// But let's make it async.
typedef AsyncGetter<T> = Future<T> Function();
Future<List<T>> asyncCollect<T>(AsyncGetter<T> getter) async {
  final List<T> list = <T>[];
  T value;
  while ((value = await getter()) != null) {
    list.add(value);
  }
  return list;
}

// Now we use it to generate and collect a bunch of scheduled tasks:
Future<void> main() async {
  int index = 0;
  final List<Future<dynamic>> tasks = await asyncCollect<Future<dynamic>>(
    () {
      if (index < 10) {
        final Completer<void> task = Completer<void>();
        Timer(Duration(seconds: index), () {
          print(index);
          task.complete();
        });
        index += 1;
        return task.future;
      }
      return null;
    },
  );
  print(tasks.length);
}