Dart CLI, progress indication while awaiting Future

main.dart

import 'dart:async';

void main() async {
  // start future
  final future = doHardWork();
  // continously post progress
  final sub = showProgress().listen((_) {});
  // wait for completion
  await future;
  // stop progress
  await sub.cancel();
  print("done");
}

Future<void> doHardWork() async {
  await Future.delayed(Duration(seconds: 5));
  print("Hard work completed");
}

Stream<void> showProgress() {
  final start = DateTime.now();

  StreamController<void> controller = StreamController();
  bool cancelled = false;
  controller.onCancel = () {
    cancelled = true;
  };

  () async {
    while (true) {
      await Future.delayed(Duration(seconds: 1));

      if (cancelled) {
        return;
      }
      print("Waiting since ${DateTime.now().difference(start).inSeconds}s");
    }
  }();

  return controller.stream;
}