Flutter run async function on stream listen

Question

I have a stream of data that gets me messages, and on each message, I want to execute an HTTP request, i.e. an async function. How can I do that?

Minimum reproducible example:

Future<int> myFuture(int n) async {
  Future.delayed(Duration(seconds: 2));  // Mock future
  print("Future finished for: $n");
  return n;
}

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

void main() async {
  var stream = countStream(10);
  stream.listen((n) { await myFuture(n); });
  // or simply: stream.listen(myFuture);
}

Essentially I can't await myFuture.

Answer 1

You can make the listen callback an async function, and then await the call to myFuture. Here's an updated version of your example code:

Future<int> myFuture(int n) async {
  await Future.delayed(Duration(seconds: 2));  // Mock future
  print("Future finished for: $n");
  return n;
}

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

void main() async {
  var stream = countStream(10);
  stream.listen((n) async {
    await myFuture(n);
  });
}

Alternatively, you could use the asyncMap method on the Stream object to apply the asynchronous operation to each element of the stream:

void main() async {
  var stream = countStream(10);
  await for (var result in stream.asyncMap((n) => myFuture(n))) {
    // do something with the result
  }
}

In this example, asyncMap applies the myFuture function to each element of the stream, and returns a new stream containing the results. The await for loop then iterates over the results of the new stream.