Stream async* and Future async in Dart

main.dart

Future<int> sumStream(Stream<int> stream) async {
  int sum = 0;
  await for (var i in stream) {
    sum += i;
  }
  return sum;
}
 
Future<int> sumStream2(Stream<int> stream) =>
    stream.reduce((previous, element) => previous + element);
 
// Stream generator 异步的 asynchronous
Stream<int> countStream(int n) async* {
  for (var i = 1; i <= n; i++) {
    yield i;
  }
}
 
// Iterable generator 同步的 synchronous
Iterable<int> count(int n) sync* {
  for (var i = 1; i <= n; i++) {
    yield i;
  }
}
 
Future<void> main() async {
  print('----- lets see Stream and Future ----');
  final stream = Stream<int>.fromIterable([1, 2, 3, 4]);
  final sum = await sumStream2(stream);
  print(sum);
 
  print('-----create stream using stream operator async* -----');
  final stream2 = countStream(4);
  final sum2 = await sumStream(stream2);
  print(sum2);
 
  print('-----create iterable using Iterable operator sync* -----');
  final myIter = count(4);
  print(myIter.reduce((value, element) => value + element));
}

output

----- lets see Stream and Future ----
10
-----create stream using stream operator async* -----
10
-----create iterable using Iterable operator sync* -----
10

Stream with Future. stream is a iterable of future instance.

hard to understand.

stream.reduce() waits for each event to be available before calling the combined function.