Managing state with streams - answer to a question https://stackoverflow.com/questions/44769472/flutter-generate-in-app-interactivity/44769847

streams_and_numbers.dart

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

void main() {
  runApp(new KeyboardApp());
}

class KeyboardApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      home: new HomePage(),
    );
  }
}

class HomePage extends StatefulWidget {
  HomePageState createState() => new HomePageState();
}

class HomePageState extends State<HomePage> {
  final StreamController<List<int>> controller = new StreamController.broadcast();

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      body: new ListView  (
        children: <Widget>[
          new NumberDisplay(numbers: controller.stream),
          new Keyboard(controller: controller),
        ],
      ),
    );
  }
}

class NumberDisplay extends StatelessWidget {
  NumberDisplay({ this.numbers });
  final Stream<List<int>> numbers;

  final Color _color = new Color(0xFFE57373);

  @override
  Widget build(BuildContext context) {
    return new Container(
      height: 145.0,
      child: new Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: <Widget>[
          new Container(
            padding: new EdgeInsets.only(right: 16.0, top: 35.0),
            child: new Row(
              mainAxisAlignment: MainAxisAlignment.end,
              children: <Widget>[
                new Text(
                  '\$  ',
                  style: new TextStyle(
                    fontFamily: 'Roboto',
                    color: new Color(0xFFFFFFFF),
                    fontSize: 20.0
                  )
                ),
                new StreamBuilder(
                  stream: numbers,
                  builder: (BuildContext context, AsyncSnapshot<List<int>> snapshot) {
                    return new Text(
                      snapshot.data?.join('') ?? '',
                      style: new TextStyle(
                        fontFamily: 'Roboto',
                        color: new Color(0xFFFFFFFF),
                        fontSize: 45.0
                      ),
                    );
                  },
                )
              ],
            ),
          ),
        ],
      ),
      color: _color,
    );
  }
}

class Keyboard extends StatefulWidget {
  Keyboard({ this.controller });
  final StreamController<List<int>> controller;
  State createState() => new KeyboardState();
}

class KeyboardState extends State<Keyboard> {
  List<int> numbers = <int>[];

  final Color _color = new Color(0xFFE57373);

  @override
  Widget build(BuildContext context) {
    return new Container(
      padding: new EdgeInsets.only(right: 15.0, left: 15.0, top: 47.0, bottom: 20.0),
      child: new Column(
        children: <Widget>[
          new Row(
            mainAxisAlignment: MainAxisAlignment.spaceAround,
            children: <Widget>[
              new FlatButton(
                textColor: _color,
                child: new Text(
                  '1',
                  style: new TextStyle(
                    fontSize: 35.0
                  ),
                ),
                onPressed: () {
                  widget.controller.add(numbers..add(1));
                },
              ),
              new FlatButton(
                textColor: _color,
                child: new Text(
                  '2',
                  style: new TextStyle(
                    fontSize: 35.0
                  ),
                ),
                onPressed: () {
                  widget.controller.add(numbers..add(2));
                },
              ),
              new FlatButton(
                textColor: _color,
                child: new Icon(
                  Icons.backspace,
                  size: 35.0
                ),
                onPressed: () {
                  widget.controller.add(numbers..removeLast());
                },
              ),
            ],
          ),
        ],
      ),
    );
  }
}