Question

我已经为测验应用程序编写了下面的代码，但我仍然坚持找到一种方法来将所选单选按钮与正确的ans进行比较，并且还将过渡延迟到下一个问题，以便直观地显示所做的选择。尝试使用switch语句和_counter变量但导致错误

The following NoSuchMethodError was thrown while handling a gesture:
I/flutter (28574): The method '[]' was called on null.

我不太熟悉，无法理解错误可能引用的位置/内容或该方法可能出错的地方（switch语句）。任何更正/方向/提示将不胜感激。感谢。

import 'dart:async';
import 'package:flutter/material.dart';

Map<String, Map<String, String>> questionBank = {
  "1": {
    "question": "What is the capital of Canada?",
    "ans1": "Toronto",
    "ans2": "Montreal",
    "ans3": "Ottawa",
    "ans4": "Vancouver",
    "coAns": "Ottawa"
  },
  "2": {
    "question": "What is the capital of the United States of America?",
    "ans1": "New York",
    "ans2": "California",
    "ans3": "Texas",
    "ans4": "Washington DC",
    "coAns": "Washington DC"
  },
  "3": {
    "question": "What is the capital of Nigeria?",
    "ans1": "Abuja",
    "ans2": "Lagos",
    "ans3": "Port Harcourt",
    "ans4": "Makurdi",
    "coAns": "Abuja"
  },
  "4": {
    "question": "What is the capital of England?",
    "ans1": "Britain",
    "ans2": "Scotland",
    "ans3": "London",
    "ans4": "Edinburgh",
    "coAns": "London"
  },
  "5": {
    "question": "What is the capital of China?",
    "ans1": "Beijing",
    "ans2": "Shanghai",
    "ans3": "Tianjin",
    "ans4": "Taiwan",
    "coAns": "Beijing"
  },
};

void main() {
  runApp(new _questionDisplay());
}

class _questionDisplay extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(home: new QuestDis());
  }
}

class QuestDis extends StatefulWidget {
  QuestDis({Key key}) : super(key: key);

  @override
  _QuestDisState createState() => new _QuestDisState();
}

class _QuestDisState extends State<QuestDis> {

  @override
  var _counter = 1;
  var bkgrdColor = Colors.blue[50];
  int radioValue = 0;

  int ans1Value = 1;
  int ans2Value = 2;
  int ans3Value = 3;
  int ans4Value = 4;

  void handleRadioValueChanged(int value) {
    setState(() {
      radioValue = value;
      /*
      switch (radioValue) {
        case 1:
          bkgrdColor = (questionBank[_counter]["coAns"] ==
                  questionBank[_counter][ans1Value])
              ? Colors.green[50]
              : Colors.red[50];
          break;
        case 2:
          bkgrdColor = (questionBank[_counter]["coAns"] ==
                  questionBank[_counter][ans2Value])
              ? Colors.green[50]
              : Colors.red[50];
          break;
        case 3:
          bkgrdColor = (questionBank[_counter]["coAns"] ==
                  questionBank[_counter][ans3Value])
              ? Colors.green[50]
              : Colors.red[50];
          break;
        case 4:
          bkgrdColor = (questionBank[_counter]["coAns"] ==
                  questionBank[_counter][ans4Value])
              ? Colors.green[50]
              : Colors.red[50];
          break;
      }
       */
      _counter++;
      radioValue = 0;
    });
  }

  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        leading: new IconButton(icon: new Icon(Icons.menu), onPressed: null),
        title: new Text('quizApp'),
      ),
      body: new Container(
        child: new Column(
          children: [
            new Expanded(
              child: new Container(
                child: new Column(
                  children: [
                    new Expanded(
                      child: new Container(
                        child: new Card(
                          color: bkgrdColor,
                          child: new Row(
                            children: <Widget>[
                              new Text(
                                  "${questionBank[_counter.toString()]["question"]}"),
                            ],
                          ),
                        ),
                      ),
                    ),
                    new Expanded(
                      child: new Container(
                        child: new Card(
                          child: new Column(
                            children: [
                              new Row(
                                children: <Widget>[
                                  new Radio<int>(
                                      value: ans1Value,
                                      groupValue: radioValue,
                                      onChanged: handleRadioValueChanged),
                                  new Text(
                                      "${questionBank[_counter.toString()]["ans1"]}")
                                ],
                              ),
                              new Divider(),
                              new Row(
                                children: <Widget>[
                                  new Radio<int>(
                                      value: ans2Value,
                                      groupValue: radioValue,
                                      onChanged: handleRadioValueChanged),
                                  new Text(
                                      "${questionBank[_counter.toString()]["ans2"]}")
                                ],
                              ),
                              new Divider(),
                              new Row(
                                children: <Widget>[
                                  new Radio<int>(
                                      value: ans3Value,
                                      groupValue: radioValue,
                                      onChanged: handleRadioValueChanged),
                                  new Text(
                                      "${questionBank[_counter.toString()]["ans3"]}")
                                ],
                              ),
                              new Divider(),
                              new Row(
                                children: <Widget>[
                                  new Radio<int>(
                                      value: ans4Value,
                                      groupValue: radioValue,
                                      onChanged: handleRadioValueChanged),
                                  new Text(
                                      "${questionBank[_counter.toString()]["ans4"]}")
                                ],
                              ),
                            ],
                          ),
                        ),
                      ),
                    ),
                  ],
                ),
              ),
            ),
          ],
        ),
      ),
    );
  }
}

Answer 1

如果我是你，我实际上会创建一个问题对象（不要认为这是理所当然的，我很快就会在我的ipad上写出来，所以语法可能没有点......）：

class Question {
  String question;
  List<String> answers;
  String correctAnswer;
  Question(question, answers, correctAnswer);
}

现在您可以使用该对象创建问题列表：

List<Question> questions = [
  new Question("What is 2+2", ["2", "3", "4"], "4"),
  // create as many questions as you want
];

你可以保持你的计数器变量，并实际将其用作索引。

假设您设法获得用户在名为selectedAnswer的字符串中选择的答案。在你的setState（）中我会做这样的事情：

if (chosenAnswer == questions[_counter].correctAnswer){
  // answer was correct
} else {
  // answer is false
}
// and dont forget to increment your counter
_counter++;

为了显示用户的问题和答案，您可以再次使用计数器变量作为索引，并访问列表中存储的Question对象中的每个可能答案。

希望我能帮助一下

在颤动中选择的音轨单选按钮

1 个答案: