未实现错误(FLUTTER)

Question

我对Flutter的编码非常陌生，接下来我将创建有状态的小部件。问题是，当我这样做时，我会得到这个抛出UnimplementedError((；它应该返回null。我不知道自己做错了什么。

my code:
import 'package:flutter/material.dart';
void main() => runApp(MyApp());
class MyApp extends StatefulWidget {
@override
State<StatefulWidget> createState() {
// TODO: implement createState
throw UnimplementedError();
}
}
class MyAppState extends State<MyApp> {
var questionIndex = 0;
void answerQuestion() {
setState(() {
questionIndex = questionIndex + 1;
});
print(questionIndex);
}
@override
Widget build(BuildContext context) {
var questions = [
"What's your favourite color?",
"What's your favourite animal?",
];
return MaterialApp(
home: Scaffold(
appBar: AppBar(
title: Text(
"My First App",
),
),
body: Column(
children: [
Text(
questions[questionIndex],
),
RaisedButton(
child: Text("Answer 1"),
onPressed: answerQuestion,
),
RaisedButton(
child: Text("Answer 2"),
onPressed: answerQuestion,
),
RaisedButton(
child: Text("Answer 3"),
onPressed: answerQuestion,
)
],
),
),
);
}
}

Answer 1

createState()方法包括一个TODO注释：

@override
State<StatefulWidget> createState() {
// TODO: implement createState
throw UnimplementedError();
}

这个评论是一个善意的提醒，我们还有一些事情要做

然而，throw UnimplementedError()使提醒不那么友好，迫使我们完成TODO任务，并删除UnimplementedError()行以使其工作：

MyAppState createState() => MyAppState();