我的 StreamBuilder 包含两个对我来说无法解决的错误

Question

我正在尝试构建一个待办事项应用程序。（学习扑动）这是代码中出现两个错误的部分：

class ToDo1 extends StatefulWidget {
  @override
  _ToDo1State createState() => _ToDo1State();
}

class _ToDo1State extends State<ToDo1> {

  var User;
  late DatabaseService database;

  Future<void> connectToFirebase() async{
    await Firebase.initializeApp();
    final FirebaseAuth auth = FirebaseAuth.instance;
    UserCredential result = await FirebaseAuth.instance.signInAnonymously();
    User = result.user;
    database = DatabaseService(User.uid);

if (!(await database.checkIfUserExists())) {
      database.setTodo('To-Do anlegen', false);
    }
  }

  void toggleDone(String key, bool value) {
    database.setTodo(key, !value);
    }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Center(
            child: Text(
          'Stufe 1',
          style: TextStyle(
              fontStyle: FontStyle.italic,
              decoration: TextDecoration.underline),
        )),
        backgroundColor: Color.fromRGBO(35, 112, 192, 1),
      ),
      body: FutureBuilder(
        future: connectToFirebase(),
            builder: (BuildContext context, AsyncSnapshot snapshot) {
          if (snapshot.connectionState == ConnectionState.waiting) {
            return Center(child: CircularProgressIndicator());
          } else {
            return StreamBuilder<DocumentSnapshot> (
              stream: database.getTodos(),
              builder: (context, AsyncSnapshot<DocumentSnapshot> snapshot) {
                if(!snapshot.hasData) {
                  return CircularProgressIndicator();
                } else {
                  Map<String, dynamic> items = snapshot.data.data;
                  return ListView.separated(
                      separatorBuilder: (BuildContext context, int index) {
                        return SizedBox(
                          height: 10,
                        );
                      },
                      padding: EdgeInsets.all(10),
                      itemCount: items.length,
                      itemBuilder: (BuildContext, i) {
                        String key = items.keys.elementAt(i);
                        return ToDoItem(
                          key,
                          items[key]!,
                              () => toggleDone(key, items[key]),
                        );
                      });
                }
              }
            );
          }
            },
      )
    );
  }
}

这是红色下划线：

Stream: database.getTodos()

还有这个：

snapshot.data.data

这是与 Firebase 交互的 class：

class DatabaseService {
  final String userID;
  DatabaseService(this.userID);

  final CollectionReference userTodos =
      FirebaseFirestore.instance.collection('userTodos');

  Future setTodo(String item, bool value) async {
    return await userTodos.doc(userID).set(
      {item:value}, SetOptions(merge: true));
  }

    Future deleteTodo(String key) async {
      return await userTodos.doc(userID).update(
        {key: FieldValue.delete(),}
      );
    }

    Future checkIfUserExists() async {
    if((await userTodos.doc(userID).get()).exists) {
      return true;
    }
      else {
        return false;
      }
    }

    Stream getTodos() {
    return userTodos.doc(userID).snapshots();
  }
}

最后，这是错误：

Error: Property 'data' cannot be accessed on 'DocumentSnapshot<Object?>?' because it is potentially null.
 - 'DocumentSnapshot' is from 'package:cloud_firestore/cloud_firestore.dart' ('/C:/flutter/.pub->cache/hosted/pub.dartlang.org/cloud_firestore-2.5.1/lib/cloud_firestore.dart').
 - 'Object' is from 'dart:core'.
Try accessing using ?. instead.
                  Map<String, dynamic> items = snapshot.data.data;
                                                             
lib/Pages/Home_Page.dart:347:62: Error: A value of type 'Object? Function()' can't be assigned to a variable of type 'Map<String, dynamic>'.
 - 'Object' is from 'dart:core'.
 - 'Map' is from 'dart:core'.
                  Map<String, dynamic> items = snapshot.data.data;
                                                             ^
lib/Pages/Home_Page.dart:342:32: Error: The argument type 'Stream<dynamic>' can't be assigned to the parameter type 'Stream<DocumentSnapshot<Object?>>?'.
 - 'Stream' is from 'dart:async'.
 - 'DocumentSnapshot' is from 'package:cloud_firestore/cloud_firestore.dart' ('/C:/flutter/.pub->cache/hosted/pub.dartlang.org/cloud_firestore-2.5.1/lib/cloud_firestore.dart').
 - 'Object' is from 'dart:core'.
              stream: database.getTodos(),
                               ^


FAILURE: Build failed with an exception.

* Where:
Script 'C:\flutter\packages\flutter_tools\gradle\flutter.gradle' line: 1035

* What went wrong:
Execution failed for task ':app:compileFlutterBuildDebug'.
 Process 'command 'C:\flutter\bin\flutter.bat'' finished with non-zero exit value 1

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

BUILD FAILED in 14s
Exception: Gradle task assembleDebug failed with exit code 1

希望我已经提供了所有必要的数据，以便可以解决这两个问题。如果没有，请写信给我，我会尽力将您需要的 material 发送给您。

Answer 1

Stream: database.getTodos() 的第一个问题是您的 getTodos() 方法需要 return 类型，例如 Stream<DocumentSnapshot<Object?>>? getTodos().

snapshot.data.data 的第二个问题是 snapshot.data 可以是 null，您无法访问最终可能是 null 的内容。但是，由于您已经在 !snapshot.hasData 中检查了它，这似乎是误报，因此您可以通过分配值 snapshot.data!.data 来消除它的可空性，! 告诉编译器在此点 snapshot.data 不能有 null 值。

我的 StreamBuilder 包含两个对我来说无法解决的错误

My StreamBuilder contains two for me unsolvable errors

firebase

flutter

google-cloud-firestore

flutter-test