Create a stream that will emit a single constant value to every subscription once

Question

I want to create a stream that can emit a single value to every subscriber, no matter when they subscribed.

Ideally, the following code should print out two 42 before done, but none of the given approaches work.

void main() async {
  final stream = createStream(42);
  stream.listen(printValue);
  await Future.delayed(Duration(seconds: 1));
  stream.listen(printValue);

  // Wait for everything to finish.
  await Future.delayed(Duration(seconds: 1));
  print('done');
}

Stream<int> createStream(int val) {
  // Error: Stream has already been listened to.
  // return Stream.value(val);

  // Only gets one output
  return Stream.value(val).asBroadcastStream();
}

void printValue(int val) {
  print(val);
}

Desired output:

42
42
done

But Stream.value() generates an error Stream has already been listened to.. While .asBroadcastStream() gives the wrong result.

---

Note: Answers using RxDart would be welcomed.

Answer 1

You can achieve the desired output by using a multi-subscription stream:

void main() async {
  final stream = createStream(42);
  stream.listen(printValue);
  await Future.delayed(Duration(seconds: 1));
  stream.listen(printValue);

  // Wait for everything to finish.
  await Future.delayed(Duration(seconds: 1));
  print('done');
}

/// Returns a multi-subscription stream.
Stream<int> createStream(int val) => Stream.multi((streamController) {
      streamController.add(val);
    });

void printValue(int val) {
  print(val);
}

The console output is listed below:

$ dart main.dart 
42
42
done

Note: Another easy way to create a multi-subscription stream is by using the factory constructor Stream.fromIterable:

/// Returns a multi-subscription stream that yields only `val`.
Stream<int> createStream(int val) => Stream.fromIterable([val]);