AsyncNotifier: how to watch only for a property change, ignore the loading/error states

[nicolasbraun]

Hello

I have a StreamProvider that is a Stream of a Firebase snapshot of a document Event.

@riverpod
Stream<Mood?> eventStream(Ref ref, EventID evenId) {
  final eventsRepository = ref.watch(eventsRepositoryProvider);
  return eventsRepository.watchEvent(id: eventId);
}

This stream is watched on the Event screen where I need to handle loading/error states wich AsyncValue.whenwhich is fine.
I also need to reed this Stream in one other controller (for the event Chat) because I need the events settings here

@riverpod
class ChatController extends _$ChatController {


  @override
  Future<void> build({required EventID eventdId}) async {

    state = AsyncValue.loading();


    final members = ref.watch(
        eventStreamProvider(eventId).select((value) => value.value?.settings));
...

Problem is when the event is edited it transitions for a bit between AsyncLoading and AsyncData causing my ChatController to rebuild.

TL;DR how can I in a select for an AsyncProvider ignore loading/error states (and maybe return the previous value)

I was thinking maybe I can store the previous value and do something like

    final members = ref.watch(
        eventStreamProvider(moodId).select((value) => value.value != null ? value.value.settings : _storedSettings));

Thank you

[gdurandrexel]

You can use ref.listen to manually control the updates to your state

[zhxst]

Use the members.valueOrNull. the state will keep the previous data even when there is an error.

Of course you will get a null before the first value comes out from stream.

[nicolasbraun]

Thanks for your quick answer ! I'll try asap.
To be sure I understand well the purpose of select is specifically to rebuild only some value changes, right?

In terms of design would it be better, from your point of view, to build a chatMembersProvider (an old StateNotifier initialized with an empty list and watching or listening to my eventProvider ?

[gdurandrexel]

To be sure I understand well the purpose of select is specifically to rebuild only some value changes, right?

select() rebuilds if the returned value is not equal (as in operator ==) to the previous one. If the objects you return do not override == then 2 different objects (instances) won't be equal, and you'll have a rebuild. That's probably why you see a rebuild.

Note that you probably don't need state = AsyncValue.loading(); in the build method. The state automatically switches to AsyncLoading when the provider rebuilds.