Stream of List with elements from another streams' current values - dart

Question

I want to create a List Stream based on another elements Stream. The List Stream should yield a new list every time an element from the list emits a new value.
Something like this:
Stream<List<Model>> getListStream(List<int> ids) async* {
final List<Model> models = [];
for (var id in ids) {
getModelStream(id).listen((event) {
models.add(event);
});
}
yield models;
}
but it always yields an empty array.
I think the problem is probably the fact the it does not react to the event listener.
How do you deal with this kind of problem?

Answer 1

You're yielding an empty list which will later be filled in.
If you want to wait for the list to be filled in before yielding it, you'll have to do that.
Either:
Stream<List<Model>> getListStream(List<int> ids) async* {
for (var id in ids) {
yield await getModelStream(id).toList();
}
}
or, if you want to start all the streams immediately, and compute them in parallel, and then emit the values when they are done (not necessarily in the original ID order), then:
Stream<List<Model>> getListStream(List<int> ids) {
var controller = StreamController<List<Model>>(sync: true);
controller.onListen = () {
var count = 0;
for (var id in ids) {
count++;
getModelStream(id).toList().then((models) {
controller.add(models);
if (--count == 0) controller.close();
});
}
};
return controller.stream;
}