如何将一个类中的变量提供给另一个类并将更改后的变量返回？

如何解决如何将一个类中的变量提供给另一个类并将更改后的变量返回？

我是新手，我正在尝试从 SetTimerPage 到 CupertinoTimePickerButton-Page 获取倒计时变量。然后将其更改为所选时间并返回。接下来，我想将更改的倒计时提供给倒计时动画开始的下一页。我也愿意提出改进我的语法的建议。谢谢:)

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(
        primarySwatch: Colors.red,),home: SetTimerPage(),);
  }
}

class SetTimerPage extends StatefulWidget {
  @override
  _SetTimerPageState createState() => _SetTimerPageState();
}

class _SetTimerPageState extends State<SetTimerPage> {
  final Duration countdown = Duration(minutes: 0,seconds: 0);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('RoundONE'),body: Container(
        padding: const EdgeInsets.all(16),child: Column(
          mainAxisAlignment: MainAxisAlignment.spaceEvenly,children: [
            CustomrowWidget(
              icon: Icons.alarm_sharp,text: 'COUNTDOWN',button: CupertinoTimePickerButton(passedValue: countdown),StartButton(countdown: countdown),],);
  }
}

class CupertinoTimePickerButton extends StatefulWidget {
  Duration passedValue;

  CupertinoTimePickerButton({
    Key key,@required this.passedValue,}) : super(key: key);

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

class _CupertinoTimePickerButtonState extends State<CupertinoTimePickerButton> {
  @override
  Widget build(BuildContext context) {
    return MaterialButton(
      child: Text(
        _formatDuration(widget.passedValue),style: TextStyle(color: Colors.white),color: Colors.redAccent,onpressed: () {
        _cupertinoTimeSetter(context);
      },);
  }

  Future _cupertinoTimeSetter(BuildContext context) {
    return showModalBottomSheet(
      context: context,builder: (BuildContext builder) {
        return Container(
          height: MediaQuery.of(context).copyWith().size.height / 2,child: CupertinoTimerPicker(
            onTimerDurationChanged: (Duration newDuration) {
              setState(() {
                widget.passedValue = newDuration;
              });
            },minuteInterval: 1,secondInterval: 5,mode: CupertinoTimerPickerMode.ms,);
      },);
  }

  String _formatDuration(Duration duration) {
    String twoDigits(int n) => n.toString().padLeft(2,"0");
    String twoDigitMinutes = twoDigits(duration.inMinutes.remainder(60));
    String twoDigitSeconds = twoDigits(duration.inSeconds.remainder(60));
    return "$twoDigitMinutes:$twoDigitSeconds";
  }
}

解决方法

最好的方法是使用回调将值返回给它的父级，在这种情况下，您可以使用 ValueChanged。然后在父之后会负责更改变量值。

class SetTimerPage extends StatefulWidget {
  @override
  _SetTimerPageState createState() => _SetTimerPageState();
}

class _SetTimerPageState extends State<SetTimerPage> {
  Duration countdown;

  @override
  void initState() {
    super.initState();
    countdown = Duration(minutes: 0,seconds: 0);
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('RoundONE'),),body: Container(
        padding: const EdgeInsets.all(16),child: Column(
          mainAxisAlignment: MainAxisAlignment.spaceEvenly,children: [
            CustomRowWidget(
              icon: Icons.alarm_sharp,text: 'COUNTDOWN',button: CupertinoTimePickerButton(
                passedValue: countdown,onChanged: (Duration duration) {
                  setState(() {
                    this.countdown = duration;
                  });
                },StartButton(countdown: countdown),],);
  }
}

class CupertinoTimePickerButton extends StatelessWidget {

  final Duration passedValue;
  final ValueChanged<Duration> onChanged;

  @override
  Widget build(BuildContext context) {
    return MaterialButton(
      child: Text(
        _formatDuration(widget.passedValue),style: TextStyle(color: Colors.white),color: Colors.redAccent,onPressed: () {
        _cupertinoTimeSetter(context);
      },);
  }

  Future _cupertinoTimeSetter(BuildContext context) {
    return showModalBottomSheet(
      context: context,builder: (BuildContext builder) {
        return Container(
          height: MediaQuery.of(context).copyWith().size.height / 2,child: CupertinoTimerPicker(
            onTimerDurationChanged: this.onChanged,minuteInterval: 1,secondInterval: 5,mode: CupertinoTimerPickerMode.ms,);
      },);
  }

  String _formatDuration(Duration duration) {
    String twoDigits(int n) => n.toString().padLeft(2,"0");
    String twoDigitMinutes = twoDigits(duration.inMinutes.remainder(60));
    String twoDigitSeconds = twoDigits(duration.inSeconds.remainder(60));
    return "$twoDigitMinutes:$twoDigitSeconds";
  }
}