Simple startup app (counter app) demonstrating the InheritedWidget

app_counter_inherited.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  const MyApp({Key key}) : super(key: key);
  @override
  Widget build(BuildContext context) => MaterialApp(
        title: 'Flutter Demo',
        theme: ThemeData(
          primarySwatch: Colors.blue,
          visualDensity: VisualDensity.adaptivePlatformDensity,
        ),
        home: MyHomePage(title: 'Flutter Demo Home Page'),
      );
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key key, this.title}) : super(key: key);
  final String title;
  @override
  State createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) => Scaffold(
        appBar: AppBar(
          title: Text(widget.title),
        ),
        body: BuildInheritedWidget(child: CounterPage()),
        floatingActionButton: FloatingActionButton(
          onPressed: () => BuildInheritedWidget.setState(() {}),
          tooltip: 'Increment',
          child: Icon(Icons.add),
        ),
      );
}

class CounterPage extends StatefulWidget {
  const CounterPage({Key key}) : super(key: key);
  @override
  State createState() => _CounterPageState();
}

class _CounterPageState extends State<CounterPage> {
  @override
  Widget build(BuildContext context) {
    var data = InheritedData.of(context).data;
    return Center(
      child: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: [
          Text(
            'You have pushed the button this many times:',
          ),
          Text(
            '$data',
            style: Theme.of(context).textTheme.headline4,
          ),
        ],
      ),
    );
  }
}

/// Builds a [InheritedWidget].
///
/// It's instantiated in a standalone widget
/// so its setState() call will **only** rebuild
/// [InheritedWidget] and consequently any of its dependents,
/// instead of rebuilding the app's entire widget tree.
class BuildInheritedWidget extends StatefulWidget {
  BuildInheritedWidget({Key key, this.child}) : super(key: key);
  final Widget child;
  static final _BuildInheritedWidgetState state = _BuildInheritedWidgetState();
  @override
  State createState() => state;

  /// Allow a 'rebuild' by the Widget and not the State.
  static void setState(VoidCallback fn) => state.setState(fn);
}

class _BuildInheritedWidgetState extends State<BuildInheritedWidget> {
  @override
  Widget build(BuildContext context) => InheritedData(child: widget.child);

  /// Override again merely to prevent the warning message.
  @override
  void setState(VoidCallback fn) => super.setState(fn);
}

class InheritedData extends InheritedWidget {
  ///
  InheritedData({Key key, Widget child})
      : object = DataField(),
        super(key: key, child: child);

  ///
  final DataField object;

  dynamic get data => object.data;

  /// Included temporarily for demonstration purposes.
  @override
  InheritedElement createElement() => InheritedElement(this);

  ///
  @override
  bool updateShouldNotify(InheritedData oldWidget) {
    object.data = oldWidget.object.data + 1;
    return true;
    return false;
  }

  ///
  static InheritedData of(BuildContext context) =>
      context.dependOnInheritedWidgetOfExactType<InheritedData>();
}

class DataField {
  int data = 0;
}