InfiniteQueryObserver<TPageData, TPageParam, TData> class Observers

Watches one infinite query, and pages it.

Everything QueryObserver does applies — subscribe, currentResult, refetch, setOptions, destroy — with an InfiniteData as the query's data (or whatever a select makes of it). On top, it adds:

• paging: fetchNextPage and fetchPreviousPage;
• whether there is more: hasNextPage and hasPreviousPage;
• which fetch is running or failed: isFetchingNextPage, isFetchingPreviousPage, isFetchNextPageError, isFetchPreviousPageError, and isRefetching / isRefetchError narrowed to refetches of the pages already held.

These live on the observer rather than on the result, because the sealed QueryResult is shared by every kind of query. A listener is still told when one of them changes, whether or not the result did, so reading them in the listener is always up to date. (TanStack Query puts them on its infinite result object instead.)

final observer = InfiniteQueryObserver(
  client,
  InfiniteQueryObserverOptions<List<Post>, int>(
    queryKey: QueryKey(['feed']),
    pageFn: (context) => api.feed(offset: context.pageParam),
    initialPageParam: 0,
    getNextPageParam: (page, pages, pageParam, pageParams) =>
        page.isEmpty ? null : pageParam + page.length,
  ),
);
final unsubscribe = observer.subscribe((result) {
  if (result case QuerySuccess(:final data)) {
    render(data.flatten<Post>(), canLoadMore: observer.hasNextPage);
  }
});
// When the user scrolls to the end:
if (observer.hasNextPage && !observer.isFetchingNextPage) {
  await observer.fetchNextPage();
}