我正在使用Flutter & Firebase创建一个测验应用程序。我已经在Firebase实时数据库中添加了150个问题，并在测验应用程序中获取了所有问题。但应用程序以相同的顺序显示所有添加的问题。我希望每次从我在Firebase实时数据库中添加的问题中生成20个随机问题。我如何实现这一点？
编码：

import 'package:flutter/material.dart';
import '../constants.dart';
import '../models/question_model.dart';
import '../widgets/question_widget.dart';
import '../widgets/next_button.dart';
import '../widgets/option_card.dart';
import '../widgets/result_box.dart';
import '../models/db_connect.dart';

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

  @override
  State<TestScreen> createState() => _TestScreenState();
}

class _TestScreenState extends State<TestScreen> {
  var db = DBconnect();
  late Future _questions;
  Future<List<Question>> getData() async {
    return db.fetchQuestions();
  }

  @override
  void initState() {
    _questions = getData();
    super.initState();
  }

  int index = 0;
  int score = 0;
  bool isPressed = false;
  bool isAlreadySelected = false;

  void nextQuestion(int questionLength) {
    if (index == questionLength - 1 || score == 12) {
      showDialog(
          context: context,
          barrierDismissible: false,
          builder: (ctx) => ResultBox(
                result: score,
                questionLength: questionLength,
              ));
    } else {
      if (isPressed) {
        setState(() {
          index++;
          isPressed = false;
          isAlreadySelected = false;
        });
      } else {
        ScaffoldMessenger.of(context).showSnackBar(SnackBar(
          content: const Text('Please select any option'),
          behavior: SnackBarBehavior.floating,
          margin: EdgeInsets.symmetric(vertical: 20.0),
        ));
      }
    }
  }

  void checkAnswerAndUpdate(bool value) {
    if (isAlreadySelected) {
      return;
    } else {
      if (value == true) {
        score++;
        setState(() {
          isPressed = true;
          isAlreadySelected = false;
        });
      } else if (value == false) {
        setState(() {
          isPressed = true;
          isAlreadySelected = false;
        });
      }
    }
  }

  void startOver() {
    setState(() {
      Text('You have already attempted the LL Test');
    });
  }

  @override
  Widget build(BuildContext context) {
    return FutureBuilder(
      future: _questions as Future<List<Question>>,
      builder: (ctx, snapshot) {
        if (snapshot.connectionState == ConnectionState.done) {
          if (snapshot.hasError) {
            return Center(
              child: Text('${snapshot.error}'),
            );
          } else if (snapshot.hasData) {
            var extractedData = snapshot.data as List<Question>;
            return Scaffold(
              backgroundColor: background,
              appBar: AppBar(
                title: const Text('LL Test'),
                backgroundColor: background,
                shadowColor: Colors.transparent,
                actions: [
                  Padding(
                    padding: const EdgeInsets.all(18.0),
                    child: Text(
                      'Score: $score',
                      style: TextStyle(fontSize: 18.0),
                    ),
                  )
                ],
              ),
              body: Container(
                width: double.infinity,
                padding: const EdgeInsets.symmetric(horizontal: 10.0),
                child: Column(
                  children: [
                    QuestionWidget(
                      question: extractedData[index].title,
                      indexAction: index,
                      totalQuestions: extractedData.length,
                    ),
                    const Divider(
                      color: neutral,
                    ),
                    const SizedBox(height: 25.0),
                    for (int i = 0;
                        i < extractedData[index].options.length;
                        i++)
                      GestureDetector(
                        onTap: () => checkAnswerAndUpdate(
                            extractedData[index].options.values.toList()[i]),
                        child: OptionCard(
                          option: extractedData[index].options.keys.toList()[i],
                          color: isPressed
                              ? extractedData[index]
                                          .options
                                          .values
                                          .toList()[i] ==
                                      true
                                  ? correct
                                  : incorrect
                              : neutral,
                        ),
                      ),
                  ],
                ),
              ),
              floatingActionButton: GestureDetector(
                onTap: () => nextQuestion(extractedData.length),
                child: Padding(
                  padding: EdgeInsets.symmetric(horizontal: 10.0),
                  child: NextButton(),
                ),
              ),
              floatingActionButtonLocation:
                  FloatingActionButtonLocation.centerFloat,
            );
          }
        } else {
          return Center(
            child: Column(
              mainAxisAlignment: MainAxisAlignment.center,
              children: [
                CircularProgressIndicator(),
                SizedBox(height: 20.0),
                Text('Please Wait While Questions Are Loading..'),
              ],
            ),
          );
        }

        return const Center(
          child: Text('NoData'),
        );
      },
    );
  }
}