vms.dart

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

main() {
  final state = State();
  final mutator = Mutator(state);
  runApp(MyApp(state: state, mutator: mutator));
}

class State {
  var currentValue = 0;
  final broadcaster = StreamController.broadcast();
  get valueStream => broadcaster.stream;
}

class Mutator {
  final State state;
  Mutator(this.state);
  increment() {
    state.broadcaster.add(++state.currentValue);
  }
}

class MyApp extends StatelessWidget {
  final State state;
  final Mutator mutator;

  const MyApp({Key key, this.state, this.mutator}) : super(key: key);

  build(context) {
    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: StreamBuilder(
            stream: state.valueStream,
            builder: (context, snapshot) {
              return Text(snapshot.data?.toString() ?? state.currentValue.toString());
            },
          ),
        ),
        floatingActionButton: FloatingActionButton(
          onPressed: () {
            mutator.increment();
          },
          child: Icon(Icons.add),
        ),
      ),
    );
  }
}