Example3

example3.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(
              home: Scaffold(
                  body: Center(
            child: Column(
                crossAxisAlignment: CrossAxisAlignment.stretch,
                children: <Widget>[
                  Expanded(
                    child: Column(
                      mainAxisAlignment: MainAxisAlignment.center,
                      children: [Text('Stateless 😑 \nnot our bro'), MyStatelessWidget()],
                    ),
                  ),
                  Expanded(
                    child: Column(
                      mainAxisAlignment: MainAxisAlignment.center,
                      children: [Text('Stateful 😎 \nour bro'), MyStatefulWidget()],
                    ),
                  ),
                  Expanded(
                    child: Column(
                      mainAxisAlignment: MainAxisAlignment.center,
                      children: [Text('another Stateful 😎 \nantoher our bro'), MyStatefulWidget2()],
                    ),
                  ),
                ]),
          )));
        });
  }
}

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()));
  }
}

class MyStatefulWidget2 extends StatefulWidget {
  @override
  _MyStatefulWidgetState2 createState() => _MyStatefulWidgetState2();
}

class _MyStatefulWidgetState2 extends State<MyStatefulWidget2> {
  var _result;

  @override
  void initState() {
    super.initState();
    Future.value(DateTime.now().second).then((result) {
      setState(() {
        _result = result;
      });
    });
  }

  @override
  Widget build(BuildContext context) {
    if (_result == null) {
      return new Container();
    }
    return Text(_result.toString());
  }
}