call method
The BatchRequest to hand to a fetcher.
Implementation
Future<BatchOutcome<TId, TValue>> call(List<TId> ids, TKey key) async {
final index = calls.length;
calls.add(List<TId>.unmodifiable(ids));
keys.add(key);
final step = steps[index < steps.length ? index : steps.length - 1];
return step(ids, key);
}