flutter - 使用无状态小部件创建简单的随机化器时出错。实例成员无法在初始化程序中访问

Question

我正在尝试创建一个非常简单的生成器，在此(无状态) Crystal 球中出现"is"，“否”或“也许”。

这是我的代码:

import 'dart:html';

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

void main() => runApp(
      MaterialApp(home: MyApp()),
    );

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Welcome to Flutter',
      home: Scaffold(
        backgroundColor: Colors.white,
        appBar: AppBar(
          title: Text('Welcome to Flutter'),
        ),
        body: Center(
          child: Image.asset('images/blu.png'),
        ),
      ),
    );
  }
}

class picker extends StatefulWidget {
  @override
  _pickerState createState() => _pickerState();
}

class _pickerState extends State<picker> {
  List yourList = ["Yes", "No", "Maybe"];
  int randomIndex = Random().nextInt(yourList.length);

  @override
  Widget build(BuildContext context) {
    return Center(
      child: TextButton(
        onPressed: () {
          setState(() {
            print(yourList[randomIndex]);
          });
        },
        child: null,
      ),
    );
  }
}

但是它显示了此错误，不会让我热重新加载它:
无法在初始化程序中访问实例成员“yourList”。
另外，我添加了child:null，因为该参数是必需的。

Answer 1

您需要像这样在yourList中调用constructor:

class _pickerState extends State<picker> {
  List yourList = ["Yes", "No", "Maybe"];

 int randomIndex;


 _pickerState() {
     randomIndex = Random().nextInt(yourList.length);
  }

...