我有两个有状态类 - FirstClass 和 SecondClass。 在 FirstClass 我有两个按钮； '+' 和 '-'，它们将接收随机数(例如 1+3 或 5-2)。 然后，这些随机数将在 SecondClass 中显示为文本。

您可以在互联网上找到一些关于如何将数据传递给有状态小部件的示例(这里有一些很好的链接)。

Passing Data to a Stateful Widget

Passing data to StatefulWidget and accessing it in it's state in Flutter

https://medium.com/@maksimrv/reactive-app-state-in-flutter-73f829bcf6a7 The best way to passing data between widgets in Flutter

所有这些示例的共同点是它们都假设您的数据是最终的。因为我的数字是随机的，所以我不能使用 final- 这就是我被卡住的地方......

我在第一个问题中有点不清楚，但目标是获得如下输出 2 + 5 =_。用户将在文本字段中填写答案。然后，在按下按钮后，答案将传回总和并查看答案是否正确。如果答案正确 -> 做这个，否则 -> 做那个。

添加了更好的键盘。

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

void main() => runApp(MyHome());

class MyHome extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'passing data',
      theme: ThemeData(primarySwatch: Colors.red),
      home: FirstClass(),
    );
  }
}

class FirstClass extends StatefulWidget {
  @override
  _FirstClassState createState() => _FirstClassState();
}

class _FirstClassState extends State<FirstClass> {
  final random = Random();
  int a, b, sum;
  String output;

  void changeData(String buttonName) {
    setState(() {
      a = random.nextInt(10);
      b = random.nextInt(10);

      if (buttonName == '+') {
        sum = a + b;
        output = '$a + $b = ';
      } else if (buttonName == '-') {
        if (a >= b) {
          sum = a - b;
          output = '$a - $b = ';
        } else if (b > a) {
          //sum cannot be negative here
          sum = b - a;
          output = '$b - $a = ';
        }
      }
      print(sum.toString());
      Navigator.of(context).popUntil(ModalRoute.withName('/'));
      Navigator.of(context).push(MaterialPageRoute(
          builder: (context) => SecondClass(
                sum: sum,
                refresh: changeData,
                output: output,
                  )));
        });
      }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('First Screen'),
      ),
      body: Center(
        child: Column(
          mainAxisSize: MainAxisSize.min,
          children: <Widget>[
            //      Text('$a + $b = $sum'),
            RaisedButton(child: Text('+'), onPressed: () => 
changeData('+')),
            RaisedButton(child: Text('-'), onPressed: () => changeData('- 
   ')),
               ],
            ),
          ),
        );
      }
    }

class SecondClass extends StatefulWidget {
  final int sum;
  final String output;
  final String correction;

  final Function refresh;

  SecondClass({this.sum, this.refresh, this.output, this.correction});

  @override
  _SecondClassState createState() => _SecondClassState();
}

class _SecondClassState extends State<SecondClass> {
  String enterAnswer;
  String correct;

  void changeAnswer() {
    if (enterAnswer.isNotEmpty) {
      if (enterAnswer == widget.sum.toString()) {
        correct = 'Correct';
        print('Correct Answer.');
      } else {
         correct = 'Wrong Answer';
         print('Wrong Answer');
      }
    } else {
      correct = 'Enter Value First';
      print('Enter Value First');
    }
  }

  @override
  void initState() {
    super.initState();
  }

  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Second Screen'),
      ),
      body: Center(
        child: Container(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              Row(
                mainAxisAlignment: MainAxisAlignment.center,
                 children: <Widget>[
                  Text(widget.output),
                  Container(
                      width: 50.0,
                      TextField(
                        keyboardType: TextInputType.number,
                        onChanged: (val) {
                          enterAnswer = val;
                         },
                      )),
                ],
              ),
              RaisedButton(
                  child: Text('Check Answer'), onPressed: () => 
changeAnswer()),
              Text('$correct'),
            ],
          ),
        ),
      ),
    );
  }
}

为什么 '$correct' 不会显示为文本？我尝试在另一个项目中做同样的事情并且成功了，那么我在这里做错了什么？

最佳答案

你需要的是回调函数来再次获取随机值。

我假设您想要在第二个 screen() 单独的类中显示随机总和值。

这与 Navigator 和类 Constructors 相关。

代码:

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

void main() => runApp(MyHome());

class MyHome extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'passing data',
      theme: ThemeData(primarySwatch: Colors.red),
      home: FirstClass(),
    );
  }
}

class FirstClass extends StatefulWidget {
  @override
  _FirstClassState createState() => _FirstClassState();
}

class _FirstClassState extends State<FirstClass> {
  final random = Random();
  int a, b, sum;
  String output;

  void changeData(String buttonName) {
    setState(() {
      a = random.nextInt(10);
      b = random.nextInt(10);

      if (buttonName == '+') {
        sum = a + b;
        output = '$a + $b = ';
      } else if (buttonName == '-') {
        if (a >= b) {
          sum = a - b;
          output = '$a - $b = ';
        } else if (b > a) {
          //sum cannot be negative here
          sum = b - a;
          output = '$b - $a = ';
        }
      }
      print(sum.toString());
      Navigator.of(context).popUntil(ModalRoute.withName('/'));
      Navigator.of(context).push(MaterialPageRoute(
          builder: (context) => SecondClass(
                sum: sum,
                refresh: changeData,
                output: output,
              )));
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('First Screen'),
      ),
      body: Center(
        child: Column(
          mainAxisSize: MainAxisSize.min,
          children: <Widget>[
            //      Text('$a + $b = $sum'),
            RaisedButton(child: Text('+'), onPressed: () => changeData('+')),
            RaisedButton(child: Text('-'), onPressed: () => changeData('-')),
          ],
        ),
      ),
    );
  }
}

class SecondClass extends StatefulWidget {
  final int sum;
  final String output;

  final Function refresh;

  SecondClass({this.sum, this.refresh, this.output});

  @override
  _SecondClassState createState() => _SecondClassState();
}

class _SecondClassState extends State<SecondClass> {
  String enterAnswer;

  @override
  void initState() {
    super.initState();
  }

  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Second Screen'),
      ),
      body: Center(
        child: Container(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              Row(
                mainAxisAlignment: MainAxisAlignment.center,
                children: <Widget>[
                  Text(widget.output),
                  Container(
                      width: 50.0,
                      child: TextField(
                        onChanged: (val) {
                          enterAnswer = val;
                        },
                      )),
                ],
              ),
              RaisedButton(
                onPressed: () {
                  if (enterAnswer.isNotEmpty) {
                    if (enterAnswer == widget.sum.toString()) {
                      print('Correct Answer.');
                    } else {
                      print('Wrong Answer');
                    }
                  } else {
                    print('Enter Value First');
                  }
                },
                child: Text('Check Answer'),
              )
            ],
          ),
        ),
      ),
    );
  }
}

关于random - 如何在有状态小部件之间传递随机数据？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/54687955/