https://stackoverflow.com/questions/74716313/dart-streams-is-it-possible-to-map-to-a-new-async-stream

74716313.dart

void main() {
  // Simulate the [signinResponse] as a simply [String].
  Stream<String> signInStream() => Stream<String>.periodic(Duration(seconds: 1), (computation) => 'Login $computation');
  
  // Simulate the [userResponse] as a simply [int].
  Stream<int> getUserDetailsStream() => Stream<int>.periodic(Duration(seconds: 1));

  final Stream<Stream<int>> itIsWhatYouAreDoing = signInStream().asyncMap((event) => getUserDetailsStream());
  
  // What you are saying you want to achieve:
  final Stream<int> userDetailsStream = signInStream().asyncMap((event) async => await getUserDetailsStream().first);

  print(itIsWhatYouAreDoing); // Stream<Stream<int>>
  print(userDetailsStream); // Stream<int>
  
  // But this has some cons: you lose any other post-event emitted by all [getUserDetailsStream] Streams.
  // You didn't provide any detail about this, so I'll assuming they are irrelevant?!?
}