flutter - 飞镖类中的错误

Question

所以我正在开发这个测验应用程序，我只是为我的问题和答案创建一个类。但是vscode一直告诉我有错误。有人可以帮帮我吗？

这是 main.dart

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

void main() {
  runApp(HomePage());
}

class HomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        backgroundColor: Colors.grey[900],
        body: SafeArea(
          child: Padding(
            padding: EdgeInsets.symmetric(horizontal: 10.0),
            child: Quizzler(),
          ),
        ),
      ),
    );
  }
}

class Quizzler extends StatefulWidget {
  @override
  QuizzlerState createState() => QuizzlerState();
}

class QuizzlerState extends State<Quizzler> {
  List<Widget> scoreKeeper = [];

  List<Domande> domandeBank = [
    Domande(d: 'Il sole è una stella', r: true),
    Domande(d: 'Il latte è verde', r: false),
    Domande(d: 'Il mare è blu', r: true),
  ];

  int qNumber = 0;

  @override
  Widget build(BuildContext context) {
    return Column(
      mainAxisAlignment: MainAxisAlignment.spaceBetween,
      crossAxisAlignment: CrossAxisAlignment.stretch,
      children: <Widget>[
        Expanded(
          flex: 5,
          child: Padding(
            padding: const EdgeInsets.all(10.0),
            child: Center(
              child: Text(
                domandeBank[qNumber].domande,
                style: TextStyle(color: Colors.white),
                textAlign: TextAlign.center,
              ),
            ),
          ),
        ),
        Expanded(
          child: Padding(
            padding: const EdgeInsets.all(10.0),
            child: TextButton(
              style: TextButton.styleFrom(padding: EdgeInsets.zero),
              child: Container(
                color: Colors.green,
                height: 50,
                width: double.infinity,
                child: Center(
                  child: Text(
                    "True",
                    style: TextStyle(color: Colors.white),
                  ),
                ),
              ),
              onPressed: () {
                setState(() {
                  bool risCorretta = domandeBank[qNumber].risposte;
                  if (risCorretta == true) {
                    scoreKeeper.add(
                      Icon(
                        Icons.check,
                        color: Colors.green,
                      ),
                    );
                  } else {
                    scoreKeeper.add(
                      Icon(
                        Icons.close,
                        color: Colors.red,
                      ),
                    );
                  }
                  qNumber++;
                });
              },
            ),
          ),
        ),
        Expanded(
          child: Padding(
            padding: const EdgeInsets.all(10.0),
            child: TextButton(
              style: TextButton.styleFrom(padding: EdgeInsets.zero),
              child: Container(
                color: Colors.red,
                height: 50,
                width: double.infinity,
                child: Center(
                  child: Text(
                    "False",
                    style: TextStyle(color: Colors.white),
                  ),
                ),
              ),
              onPressed: () {
                setState(() {
                  bool risCorretta = domandeBank[qNumber].risposte;
                  if (risCorretta == false) {
                    scoreKeeper.add(
                      Icon(
                        Icons.check,
                        color: Colors.green,
                      ),
                    );
                  } else {
                    scoreKeeper.add(
                      Icon(
                        Icons.close,
                        color: Colors.red,
                      ),
                    );
                  }
                  qNumber++;
                });
              },
            ),
          ),
        ),
        Row(
          children: scoreKeeper,
        ),
      ],
    );
  }
}

这是 question.dart 类

class Domande {
  String domande;
  bool risposte;

  Domande({String d, bool r}) {
    domande = d;
    risposte = r;
  }
}

我得到的错误：

Non-nullable instance field 'domande' must be initialized.
Try adding an initializer expression, or add a field initializer in this constructor, or mark it 'late'.


Non-nullable instance field 'risposte' must be initialized.
Try adding an initializer expression, or add a field initializer in this constructor, or mark it 'late'.


The parameter 'd' can't have a value of 'null' because of its type, but the implicit default value is 'null'.
Try adding either an explicit non-'null' default value or the 'required' modifier.

The parameter 'r' can't have a value of 'null' because of its type, but the implicit default value is 'null'.
Try adding either an explicit non-'null' default value or the 'required' modifier.

标签： flutterdartdart-null-safety

Answer 1

首先你可以省略这部分。

domande = d;
risposte = r;

将构造函数参数分配给实例变量的模式非常常见，但是使用 dart 你可以像下面的代码片段那样做。
现在我们来解决您的问题。首先，您使用的是null-safety并将您的变量创建为 not nullable。然后你创建了你的构造函数optional parameters，这意味着这些参数可以是null，但你的类变量不能为空。这就是您发生此错误的原因。您可以通过使用required
关键字来解决此问题，这意味着您的变量是.mandatory

class Domande {
  String domande;
  bool risposte;

  Domande({required this.domande, required this.risposte});
}

解决此问题的另一种方法是nullable使用?. 但是，如果这些变量是null.

class Domande {
  String? domande;
  bool? risposte;

  Domande({this.domande, this.risposte});
}