[flutter_timer] Timer Bloc tick event

timer_bloc.dart

import 'dart:async';
import 'package:meta/meta.dart';
import 'package:bloc/bloc.dart';
import 'package:flutter_timer/bloc/bloc.dart';
import 'package:flutter_timer/ticker.dart';

class TimerBloc extends Bloc<TimerEvent, TimerState> {
  final Ticker _ticker;
  final int _duration = 60;
  
  StreamSubscription<int> _tickerSubscription;
  
  TimerBloc({@required Ticker ticker})
      : assert(ticker != null),
        _ticker = ticker;
  
  @override
  TimerState get initialState => Ready(_duration);

  @override
  Stream<TimerState> mapEventToState(
    TimerEvent event,
  ) async* {
    if (event is Start) {
      yield* _mapStartToState(event);
    } else if (event is Tick) {
      yield* _mapTickToState(event);
    }
  }
  
  @override
  void dispose() {
    _tickerSubscription?.cancel();
    super.dispose();
  }

  Stream<TimerState> _mapStartToState(Start start) async* {
    yield Running(start.duration);
    _tickerSubscription?.cancel();
    _tickerSubscription = _ticker.tick(ticks: start.duration).listen(
      (duration) {
        dispatch(Tick(duration: duration));
      },
    );
  }
  
  Stream<TimerState> _mapTickToState(Tick tick) async* {
    yield tick.duration > 0 ? Running(tick.duration) : Finished();
  }
}