精度，例如“1.23”，因此當它轉換為地圖時，它被轉換為 a double，它是 的子型別double?。

當它不作業時，它很可能處理整數余額，例如“25” - 然后存盤在映射中的值將具有型別int，它不是 的子型別double?，因為int不是 的子型別double。

要解決此問題，您可以簡單地通過呼叫toDouble()該值來確保始終處理雙精度數。

// receive data from the server
factory UserModel.fromMap(map) {
  return UserModel(
    uid: map['uid'],
    username: map['username'],
    email: map['email'],
    type: map['type'],
    balance: map['balance'].toDouble(), // <- Change this line
    usertag: map['usertag'],
    connected: map['connected']
  );
}

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