A CountDown Timer Because Dart Team Couldn't Be Bothered

countdown_timer.dart

class CountdownTimer extends ValueListenable<Duration> with ChangeNotifier {
  CountdownTimer(this.duration, {this.frequency});

  final Duration duration;

  final Duration? frequency;

  final Stopwatch _stopwatch = Stopwatch();

  Timer? _timer;

  bool get timedOut => value == Duration.zero;

  bool get isRunning => _timer != null;

  void start() {
    assert(_timer == null);
    // Yeah there's probably like a sub microsecond error here because stopwatch
    // and timer aren't initialized in the same CPU cycle, sue me.
    _stopwatch.start();
    _timer = Timer.periodic(
      frequency ?? const Duration(milliseconds: 10),
      (t) {
        notifyListeners();
      },
    );
  }

  void pause() {
    _timer?.cancel();
  }

  void stop() {
    _timer?.cancel();
    _timer = null;
  }

  @override
  Duration get value {
    final Duration timeLeft = duration - _stopwatch.elapsed;
    if (timeLeft.isNegative) {
      stop();
      return Duration.zero;
    }
    return timeLeft;
  }
}