Can't yield in forEachAsync inside Stream in dart/flutter - flutter

Question

I have a forEachAsync inside an async* Stream and can't yield.
Stream<ProjectState> _mapProjectSelectedEventToState(ProjectSelected event) async* {
try {
yield ProjectLoading(
message: 'Fetching database',
fetchedCount: 0,
totalCount: 1,
);
await forEachAsync(fileModels, (FileEntity fileModel) async {
await downloader.download(filename: fileModel.hashName);
_totalMediaFilesFetched++;
//// ERROR - THIS DOES NOT WORK ////
yield (ProjectLoadingTick(
_totalMediaFiles,
_totalMediaFilesFetched,
));
}, maxTasks: 5);
} catch (error, stacktrace) {
yield ProjectFailure(error: error);
}
}
I've tried other means by dispatching the message and converting it to a state but it doesn't work as well. It seems like the whole app is blocked by this await forEachAsync.
I'm using the bloc pattern which reacts to the emited ProjectStates based on the current ProjectSelected event

Answer 1

Your attempt doesn't work because you're using yield in a callback, not in the function that's returning a Stream. That is, you're attempting the equivalent of:
Stream<ProjectState> _mapProjectSelectedEventToState(ProjectSelected event) async* {
...
await forEachAsync(fileModels, helperFunction);
...
}
Future helperFunction(FileEntity fileModel) async {
...
yield ProjectLoadingTick(...);
}
which doesn't make sense.
Since care about forEachAsync's ability to set a maximum limit to the number of outstanding asynchronous operations,
you might be better off using a StreamController that you can manually add events to:
var controller = StreamController<ProjectState>();
// Note that this is not `await`ed.
forEachAsync(fileModels, (FileEntity fileModel) async {
await downloader.download(filename: fileModel.hashName);
_totalMediaFilesFetched++;
controller.add(ProjectLoadingTick(
_totalMediaFiles,
_totalMediaFilesFetched,
));
},
maxTasks: 5);
yield* controller.stream;