stream - Difference between await for and listen in Dart

Question

I am trying to create a web server stream. Here is the code:

import 'dart:io';

main() async {
  HttpServer requestServer = await HttpServer.bind(InternetAddress.LOOPBACK_IP_V4, 8000);

requestServer.listen((request) {   //comment out this or the await for to work
  request.response
    ..write("This is a listen stream")
    ..close();
});

  await for (HttpRequest request in requestServer) {
  request.response
    ..write("This is an await for stream")
    ..close();
  }
}

What is the difference between listen and await for? They both do not work at the same time. You need to comment out one or the other to work, but there doesn't seem to be a difference in function here. Are there circumstances where there is a difference, and when should you use one over the other?

Answer 1

Given:

Stream<String> stream = new Stream<String>.fromIterable(['mene', 'mene', 'tekel', 'parsin']);

then:

print('BEFORE');
stream.listen((s) { print(s); });
print('AFTER');

yields:

BEFORE
AFTER
mene
mene
tekel
parsin

whereas:

print('BEFORE');
await for(String s in stream) { print(s); }
print('AFTER');

yields:

BEFORE
mene
mene
tekel
parsin
AFTER

stream.listen() sets up code that will be put on the event queue when an event arrives, then following code is executed.

await for suspends between events and keeps doing so until the stream is done, so code following it will not be executed until that happens.

I use `await for when I have a stream that I know will have finite events, and I need to process them before doing anything else (essentially as if I'm dealing with a list of futures).

Check https://www.dartlang.org/articles/language/beyond-async for a description of await for.