Merge Firestore's separate queries Streams in Dart - asynchronous

Question

I'm implementing a Todo Application in Flutter. I need to merge a double query in client, in order to perform an OR request in Firestore.
One hand, I have the following code that performs the double queries.
Future<Stream> combineStreams() async {
Stream stream1 = todoCollection
.where("owners", arrayContains: userId)
.snapshots()
.map((snapshot) {
return snapshot.documents
.map((doc) => Todo.fromEntity(TodoEntity.fromSnapshot(doc)))
.toList();
});
Stream stream2 = todoCollection
.where("contributors", arrayContains: userId)
.snapshots()
.map((snapshot) {
return snapshot.documents
.map((doc) => Todo.fromEntity(TodoEntity.fromSnapshot(doc)))
.toList();
});
return StreamZip(([stream1, stream2])).asBroadcastStream();
}
And other hand, I have the following code that will perform the update of view with the Bloc pattern.
Stream<TodosState> _mapLoadTodosToState(LoadTodos event) async* {
_todosSubscription?.cancel();
var res = await _todosRepository.todos(event.userId);
_todosSubscription = res.listen(
(todos) {
dispatch(
TodosUpdated(todos));
},
);
}
I have the following error.
flutter: Instance of '_AsBroadcastStream<List<List<Todo>>>'
[VERBOSE-2:ui_dart_state.cc(148)] Unhandled Exception: type 'List<List<Todo>>' is not a subtype of type 'List<Todo>'
I tried to look for more infos with the debugger and it turned out that my StreamZip source contains the 2 stream separately.
For the moment I can get one stream at a time.
I don't know how to proceed in order to get the 2 streams and display them.

Answer 1

You're doing a map of a map, which returns a List of another List.
You should zip the QuerySnapshot streams and do the mapping after creating the subscription, and then you can create a new Stream<List<Todo>> from it.
///private method to zip QuerySnapshot streams
Stream<List<QuerySnapshot>> _combineStreams() async {
Stream stream1 = todoCollection
.where("owners", arrayContains: userId)
.snapshots()
});
Stream stream2 = todoCollection
.where("contributors", arrayContains: userId)
.snapshots()
});
return StreamZip(([stream1, stream2])).asBroadcastStream();
}
///exposed method to be consumed by repository
Stream<List<Todo>> todosStream() {
var controller = StreamController<List<Todo>>();
_combineStreams().listen((snapshots) {
List<DocumentSnapshot> documents;
snapshots.forEach((snapshot) {
documents.addAll(snapshot.documents);
});
final todos = documents.map((document) {
return Todo.fromEntity(TodoEntity.fromSnapshot(doc));
}).toList();
controller.add(todos);
});
return controller.stream;
}