Merge Pagination Data

Problem

You need to implement load more functionality that merges new paginated data with existing cached results without duplicates.

Load More Implementation

The datasource provides a loadMore method that uses updateResult for merging:

class HotelBookingsDatasource {
  @override
  void loadMore({
    required int skip,
    required int take,
    GhotelbookingsFilters? filter,
    GhotelbookingsOrder? sort,
  }) {
    final listHolder = requestContext.getHolder<GQueryHotelBookingsReq>(
      HotelBookingsRequestStrategyKeys.hotelBookingsList.name,
    );

    final params =
        ListRequestParams<GhotelbookingsFilters, GhotelbookingsOrder>(
          skip: skip,
          take: take,
          filter: filter,
          sort: sort,
        );

    final nextRequest =
        listHolder?.request?.rebuild(
          (b) => b
            ..vars.G_skip = skip
            ..updateResult = (previous, result) {
              if (previous == null) return result;
              return previous.rebuild(
                (b) => b
                  ..hotelbookings.hotelbookingsItems.addAll(
                    result?.hotelbookings?.hotelbookingsItems?.toList() ?? [],
                  ),
              );
            },
        ) ??
        requestContext.execute<
          GQueryHotelBookingsReq,
          ListRequestParams<GhotelbookingsFilters, GhotelbookingsOrder>
        >(HotelBookingsRequestStrategyKeys.hotelBookingsList.name, params);

    client.requestController.add(nextRequest);
  }
}

UpdateResult Pattern

The key to pagination merging is the updateResult function that combines previous and new data:

Key Components

Request Holder - Get the existing request to rebuild it
Skip Update - Update the skip parameter for the next page
UpdateResult Function - Merge previous data with new results
Request Controller - Add the rebuilt request to the queue

UpdateResult Function Breakdown

..updateResult = (previous, result) {
  if (previous == null) return result;
  return previous.rebuild(
    (b) => b
      ..hotelbookings.hotelbookingsItems.addAll(
        result?.hotelbookings?.hotelbookingsItems?.toList() ?? [],
      ),
  );
}

This function:

Returns new result if no previous data exists
Rebuilds previous data by adding new items to the existing list
Handles null safety with the ?? [] operator

Duplicate Handling

For scenarios requiring duplicate prevention, enhance the updateResult function:

..updateResult = (previous, result) {
  if (previous == null) return result;

  final existingItems = previous.hotelbookings?.hotelbookingsItems?.toList() ?? [];
  final newItems = result?.hotelbookings?.hotelbookingsItems?.toList() ?? [];

  final existingIds = existingItems
      .where((item) => item?.id != null)
      .map((item) => item!.id!)
      .toSet();

  final uniqueNewItems = newItems
      .where((item) => item?.id != null && !existingIds.contains(item!.id!))
      .toList();

  return previous.rebuild(
    (b) => b
      ..hotelbookings.hotelbookingsItems.addAll(uniqueNewItems)
      ..hotelbookings.totalCount = result?.hotelbookings?.totalCount,
  );
}

## Repository Load More

The repository exposes the load more functionality:

```dart
class HotelBookingsRepository {
  @override
  void loadMore({
    required int skip,
    required int take,
    HotelBookingFilterCriteria? filter,
    HotelBookingSortCriteria? sort,
  }) {
    datasource.loadMore(
      skip: skip,
      take: take,
      filter: filter?.toGql,
      sort: sort?.toGql,
    );
  }
}