form_page.dart

## form_page.dart
import 'package:flutter/material.dart';
import 'package:simple_inherit/state_container.dart';

class UpdateUserScreen extends StatelessWidget {
  static final GlobalKey<FormState> formKey = new GlobalKey<FormState>();
  static final GlobalKey<FormFieldState<String>> firstNameKey =
  new GlobalKey<FormFieldState<String>>();
  static final GlobalKey<FormFieldState<String>> lastNameKey =
  new GlobalKey<FormFieldState<String>>();
  static final GlobalKey<FormFieldState<String>> emailKey =
  new GlobalKey<FormFieldState<String>>();

  final User user;

  const UpdateUserScreen({Key key, this.user}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    final container = StateContainer.of(context);
    return new Scaffold(
      appBar: new AppBar(
        title: new Text('Edit User Info'),
      ),
      body: new Padding(
        padding: new EdgeInsets.all(16.0),
        child: new Form(
          key: formKey,
          autovalidate: false,
          child: new ListView(
            children: [
              new TextFormField(
                key: firstNameKey,
                style: Theme.of(context).textTheme.headline,
                decoration: new InputDecoration(
                  hintText: 'First Name',
                ),
              ),
              new TextFormField(
                key: lastNameKey,
                style: Theme.of(context).textTheme.headline,
                decoration: new InputDecoration(
                  hintText: 'Last Name',
                ),
              ),
              new TextFormField(
                key: emailKey,
                style: Theme.of(context).textTheme.headline,
                decoration: new InputDecoration(
                  hintText: 'Email Address',
                ),
              )
            ],
          ),
        ),
      ),
      floatingActionButton: new FloatingActionButton(
        child: new Icon(Icons.add),
        onPressed: () {
          final form = formKey.currentState;
          if (form.validate()) {
            var firstName = firstNameKey.currentState.value;
            var lastName = lastNameKey.currentState.value;
            var email = emailKey.currentState.value;

            if (firstName == '') {
              firstName = null;
            }
            if (lastName == '') {
              lastName = null;
            }
            if (email == '') {
              email = null;
            }

            container.updateUserInfo(
              firstName: firstName,
              lastName: lastName,
              email: email,
            );

            Navigator.pop(context);
          }
        },
      ),
    );
  }
}

## main.dart
import 'package:flutter/material.dart';
import 'package:simple_inherit/form_page.dart';
import 'package:simple_inherit/state_container.dart';

void main() {
  runApp(new StateContainer(child: new TodoApp()));
}

class TodoApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Some Todos',
      home: new HomeScreen(),
    );
  }
}

class HomeScreen extends StatefulWidget {
  @override
  HomeScreenState createState() => new HomeScreenState();
}

class HomeScreenState extends State<HomeScreen> {
  User user;

  Widget get _userInfo {
    return new Center(
      child: new Column(
        mainAxisAlignment: MainAxisAlignment.center,
        crossAxisAlignment: CrossAxisAlignment.center,
        children: <Widget>[
          new Text("${user.firstName} ${user.lastName}",
              style: new TextStyle(fontSize: 24.0)),
          new Text(user.email, style: new TextStyle(fontSize: 24.0)),
        ],
      ),
    );
  }

  Widget get _logInPrompt {
    return new Center(
      child: new Column(
        mainAxisAlignment: MainAxisAlignment.center,
        crossAxisAlignment: CrossAxisAlignment.center,
        children: <Widget>[
          new Text('Please add user information'),
        ],
      ),
    );
  }

  void _updateUser(BuildContext context) {
    Navigator.push(
      context,
      new MaterialPageRoute(
        fullscreenDialog: true,
        builder: (context) {
          return new UpdateUserScreen();
        },
      ),
    );
  }

  @override
  Widget build(BuildContext context) {
    final container = StateContainer.of(context);
    user = container.user;
    var body = user != null ? _userInfo : _logInPrompt;
    return new Scaffold(
      appBar: new AppBar(
        title: new Text('Inherited Widget Test'),
      ),
      body: body,
      floatingActionButton: new FloatingActionButton(
        onPressed: () => _updateUser(context),
        child: new Icon(Icons.edit),
      ),
    );
  }
}

## state_container.dart
import 'package:flutter/cupertino.dart';
import 'package:flutter/foundation.dart';

class User {
  String firstName;
  String lastName;
  String email;

  User(this.firstName, this.lastName, this.email);
}

class StateContainer extends StatefulWidget {
  final Widget child;
  final User user;

  StateContainer({
    @required this.child,
    this.user,
  });

  static StateContainerState of(BuildContext context) {
    return (context.inheritFromWidgetOfExactType(_InheritedStateContainer)
            as _InheritedStateContainer)
        .data;
  }

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

class StateContainerState extends State<StateContainer> {
  User user;

  void updateUserInfo({firstName, lastName, email}) {
    if (user == null) {
      user = new User(firstName, lastName, email);
      setState(() {
        user = user;
      });
    } else {
      setState(() {
        user.firstName = firstName ?? user.firstName;
        user.lastName = lastName ?? user.lastName;
        user.email = email ?? user.email;
      });
    }
  }

  @override
  Widget build(BuildContext context) {
    return new _InheritedStateContainer(
      data: this,
      child: widget.child,
    );
  }
}

class _InheritedStateContainer extends InheritedWidget {
  final StateContainerState data;

  _InheritedStateContainer({
    Key key,
    @required this.data,
    @required Widget child,
  }) : super(key: key, child: child);

  @override
  bool updateShouldNotify(_InheritedStateContainer old) => true;
}