Firebase 如何在映射函数中等待我的未来函数？（颤振）

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TestProvider类中做的是从firestore获取数据（getQuestionFromFirebase（）函数正在做什么），然后，我想从DocumentSnapshot创建一个映射（questionMapFromFirebase（）函数正在做什么）。出现了一个错误，因为我不能在map函数中异步，所以我的函数不会等待前一个函数的结果，并返回null。有什么解决办法吗*我试图从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;
  }
}

还有其他建议吗？我该怎么办

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

您的

\u loadQuestions

功能如下

_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
}

\u loadQuestions（）异步{
列表范围=numberRange（amountOquestions）；
问题列表=[]；
对于（int i=1；我喜欢我在*中所说的，它必须是一个纯映射，因为稍后我会得到：
参数类型“Future”不能分配给参数类型“map”。
error您将在哪里使用这个纯映射？有很多方法可以获得实际结果，而不是包装到Future中。我扩展了我的问题。l将显示istOfQuestions？是此列表将被使用
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();
  }
}

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;
  }
}

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")

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

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

_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
}