CounterBloc.dart

import 'dart:async';

class CounterBloc {
  int _counter = 0;
  final _stateController = StreamController<int>();

  // This one goes to the StreamBuilder
  Stream<int> get counterStream => _stateController.stream;

  // This is one of posible modificators
  // I chose String for simplicity. We can pass a param of any type and the check it.
  void modifyCounter(String direction) {
    if (direction == 'up') {
      _counter++;
    } else {
      _counter--;
    }

    // This causes the StreamBuilder to rerender itself
    _stateController.sink.add(_counter);
  }

  void dispose() {
    _stateController.close();
  }
}