InheritedWidget

main_inherited_widget.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyInheritedWidget(
        counter: 0,
        child: MyHomePage(title: 'Flutter Demo Home Page'),
      ),
    );
  }
}

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

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

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    final MyInheritedWidget myInheritedWidget = MyInheritedWidget.of(context);

    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '${myInheritedWidget.counter}',
              style: Theme.of(context).textTheme.display1,
            ),
            TestStateless(),
            TestStateful()
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
        },
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}

class MyInheritedWidget extends InheritedWidget {
  MyInheritedWidget({
    Key key,
    @required Widget child,
    this.counter,
  }) : super(key: key, child: child);

  final int counter;

  static MyInheritedWidget of(BuildContext context) {
    return context.inheritFromWidgetOfExactType(MyInheritedWidget);
  }

  @override
  bool updateShouldNotify(MyInheritedWidget oldWidget) =>
      counter != oldWidget.counter;
}

class TestStateless extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    print('build TestStateless');
    return Text('TestStateless');
  }
}

class TestStateful extends StatefulWidget {
  @override
  _TestStatefulState createState() => _TestStatefulState();
}

class _TestStatefulState extends State<TestStateful> {
  @override
  Widget build(BuildContext context) {
    print('build TestStateful');
    return Text('_TestStatefulState');
  }
}