interval<T>  method 
Implementation
ZenWorkerHandle interval<T>(
  ValueNotifier<T> obs,
  void Function(T) callback,
  Duration duration,
) {
  if (duration.isNegative || duration == Duration.zero) {
    throw ArgumentError('Interval duration must be positive');
  }
  return _createWorker(
      () => ZenWorkers.interval<T>(obs, callback, duration), 'interval');
}