https://benzneststudios.com/blog/flutter/summary-about-build-context-widget-state-key-in-flutter/

main.dart

import 'package:flutter/material.dart';
import 'package:flutter_hello/my_inherited_widget_data.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: HomePage(),
    );
  }
}

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => new _HomePageState();
}

class _HomePageState extends State<HomePage> {
  @override
  Widget build(BuildContext context) {
    return MyInheritedWidgetData(
      child: Scaffold(
        appBar: AppBar(
          title: Text('My App'),
        ),
        body: Column(
          children: <Widget>[
            FlatButton(
              child: Text('rebuild all'),
              color: Colors.red,
              textColor: Colors.white,
              onPressed: () {
                setState(() {});
              },
            ),
            WidgetA(),
            Container(
              child: Column(
                children: <Widget>[
                  WidgetB(),
                  WidgetC(),
                ],
              ),
            ),
          ],
        ),
      ),
    );
  }
}

class WidgetA extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    print("build Widget A");
    final MyInheritedWidgetDataState state = MyInheritedWidgetData.of(context, false);
    return Container(
      child: FlatButton(
        child: Text('Commit'),
        color: Colors.orange,
        textColor: Colors.white,
        onPressed: () {
          state.setData("Second commit");
        },
      ),
    );
  }
}

class WidgetB extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    print("build Widget B");
    final MyInheritedWidgetDataState widget = MyInheritedWidgetData.of(context);
    return Text('${widget.data}');
  }
}

class WidgetC extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    print("build Widget C");
    return Text('I am robot.');
  }
}

my_inherited_widget.dart

import 'package:flutter/material.dart';
import 'package:flutter_hello/my_inherited_widget_data.dart';

class MyInheritedWidget extends InheritedWidget {
  final Widget child;
  final MyInheritedWidgetDataState state;

  MyInheritedWidget({Key key, @required this.child, @required this.state})
      : super(key: key, child: child);

  @override
  bool updateShouldNotify(MyInheritedWidget oldWidget) {
    return true;
  }
}

my_inherited_widget_data.dart

import 'package:flutter/material.dart';
import 'package:flutter_hello/my_inherited_widget.dart';

class MyInheritedWidgetData extends StatefulWidget {
  final Widget child;

  MyInheritedWidgetData({@required this.child});

  @override
  MyInheritedWidgetDataState createState() => MyInheritedWidgetDataState();
//
//  static MyInheritedWidgetDataState of(BuildContext context) {
//    return (context.inheritFromWidgetOfExactType(MyInheritedWidget)
//            as MyInheritedWidget)
//        .state;
//  }

  static MyInheritedWidgetDataState of([BuildContext context, bool rebuild = true]){
    return (rebuild ? context.inheritFromWidgetOfExactType(MyInheritedWidget) as MyInheritedWidget
        : context.ancestorWidgetOfExactType(MyInheritedWidget) as MyInheritedWidget).state;
  }
}

class MyInheritedWidgetDataState extends State<MyInheritedWidgetData> {
  String data;

  @override
  void initState() {
    data = "First commit.";
    super.initState();
  }

  setData(String newData) {
    setState(() {
      data = newData;
    });
  }

  @override
  Widget build(BuildContext context) {
    return MyInheritedWidget(child: widget.child, state: this);
  }
}