How can I know when two Streams are done?

Question

I am using Dart's Streams for async events. I have two streams, and I want to know when both streams are complete (aka closed). I'd like to do this with Futures. How do I do it?

Answer 1

A StreamSubscription can return a Future with asFuture. When the stream is closed, the subscription's future completes.

Use Future.wait to wait for both streams' futures to complete.

Here is an example:

import 'dart:async';

Future listenOnTwoStreams(Stream s1, Stream s2) {
  StreamSubscription sub1 = s1.listen((e) => print(e));
  StreamSubscription sub2 = s2.listen((e) => print(e));
  
  Future f1 = sub1.asFuture();
  Future f2 = sub2.asFuture();
  
  return Future.wait([f1, f2]).then((e) => print("all done!"));
}

void main() {
  StreamController controller1 = StreamController();
  StreamController controller2 = StreamController();
  
  Stream s1 = controller1.stream;
  Stream s2 = controller2.stream;
  
  listenOnTwoStreams(s1, s2);
  
  Timer.run(() {
    controller1.add("1");
    controller2.add("2");
    
    controller1.close();
    controller2.close();
  });
}