无法从 Flutter 中的 Firebase Firestore 返回列表

Question

我目前正在 Flutter 中开发一个测验应用程序。 我正在从 firebase 获取数据并成功创建了包含我所需数据的地图列表。

但问题是我正在对snapshot.docs执行forEach循环并将值添加到空的AllQuestionsOfQuiz列表中，但我无法从函数中返回它。

我不知道我在这里缺少的是代码：

import 'dart:convert';

import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:kbc/screen/question.dart';

class QuizCreator {

static List AllQuestionsOfQuiz = [];

  static Future<List<String>> fetchOptions(
      QueryDocumentSnapshot<Map<String, dynamic>> questionSnap) async {
    List<String> queOptions = [];
    await questionSnap.reference
        .collection("options")
        .get()
        .then((value) async {
      value.docs.forEach((element) {
        queOptions.add(element.data()["opt1"]);
        queOptions.add(element.data()["opt2"]);
        queOptions.add(element.data()["opt3"]);
        queOptions.add(element.data()["opt4"]);
      });
    });
    return queOptions;
  }

  static Future<Map> fetchQuestion(
      QueryDocumentSnapshot<Map<String, dynamic>> questionSnap,
      String QuePrize) async {
    Map questionNOpt = {};
    String question;
    await questionSnap.reference
        .collection("questions")
        .get()
        .then((value) async {
      question = value.docs.elementAt(0)["question"];
      questionNOpt["prize"] = QuePrize;
      questionNOpt["question"] = question;
      questionNOpt["options"] = await fetchOptions(value.docs.elementAt(0));
    });
    return questionNOpt;
  }

  static Future<List> quizCreator(String quizID) async {
 List QuestionOfQuizzes = [];
    await FirebaseFirestore.instance
        .collection("quizzes")
        .doc(quizID)
        .collection("prizes")
        .orderBy("prize", descending: false)
        .snapshots()
        .forEach((snapshot)  {
      snapshot.docs.forEach((element) async  {
         await fetchQuestion(element, element.data()["prize"].toString())
            .then((value)async {
           AllQuestionsOfQuiz.add(value);

          
        });
print(AllQuestionsOfQuiz);
      });
      
    });

  return AllQuestionsOfQuiz;
  }








}

通过这段代码，我在终端中得到以下输出：

I/flutter ( 8913): [{prize: 20000, question: How many workers was there to create the TAL MAHAL?, options: [60000, 70888, 75600, 789000]}]
I/flutter ( 8913): [{prize: 20000, question: How many workers was there to create the TAL MAHAL?, options: [60000, 70888, 75600, 789000]}, {prize: 10000, question: Who was awarded by the Padma Shri In The Field Of Physics ?, options: [Tulasi Gowda, Sujoy K. Guha, Harekala Hajabba, HC Verma]}]
I/flutter ( 8913): [{prize: 20000, question: How many workers was there to create the TAL MAHAL?, options: [60000, 70888, 75600, 789000]}, {prize: 10000, question: Who was awarded by the Padma Shri In The Field Of Physics ?, options: [Tulasi Gowda, Sujoy K. Guha, Harekala Hajabba, HC Verma]}, {prize: 5000, question: Who is Jethalal in TMOCK ?, options: [Husband of daya, Param Mitra Of Mehta, Son of bapuji, Father of tapu]}]

我希望在我的函数quizCreator返回最后一个迭代值：

[{prize: 20000, question: How many workers was there to create the TAL MAHAL?, options: [60000, 70888, 75600, 789000]}, {prize: 10000, question: Who was awarded by the Padma Shri In The Field Of Physics ?, options: [Tulasi Gowda, Sujoy K. Guha, Harekala Hajabba, HC Verma]}, {prize: 5000, question: Who is Jethalal in TMOCK ?, options: [Husband of daya, Param Mitra Of Mehta, Son of bapuji, Father of tapu]}]

请帮助我，我在过去 2 天一直被这个错误困住。

Answer 1

查看文档，有两种方法可以检索存储在Cloud Firestore 中的数据。 这些方法中的任何一种都可以用于文档、文档集合或查询结果。

当您使用get()时，您只能“获取单个文档的内容”一次。 这是一种“一劳永逸”的情况：如果（后端）Firestore 数据库中的文档发生变化，您将不得不再次执行get()才能看到它。

另一方面，如果您使用onSnapshot()方法，您将一直在收听文档，如文档中所述

您可以使用Promise来包围 forEach 循环，如果您希望在forEach()循环中更新多个文档，也需要使用Promise.all 。

您也可以使用以下语法返回 Promise.all()：

.... return Promise.all(promises); }) });