Flutter: call a method from another class / setState a page from outside that class with StreamController

This Flutter demo shows how to call a method from another class or how to setState a page from outside that class, using StreamController.

main.dart

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

StreamController<int> streamController = StreamController<int>();

void main() {
  runApp(MyApp());
}

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

class MyHomePage extends StatefulWidget {
  MyHomePage(this.title, this.stream);
  final String title;
  final Stream<int> stream;

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

class _MyHomePageState extends State<MyHomePage> {
  String menuName = 'A';

  @override
  void initState() {
    super.initState();
    widget.stream.listen((index) {
      mySetState(index);
    });
  }

  void mySetState(int index) {
    List menuList = ['A', 'B', 'C'];
    setState(() {
      menuName = menuList[index];
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          children: [
            Padding(
              padding: const EdgeInsets.all(30.0),
              child: Text(
                'Today\'s special is:',
                style: TextStyle(fontSize: 20),
              ),
            ),
            Padding(
              padding: const EdgeInsets.all(10.0),
              child: Text(
                'Combo ' + menuName,
                style: TextStyle(fontSize: 30, color: Colors.blue),
              ),
            ),
            SizedBox(
              height: 50,
            ),
            ElevatedButton(
              style: ButtonStyle(
                backgroundColor:
                    MaterialStateProperty.all<Color>(Colors.blue[700]),
              ),
              child: Text(
                'Settings',
                style: TextStyle(color: Colors.white),
              ),
              onPressed: () {
                Navigator.push(
                  context,
                  MaterialPageRoute(builder: (context) => SecondPage()),
                );
              },
            ),
          ],
        ),
      ),
    );
  }
}

class SecondPage extends StatefulWidget {
  @override
  _SecondPageState createState() => _SecondPageState();
}

class _SecondPageState extends State<SecondPage> {
  List<bool> isSelected = [true, false, false];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Second Page'),
      ),
      body: Center(
        child: Column(
          children: [
            Padding(
              padding: const EdgeInsets.all(30.0),
              child: Text(
                'Select a menu name:',
                style: TextStyle(fontSize: 20),
              ),
            ),
            Padding(
              padding: const EdgeInsets.all(20.0),
              child: ToggleButtons(
                children: <Widget>[
                  Text('A'),
                  Text('B'),
                  Text('C'),
                ],
                onPressed: (int index) {
                  setState(() {
                    for (int buttonIndex = 0;
                        buttonIndex < isSelected.length;
                        buttonIndex++) {
                      if (buttonIndex == index) {
                        isSelected[buttonIndex] = true;
                      } else {
                        isSelected[buttonIndex] = false;
                      }
                    }
                  });
                  streamController.add(index);
                },
                isSelected: isSelected,
              ),
            ),
          ],
        ),
      ),
    );
  }
}