Isolates emiting numbers and another stream sums them

isolates.dart

import 'dart:async';
import 'dart:collection';
import 'dart:isolate';
import 'package:rxdart/rxdart.dart';

void main() async {
  await sumTwoLists();
  print("done");
}

///
/// Create two isolates and passes to them a list of integers
/// Then it listens to the Isolate streams and prints the sum of numbers from first and second lists
/// Once both lists are finished the function finishes.
///
Future sumTwoLists() async {
  final p = ReceivePort();
  final e = ReceivePort();

  await Isolate.spawn(_emitNumber, [
    p.sendPort,
    [0, 1, 2, 3]
  ]);
  await Isolate.spawn(_emitNumber, [
    e.sendPort,
    [10, 11, 12, 13]
  ]);

  await sumListsInIsolates(p, e);
}

Future sumListsInIsolates(ReceivePort a, ReceivePort b) {
  // create a promise so we can notify caller that both streams are empty and we
  // summed all the numbers in both lists.
  var completer = Completer();
  // zipper with combine numbers from both streams into a List with two values
  // It will combine only values with the same index in stream
  // null means the Isolate was killed
  a.zipWith(b, (f, s) {
    if (f == null || s == null) {
      // !!! don't forget to close the ports as the application will never Exit with Code 0 !!!
      // otherwise it will be forever running
      a.close();
      b.close();
      completer.complete(null);
      return;
    }
    return f + s;
  }).listen(print);
  // return defer object
  return completer.future;
}

// emits given numbers one by one each second. Isolate is exit when no numbers left
void _emitNumber(List<dynamic> args) {
  SendPort p = args[0];
  List<int> numbers = args[1];
  // use queue to easier empty the list
  var queue = Queue.from(numbers);
  var stream = Stream.periodic(const Duration(seconds: 1), (count) {
    if (queue.isNotEmpty) {
      return queue.removeFirst();
    } else {
      Isolate.exit(p);
    }
  });
  stream.listen((event) {
    p.send(event);
  });
}