ref.listen not invoked when state changed with the same value

[minhnt3]

Problem

I know that ref.listen will not invoked when state changed with the same value from Riverpod 2.x. This is a feature in most cases but it may lead to a bug in some cases. Let's assume we are implementing the pagination feature. We have to listen to state changes to add data to PagingController.

final provider = StateProvider<List<int>>((ref) => []);

ref.listen(
        provider,
        (previous, next) {
         if (next.isEmpty) pagingController.appendLastPage(next);
         else pagingController.appendPage(next);
        },
);

Of course, the default state is an empty list. When we fetch data of the first page, if it also returns an empty list, the ref.listen will not triggered and thus the pagingController.appendLastPage will never be called.

In another case, when data of the first page is [1,2,3] and data of the second page is [1,2,3] as well, the ref.listen will not triggered and we will lose data of the second page. It's a bug. Of course, we can fix them by using some flags but it make code so messy.

Solution

I think we should add a boolean parameter to ref.listen that allow us using it flexiblely or we create a new listen method to use in some special cases like these. Thanks.

[rrousselGit]

This is unrelated to ref.listen. ref.listen doesn't filter anything.
Your issue is how you modify your state. It's the provider that filters notification.

For that purpose, there's already ref.notifyListeners() which providers can call if they mutated their state instead of cloning it. Sounds like you want a way to call notifyListeners when using StateProvider.

I'd suggest using Provider instead of StateProvider, and making a generic object that exposes notifyListeners.

class Box<T> {
  Box(this._ref, this.value);
  final Ref<T> _ref;

  T value;

  void notifyListeners() => _ref.notifyListeners();
}

Then used as:

final provider = Provider<Box<List<int>>>((ref) => Box(ref, []));


ref.read(provider)
  ..value.add(42)
  ..notifyListeners();