start method

void start()

Implementation

void start() {
  if (extendable) reset();
  if (_timer == null) {
    reset();
    _onTick();
    _timer = Timer.periodic(period, (_) {
      if (countDown && _elapsed.value <= Duration.zero ||
          !countDown &&
              duration != Duration.zero &&
              _elapsed.value >= duration) {
        _onCancel();
      } else {
        _onTick();
      }
    });
  }
}