Dart program exits without executing last statement

问题

I'm trying to understand Streams and wrote some code. Everything seems to work, the program exits with status code 0. But it doesn't print the 'loop done' and 'main done' strings. I can't figure out why.

import 'dart:async';

Stream<int> countStream(int to) async* {
      for (int i = 1; i <= to; i++) {
              yield i;
      }
}

class Retry {
    StreamController<int> _outgoing;

    Retry(Stream<int> incoming) {
        _outgoing = StreamController<int>();
        _outgoing.addStream(incoming);
    }

    Future<void> process() async {
        await for (final i in _outgoing.stream) {
            print("got $i");
        }
        print('loop done'); // Not printed
    }
}

void main() async {
  var stream = countStream(4);
  var retry = Retry(stream);
  await retry.process();
  print('main done'); // Not printed
}

回答1:

The _outgoing.stream is never closed, so code after the await for will never execute. The VM does notice that there also won't be any new events on that stream so nothing else will ever happen, and it can exit. You could fix the bug with:

_outgoing.addStream(incoming).whenComplete(() {
    _outgoing.close();
});

来源：https://stackoverflow.com/questions/56169492/dart-program-exits-without-executing-last-statement