batch_fetcher_flutter #

Flutter bindings for batch_fetcher: a ChangeNotifier facade and a builder that rebuilds one row at a time.

The engine is pure Dart on purpose, so it can drive a bloc without a ChangeNotifier in the middle. This package is for the case where you do want one.

final costs = BatchFetchNotifier<String, double, BatchKey>(
  request: (ids, key) async =>
      BatchOutcome.resolved(await api.costs(ids, at: key)),
);

// In a row, rebuilding only when *this* id changes:
BatchFetchBuilder<String, double>(
  source: costs,
  id: event.id,
  builder: (context, entry) => switch (entry) {
    FetchPresent(:final value) => Text(format(value)),
    FetchLoading() => const CostSkeleton(),
    FetchFailed(:final willRetry) => RetryChip(enabled: !willRetry),
    FetchAbsent() || FetchIdle() => const Text('—'),
  },
);

Why the builder, and not AnimatedBuilder #

A notifier fires once per change, and a batch changes many ids at once. An AnimatedBuilder over the notifier therefore rebuilds all 120 rows whenever any one id resolves. BatchFetchBuilder compares the entry for its own id and rebuilds only on a real change — the selector pattern, packaged, so no feature has to remember it.

Entries have value equality precisely so this works.

Ownership #

BatchFetchNotifier(...) builds the fetcher and owns it: disposing the notifier disposes the fetcher. When the fetcher outlives the notifier — it is registered in a service locator, say — wrap it instead and say so:

BatchFetchNotifier.wrapping(sharedFetcher, owns: false);

BatchFetchListenable #

BatchFetchBuilder takes a BatchFetchListenable<TId, TValue> rather than a BatchFetchNotifier<TId, TValue, TKey>. A widget needs the state of one id and a signal when it changes; neither depends on how requests are grouped. Erasing TKey at that boundary is what lets one widget serve fetchers with different key types — the alternative people reach for is an interface that erases the value type to dynamic too, which throws away the type safety that made the generics worth having.

Implement it directly if you have a source that is not a BatchFetcher.