How to do it...

We will create a very simple countdown app, which goes from 60 to 0, using a BLoC:

1. Create a new file in the lib folder of your project and call it countdown_bloc.dart.
2. In the countdown_bloc.dart file, import the dart:async library:

import 'dart:async';

3. Still in the countdown_bloc.dart file, create a new class, called TimerBLoC: 

class TimerBLoC {}

4. At the top of the TimerBLoC class, declare an integer, StreamController, a Stream getter, and StreamSink:

  int seconds = 60;
  StreamController _secondsStreamController = StreamController();
  Stream get secondsStream =>
      _secondsStreamController.stream.asBroadcastStream();
  StreamSink get secondsSink => _secondsStreamController.sink;

5. In the TimerBLoC class, create an async method called decreaseSeconds, which decreases the seconds and adds the new value to the sink: 

  Future decreaseSeconds() async {
    await Future.delayed(const Duration(seconds: 1));
    seconds...