import 'dart:async';

// Consume a stream.
Future<int> sumStream(Stream<int> stream) async {
  var sum = 0;
  await for (var value in stream) {
    sum += value;
  }
  return sum;
}

// Create a stream.
Stream<int> countStream(int to) async* {
  for (int i = 1; i <= to; i++) {
    // add delay to generation of each value
    // await Future.delayed(Duration(milliseconds: 500));
    yield i;
  }
}

main() async {
  var stream = countStream(10);
  var sum = await sumStream(stream);
  print(sum); // 55
}