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 the parent knows the value of the switch!

import 'package:flutter/material.dart';

class Switchy extends StatefulWidget{
  Switchy({Key key}) : super(key: key);

  @override
  State<StatefulWidget> createState() => new _SwitchyState();
  }

class _SwitchyState extends State<Switchy> {
  var myvalue = true;

  void onchange(bool value) {
    setState(() {
      this.myvalue = value;      // I need the parent to receive this one!
      print('value is: $value');
    });
  }

  @override
  Widget build(BuildContext context) {
    return             
      new Card(
      child: new Container(
        child: new Row(
          mainAxisAlignment: MainAxisAlignment.end,
          children: <Widget>[
            new Text("Enable/Disable the app in the background",
              textAlign: TextAlign.left,
              textDirection: TextDirection.ltr,),
            new Switch(value: myvalue, onChanged: (bool value) => onchange(value)),
          ],
        ),
      ),
    );
  }
}

In the main.dart (parent) file, I started with this:

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

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(
        primarySwatch: Colors.deepOrange,
      ),
      home: new MyHomePage(title: 'My App settup'),
    );
  }
}

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

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {

  Widget e = new Switchy();
  //...

}

Answer 1

Output:

Generally you use Navigator.pop(context, "your data here"); but you can also pass the data in this manner

class ParentScreen extends StatefulWidget {
  @override
  _ParentScreenState createState() => _ParentScreenState();
}

class _ParentScreenState extends State<ParentScreen> {
  String _text = "";

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text("Parent screen")),
      body: Column(
        children: <Widget>[
          RaisedButton(
            child: Text("GO TO CHILD"),
            onPressed: () => Navigator.push(context, MaterialPageRoute(builder: (_) => ChildScreen(func: function))),
          ),
          Text(_text),
        ],
      ),
    );
  }

  function(value) => setState(() => _text = value);
}

class ChildScreen extends StatelessWidget {
  final Function func;

  const ChildScreen({Key key, this.func}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text("Child screen")),
      body: RaisedButton(
        child: Text("BACK TO PARENT"),
        onPressed: () {
          func("This is the data which child has passed"); // passing data to parent
          Navigator.pop(context);
        },
      ),
    );
  }
}

Answer 2

The first possibility is to pass a callback into your child, and the second is to use the of pattern for your stateful widget. See below.

import 'package:flutter/material.dart';

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

  static MyStatefulWidgetState of(BuildContext context) {
    final MyStatefulWidgetState navigator =
        context.ancestorStateOfType(const TypeMatcher<MyStatefulWidgetState>());

    assert(() {
      if (navigator == null) {
        throw new FlutterError(
            'MyStatefulWidgetState operation requested with a context that does '
            'not include a MyStatefulWidget.');
      }
      return true;
    }());

    return navigator;
  }
}

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

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

  @override
  Widget build(BuildContext context) {
    return new Column(
      children: <Widget>[
        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: <Widget>[
        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(),
      ),
    );

There is also the alternative of using an InheritedWidget instead of a StatefulWidget; this is particularly useful if you want your child widgets to rebuild if the parent widget's data changes and the parent isn't a direct parent. See the inherited widget documentation