Is there a way to get notified when a dart stream gets its first result? - flutter

Question

I currently have an async function that does the following:
Initializes the stream
Call stream.listen() and provide a function to listen to the stream.
await for the stream to get its first result.
The following is some pseudo code of my function:
Future<void> initStream() async {
// initialize stream
var stream = getStream();
// listen
stream.listen((result) {
// do some stuff here
});
// await until first result
await stream.first; // gives warning
}
Unfortunately it seems that calling stream.first counts as listening to the stream, and streams are not allowed to be listened by multiple...listeners?
I tried a different approach by using await Future.doWhile()
Something like the following:
bool gotFirstResult = false;
Future<void> initStream() async {
var stream = getStream();
stream.listen((result) {
// do some stuff here
gotFirstResult = true;
});
await Future.doWhile(() => !gotFirstResult);
}
This didn't work for me, and I still don't know why. Future.doWhile() was successfully called, but then the function provided to stream.listen() was never called in this case.
Is there a way to wait for the first result of a stream?
(I'm sorry if I didn't describe my question well enough. I'll definitely add other details if needed.)
Thanks in advance!

Answer 1

One way is converting your stream to broadcast one:
var stream = getStream().asBroadcastStream();
stream.listen((result) {
// do some stuff here
});
await stream.first;

Answer 2

Another way, without creating new stream, is to use Completer. It allows you to return a Future which you can complete (send value) later. Caller will be able to await this Future as usual.
Simple example:
Future<int> getValueAsync() {
var completer = Completer<int>();
Future.delayed(Duration(seconds: 1))
.then((_) {
completer.complete(42);
});
return completer.future;
}
is equivalent of
Future<int> getValueAsync() async {
await Future.delayed(Duration(seconds: 1));
return 42;
}
In your case:
Future<void> initStream() {
var stream = getStream();
var firstValueReceived = Completer<void>();
stream.listen((val) {
if (!firstValueReceived.isCompleted) {
firstValueReceived.complete();
}
// do some stuff here
});
return firstValueReceived.future;
}