Why doesn't my Dart async function start working immediately-CodePudding

I have this program that I'm using to learn about Dart's async programming.

import 'dart:io';

Future<int> sumStream(Stream<int> stream) async {
  var sum = 0;
  await for (final value in stream) {
    print('consuming event $value');
    sum  = value;
  }
  return sum;
}

Stream<int> countStream(int to) async* {
  for (int i = 1; i <= to; i  ) {
    sleep(const Duration(milliseconds: 400));
    print('publishing event $i');
    yield i;
  }
}

Future<void> main() async {
  var stream = countStream(10);
  var sum = sumStream(stream);

  print('working...');
  sleep(const Duration(milliseconds: 500));
  print('working...');
  sleep(const Duration(milliseconds: 500));
  print('working...');
  sleep(const Duration(milliseconds: 500));

  print(await sum); // 55
}

Output:

working...
working...
working...
publishing event 1
consuming event 1
publishing event 2
consuming event 2
publishing event 3
consuming event 3
publishing event 4
consuming event 4
publishing event 5
consuming event 5
publishing event 6
consuming event 6
publishing event 7
consuming event 7
publishing event 8
consuming event 8
publishing event 9
consuming event 9
publishing event 10
consuming event 10
55

In the code above, I chose not to await for the result of sumStream() immediately because I wanted to do some additional work while sumStream() is busy consuming events from the provided stream. So my expectation was that sumStream() would start running immediately while main() is running. I expected the output to look something like below text. I expected the text working... to be interleaved with the other print outs from the publisher countStream() and the consumer sumStream().

working...
publishing event 1
consuming event 1
publishing event 2
consuming event 2
publishing event 3
consuming event 3
working...
publishing event 4
consuming event 4
publishing event 5
consuming event 5
publishing event 6
working...
consuming event 6
publishing event 7
consuming event 7
publishing event 8
consuming event 8
publishing event 9
consuming event 9
publishing event 10
consuming event 10
55

Is this because Dart is single threaded and thus it can't run main(), countStream() and sumStream() at the same time? If so, how could I change my program so that sumStream() would run in parallel with main() (make it multi-threaded)?

CodePudding user response：

I am pretty sure this is due to the sleep function. Sleep function documentation:

Use this with care, as no asynchronous operations can be processed in a isolate while it is blocked in a sleep call.

I suggest trying this instead:

await Future.delayed(const Duration(milliseconds: 500));

Which won't block the processing of all the other async operations going on.