How to pass data from child widget to its parent

Question

I\'ve the below custom widget that make a Switch and reads its status (true/false)

Then I add this one to my main app widget (parent), how can I make th

Answer 1

In 2020, the function in the highest voted answer is marked deprecated. So here is the modified solution based on that answer:

import 'package:flutter/material.dart';

class MyStatefulWidget extends StatefulWidget {
  @override
  State createState() => new MyStatefulWidgetState();

  // --> NOTE this! <--
  static MyStatefulWidgetState of(BuildContext context) =>
    context.findAncestorStateOfType();
}

class MyStatefulWidgetState extends State {
  String _string = "Not set yet";

  set string(String value) => setState(() => _string = value);

  @override
  Widget build(BuildContext context) {
    return new Column(
      children: [
        new Text(_string),
        new MyChildClass(callback: (val) => setState(() => _string = val))
      ],
    );
  }
}

typedef void StringCallback(String val);

class MyChildClass extends StatelessWidget {
  final StringCallback callback;

  MyChildClass({this.callback});

  @override
  Widget build(BuildContext context) {
    return new Column(
      children: [
        new FlatButton(
          onPressed: () {
            callback("String from method 1");
          },
          child: new Text("Method 1"),
        ),
        new FlatButton(
          onPressed: () {
            MyStatefulWidget.of(context).string = "String from method 2";
          },
          child: new Text("Method 2"),
        )
      ],
    );
  }
}

void main() => runApp(
      new MaterialApp(
        builder: (context, child) => new SafeArea(child: new Material(color: Colors.white, child: child)),
        home: new MyStatefulWidget(),
      ),
    );