main.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(context) {
    return StreamBuilder<void>(
      stream: Stream.periodic(Duration(seconds: 3)),
      builder: (context, snapshot) {
        print('rebuilding MaterialApp');
        return MaterialApp(
          theme: ThemeData(
            textTheme: TextTheme(
              body1: TextStyle(fontSize: 40)
            ),
          ),
          home: Scaffold(
            body: Center(
              child: Column(
                crossAxisAlignment: CrossAxisAlignment.stretch,
                children: <Widget>[
                  Expanded(
                    child: Column(
                      mainAxisAlignment: MainAxisAlignment.center,
                      children: [Text('Stateless 😩'), MyStatelessWidget()],
                    ),
                  ),
                  Expanded(
                    child: Column(
                      mainAxisAlignment: MainAxisAlignment.center,
                      children: [Text('Stateful 👌'), MyStatefulWidget()],
                    ),
                  ),
                ]
              ),
            )
          )
        );
      }
    );
  }
}

class MyStatelessWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return FutureBuilder<int>(
      future: Future.value(DateTime.now().second),
      builder: (context, snapshot) => Text(snapshot.data.toString())
    );
  }
}


class MyStatefulWidget extends StatefulWidget {
  @override
  _MyStatefulWidgetState createState() => _MyStatefulWidgetState();
}

class _MyStatefulWidgetState extends State<MyStatefulWidget> {

  Future<int> _future;

  @override
  void initState() {
    super.initState();
    _future = Future.value(DateTime.now().second);
  }

  @override
  Widget build(BuildContext context) {
    return FutureBuilder<int>(
      future: _future,
      builder: (context, snapshot) => Text(snapshot.data.toString())
    );
  }
}