ボタンが親にあるときにボタンをクリックすると、子から親に値を取得するにはどうすればよいですか？

以下の完全なコードをコピーして貼り付ける
ことができます。GlobalKeyを取得するためdeliveryStateに使用でき、の属性を取得するためにDelivery使用deliveryStateできDeliveryます。ここでの属性はRadioModel selected
ステップ1：使用するGlobalKey _key = GlobalKey();

class _MyHomePageState extends State<DeliveryTimeline> {
  ...
  GlobalKey _key = GlobalKey();

ステップ2：アイテムdeliveryStateを取得するために使用するseleted

onStepContinue: () {
          setState(() {
            if (this._currentStep == 0) {
              this._currentStep = this._currentStep + 1;

              final _DeliveryState deliveryState =
                  _key.currentState;
              print("hi ${deliveryState.selected.title} ${deliveryState.selected.label} ");

ステップ3：Delivery必要key

child: Delivery(
              key: _key,
              onShipingTypeClicked: (shippingtype) {
 ...
 Delivery({Key key, this.onShipingTypeClicked}) : super(key:key);

ステップ4：変数を設定する selected

RadioModel selected = null;
...
return InkWell(
              onTap: () {
                setState(() {
                  ...
                  selected = sampleData[index];  

作業デモ

作業デモの出力

I/flutter ( 6246): hi Standard Delivery Order will be delivered between 3 - 5 business days 

完全なコード

import 'package:flutter/material.dart';

class DeliveryTimeline extends StatefulWidget {
  DeliveryTimeline({Key key, this.title}) : super(key: key);

  final String title;

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

class _MyHomePageState extends State<DeliveryTimeline> {
  int _currentStep = 0;
  String shippingtype;
  GlobalKey _key = GlobalKey();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          backgroundColor: Colors.white,
          centerTitle: true,
          iconTheme: IconThemeData(color: Colors.black),
          elevation: 0,
          title: Text(
            "Checkout",
            style: TextStyle(color: Colors.black),
          ),
        ),
        body: Stepper(
            type: StepperType.horizontal,
            steps: _mySteps(),
            currentStep: this._currentStep,
            onStepTapped: (step) {
              setState(() {
                this._currentStep = step;
              });
            },
            onStepContinue: () {
              setState(() {
                if (this._currentStep == 0) {
                  this._currentStep = this._currentStep + 1;

                  final _DeliveryState deliveryState =
                      _key.currentState;
                  print("hi ${deliveryState.selected.title} ${deliveryState.selected.label} ");

                } else if (this._currentStep == 1) {
                  this._currentStep = this._currentStep + 1;
                } else {
                  print('Completed, check fields.');
                }
              });
            },
            onStepCancel: () {
              setState(() {
                if (this._currentStep > 0) {
                  this._currentStep = this._currentStep - 1;
                } else {
                  this._currentStep = 0;
                }
              });
            },
            controlsBuilder: (BuildContext context,
                {VoidCallback onStepContinue,
                VoidCallback onStepCancel,
                Function onShippingNextClick}) {
              return Row(
                mainAxisAlignment: MainAxisAlignment.spaceBetween,
                children: <Widget>[
                  OutlineButton(
                      child: Text("Back"),
                      onPressed: onStepCancel,
                      shape: RoundedRectangleBorder(
                          borderRadius: BorderRadius.circular(4.0))),
                  MaterialButton(
                    child: Text("Next"),
                    color: Colors.blue,
                    textColor: Colors.white,
                    onPressed: onStepContinue,
                  ),
                ],
              );
            }));
  }

  List<Step> _mySteps() {
    List<Step> _steps = [
      Step(
        title: Text('Delivery'),
        content: Center(
          child: Container(
            height: MediaQuery.of(context).size.height / 1.5,
            child: Delivery(
              key: _key,
              onShipingTypeClicked: (shippingtype) {
                shippingtype = shippingtype;
                print("myvalue${shippingtype}");
              },
            ),
          ),
        ),
        isActive: _currentStep >= 0,
      ),
      Step(
        title: Text('Address'),
        content: Text("Address()"),
        isActive: _currentStep >= 1,
      ),
      Step(
        title: Text('Payment'),
        content: Text("Payment()"),
        isActive: _currentStep >= 2,
      )
    ];
    return _steps;
  }
}

class Delivery extends StatefulWidget {
  final ValueChanged<String> onShipingTypeClicked;

  Delivery({Key key, this.onShipingTypeClicked}) : super(key:key);

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

class _DeliveryState extends State<Delivery> {
  List<RadioModel> sampleData = List<RadioModel>();

  @override
  void initState() {
    // TODO: implement initState
    super.initState();
    sampleData.add(RadioModel(false, 'A', 0xffe6194B, "Standard Delivery",
        "Order will be delivered between 3 - 5 business days", 1));
    sampleData.add(RadioModel(
        true,
        'A',
        0xffe6194B,
        "Next Day Delivery",
        "Place your order before 6pm and your items will be delivered the next day",
        2));
    sampleData.add(RadioModel(
        false,
        'A',
        0xffe6194B,
        "Nominated Delivery",
        "Pick a particular date from the calendar and order will be delivered on selected date",
        3));
  }

  RadioModel selected = null;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: ListView.builder(
        itemCount: sampleData.length,
        shrinkWrap: true,
        physics: NeverScrollableScrollPhysics(),
        itemBuilder: (BuildContext context, int index) {
          return InkWell(
              onTap: () {
                setState(() {
                  sampleData.forEach((element) => element.isSelected = false);
                  sampleData[index].isSelected = true;
                  selected = sampleData[index];
                  widget.onShipingTypeClicked(sampleData[index].buttonText);
                });
              },
              child: Column(
                crossAxisAlignment: CrossAxisAlignment.start,
                children: <Widget>[
                  ListTile(
                    title: Text(sampleData[index].title),
                  ),
                  Padding(
                    padding: const EdgeInsets.only(bottom: 20.0),
                    child: Row(
                      mainAxisAlignment: MainAxisAlignment.spaceAround,
                      children: <Widget>[
                        Flexible(
                          child: Text(sampleData[index].label),
                        ),
                        RadioItem(sampleData[index]),
                      ],
                    ),
                  )
                ],
              ));
        },
      ),
    );
  }
}

class RadioItem extends StatelessWidget {
  final RadioModel _item;

  RadioItem(this._item);

  @override
  Widget build(BuildContext context) {
    return Container(
      margin: EdgeInsets.all(15.0),
      child: Row(
        mainAxisSize: MainAxisSize.max,
        children: <Widget>[
          Container(
            height: 25.0,
            width: 25.0,
            alignment: Alignment.center,
            child: Container(
                height: 15.0,
                width: 15.0,
                decoration: BoxDecoration(
                  color: Colors.blue,
                  borderRadius:
                      const BorderRadius.all(const Radius.circular(15)),
                )),
            decoration: BoxDecoration(
              color: Colors.transparent,
              border: Border.all(
                  width: 3.0,
                  color: _item.isSelected ? Colors.blue : Colors.transparent),
              borderRadius: const BorderRadius.all(const Radius.circular(25)),
            ),
          ),
          Container(margin: EdgeInsets.only(left: 10.0))
        ],
      ),
    );
  }
}

class RadioModel {
  bool isSelected;
  final String buttonText;
  final int colorCode;
  final String title, label;
  final int buttonid;

  RadioModel(this.isSelected, this.buttonText, this.colorCode, this.title,
      this.label, this.buttonid);
}

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: DeliveryTimeline(),
    );
  }
}