firebase - 我怎样才能在 map 功能中等待我 future 的功能？ ( flutter )

Question

所以，我在 Flutter 中构建我的应用程序，不幸的是，我最近遇到了一个错误。所以我想在我的 TestProvider 类中做的是从 firestore 获取数据(getQuestionFromFirebase() 函数在做什么)，在那之后，我想从 DocumentSnapshot 创建一个 map (questionMapFromFirebase() 函数在做什么)。并且出现错误，因为我不能在 map 函数中异步，所以我的函数不等待前一个函数的结果，而是返回 null。任何解决方案？ *我尝试从 getQuestionFromFirebase() - Future 返回 map ，但后来我无法使用它的值，因为我的函数需要纯 map 。

 class TestProvider {
  FirebaseFirestore _firestore = FirebaseFirestore.instance;

  Future<DocumentSnapshot> getQuestionFromFirebase(String documentId) async {
    return await _firestore.collection('questions').doc(documentId).get();
  }

  Map questionMapFromFirebase(String documentId) {
    Map questionMapFromFirebase;
    getQuestionFromFirebase(documentId).then((DocumentSnapshot carSnapshot) => {
          questionMapFromFirebase = carSnapshot.data(),
        });
    return questionMapFromFirebase;
  }
}

后来我在那里使用这个函数:我稍后会在那里使用此功能

List<Question> listOfQuestions() {
    List<int> range = numberInRange(amountOfQuestions);
    List<Question> listOfQuestions;
    for (int i = 1; i <= amountOfQuestions; i++) {
      listOfQuestions.add(Question.fromMap(
          _testProvider.questionMapFromFirebase(range[1].toString())));
    }
    return listOfQuestions;
  }

这会在 Future 发生时产生错误。

 The argument type 'Future<Map<dynamic, dynamic>>' can't be assigned to the parameter type 'Map<String, dynamic>'.

编辑:所以最近我对我的代码做了一些更改，现在看起来像这样

class TestProvider {
  FirebaseFirestore _firestore = FirebaseFirestore.instance;

  Future<DocumentSnapshot> getQuestionFromFirebase(String documentId) async {
    return await _firestore.collection('questions').doc(documentId).get();
  }

  Future<Map> questionMapFromFirebase(String documentId) async {
    DocumentSnapshot ds = await getQuestionFromFirebase(documentId);
    return ds.data();
  }
}

和存储库

class TestRepository {
  final int amountOfQuestions;
  TestRepository({
    @required this.amountOfQuestions,
  });

  TestProvider _testProvider;

  Future listOfQuestions() async {
   List<int> range = numberInRange(amountOfQuestions);
   List<Question> listOfQuestions;
   for (int i = 1; i <= amountOfQuestions; i++) {
    listOfQuestions.add(Question.fromMap(
      await _testProvider.questionMapFromFirebase(range[i].toString())));
   }
  return listOfQuestions;
  }
}

我开始看到的问题是，每次我尝试从 TestProvider 调用函数 questionMapFromFirebase 时，它​​都运行良好。但是当我试图从 TestRepository 调用它时，它抛出了错误:

E/flutter (13348): [ERROR:flutter/lib/ui/ui_dart_state.cc(177)] Unhandled Exception: NoSuchMethodError: The method 'questionMapFromFirebase' was called on null.
E/flutter (13348): Receiver: null
E/flutter (13348): Tried calling: questionMapFromFirebase("2")

还有其他问题我该如何处理？

Answer 1

Future<Map> questionMapFromFirebase(String documentId) async {
   DocumentSnapshot ds = await getQuestionFromFirebase(documentId);
   return ds.data();
  }

编辑检查FutureBuilder类

例如，它将在您的小部件树中需要显示列表的地方。

return FutureBuilder(
  future: _loadQuestions(),
  builder: (context, snapshot) {
    if(snapshot.connectionState == ConnectionState.done){
      return widgetForListing(snapshot.data);
    }
    return Center(child: Text('Loading...'));
  },
);

你的_loadQuestions 函数将是

_loadQuestions() async {
    List<int> range = numberInRange(amountOfQuestions);
    List<Question> listOfQuestions = [];
    for (int i = 1; i <= amountOfQuestions; i++) {
        listOfQuestions.add(Question.fromMap(
      await _testProvider.questionMapFromFirebase(range[1].toString())));
    }
    return listOfQuestions; //you can get this list in **snapshot.data** of future builder
}