River_pod: Listening to selected states of StateNotifierProvider

[MarcinusX]

Hey Remi 👋
Got a question for you :)

I am having a scenario where I have a freezed union which is a state of StateNotifier.
In 2 of possible states, there is a field I want to expose in another provider.

final channelProvider = StateProvider<Channel>((ref) {
  final state = ref.watch(sendbirdServiceProvider.state);
  return state.maybeWhen(
    channelChosen: (_, channel) => channel,
    messagesLoaded: (_, channel, __) => channel,
    orElse: () => null, //Use previous state (don't rebuild) if possible
  );
});

And this works.
Although I wish to know if it's possible to conditionally return the previous instance of a provider instead of creating a new one?
Basically in orElse I would like to return a cached channel if it's possible.

The other way to approach it is to watch for only specific states of stateNotifierProvider which contain channel I want to expose and don't cause rebuilds in other scenarios. Is any of those possible?

Thank you 💙

[smiLLe]

Your channel can be null.

return state.maybeWhen(
    channelChosen: (_, channel) => channel,
    messagesLoaded: (_, channel, __) => channel,
    orElse: () => null, //Use previous state (don't rebuild) if possible
  );

orElse might be the first thing to be called in this statement and if not, why do you expose more than channelChosen and messagesLoaded?

[MarcinusX]

orElse might be the first thing to be called in this statement

Yeah, I know I can't always prevent it but accessing the previous state would be great with all the non-first scenarios.

[rrousselGit]

That's a case I'm working on

I'm thinking of two things:

ref.listen<T>(myProvider, (value) {
  print(value);
});

as an alternative to ref.watch, as your typical onChange (but without having to care about removing the listener).

Then combined with ref.setState(newState)

So we'd have

final example = StateProvider<Model>((ref) {
  ref.listen<Another>(another, (value) {
    ref.setState(<whatever>);
  });
});

[rrousselGit]

For now you can do:

final example = StateNotifierProvider<StateController<Channel>>((ref) {
  final notifier = StateController<Channel>();

  final sendbird = ref.watch(sendbirdServiceProvider);
  final removeListener = sendbird.addListener((value) {
    notifier.state = value.channel;
  });
  ref.onDispose(removeListener);

  return notifier;
});

[MarcinusX]

Thank you! <3