PagedListAdapter jumps to beginning of the list on receiving new PagedList

Question

I'm using Paging Library to load data from network using ItemKeyedDataSource. After fetching items user can edit them, this updates are done inside in Memory cache (no database like Room is used).

Now since the PagedList itself cannot be updated (discussed here) I have to recreate PagedList and pass it to the PagedListAdapter.

The update itself is no problem but after updating the recyclerView with the new PagedList, the list jumps to the beginning of the list destroying previous scroll position. Is there anyway to update PagedList while keeping scroll position (like how it works with Room)?

DataSource is implemented this way:

public class MentionKeyedDataSource extends ItemKeyedDataSource<Long, Mention> {

    private Repository repository;
    ...
    private List<Mention> cachedItems;

    public MentionKeyedDataSource(Repository repository, ..., List<Mention> cachedItems){
        super();

        this.repository = repository;
        this.teamId = teamId;
        this.inboxId = inboxId;
        this.filter = filter;
        this.cachedItems = new ArrayList<>(cachedItems);
    }

    @Override
    public void loadInitial(@NonNull LoadInitialParams<Long> params, final @NonNull ItemKeyedDataSource.LoadInitialCallback<Mention> callback) {
        Observable.just(cachedItems)
                .filter(() -> return cachedItems != null && !cachedItems.isEmpty())
                .switchIfEmpty(repository.getItems(..., params.requestedLoadSize).map(...))
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(response -> callback.onResult(response.data.list));
    }

    @Override
    public void loadAfter(@NonNull LoadParams<Long> params, final @NonNull ItemKeyedDataSource.LoadCallback<Mention> callback) {
        repository.getOlderItems(..., params.key, params.requestedLoadSize)
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(response -> callback.onResult(response.data.list));
    }

    @Override
    public void loadBefore(@NonNull LoadParams<Long> params, final @NonNull ItemKeyedDataSource.LoadCallback<Mention> callback) {
        repository.getNewerItems(..., params.key, params.requestedLoadSize)
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(response -> callback.onResult(response.data.list));
    }

    @NonNull
    @Override
    public Long getKey(@NonNull Mention item) {
        return item.id;
    }
}

The PagedList created like this:

PagedList.Config config = new PagedList.Config.Builder()
        .setPageSize(PAGE_SIZE)
        .setInitialLoadSizeHint(preFetchedItems != null && !preFetchedItems.isEmpty()
                ? preFetchedItems.size()
                : PAGE_SIZE * 2
        ).build();

pagedMentionsList = new PagedList.Builder<>(new MentionKeyedDataSource(mRepository, team.id, inbox.id, mCurrentFilter, preFetchedItems)
        , config)
        .setFetchExecutor(ApplicationThreadPool.getBackgroundThreadExecutor())
        .setNotifyExecutor(ApplicationThreadPool.getUIThreadExecutor())
        .build();

The PagedListAdapter is created like this:

public class ItemAdapter extends PagedListAdapter<Item, ItemAdapter.ItemHolder> { //Adapter from google guide, Nothing special here.. }

mAdapter = new ItemAdapter(new DiffUtil.ItemCallback<Mention>() {
            @Override
            public boolean areItemsTheSame(Item oldItem, Item newItem) {
                return oldItem.id == newItem.id;
            }

            @Override
            public boolean areContentsTheSame(Item oldItem, Item newItem) {
                return oldItem.equals(newItem);
            }
        });

, and updated like this:

mAdapter.submitList(pagedList);

Answer 1

You should use a blocking call on your observable. If you don't submit the result in the same thread as loadInitial, loadAfter or loadBefore, what happens is that the adapter will compute the diff of the existing list items against an empty list first, and then against the newly loaded items. So effectively it's as if all items were deleted and then inserted again, that is why the list seems to jump to the beginning.

Answer 2

You're not using androidx.paging.ItemKeyedDataSource.LoadInitialParams#requestedInitialKey in your implementation of loadInitial, and I think you should be.

I took a look at another implementation of ItemKeyedDataSource, the one used by autogenerated Room DAO code: LimitOffsetDataSource. Its implementation of loadInitial contains (Apache 2.0 licensed code follows):

// bound the size requested, based on known count final int firstLoadPosition = computeInitialLoadPosition(params, totalCount); final int firstLoadSize = computeInitialLoadSize(params, firstLoadPosition, totalCount);

... where those functions do something with params.requestedStartPosition, params.requestedLoadSize and params.pageSize.

So what's going wrong?

Whenever you pass a new PagedList, you need to make sure that it contains the elements that the user is currently scrolled to. Otherwise, your PagedListAdapter will treat this as a removal of these elements. Then, later, when your loadAfter or loadBefore items load those elements, it will treat them as a subsequent insertion of these elements. You need to avoid doing this removal and insertion of any visible items. Since it sounds like you're scrolling to the top, maybe you're accidentally removing all items and inserting them all.

The way I think this works when using Room with PagedLists is:

1. The database is updated.
2. A Room observer invalidates the data source.
3. The PagedListAdapter code spots the invalidation and uses the factory to create a new data source, and calls loadInitial with the params.requestedStartPosition set to a visible element.
4. A new PagedList is provided to the PagedListAdapter, who runs the diff checking code to see what's actually changed. Usually, nothing has changed to what's visible, but maybe an element has been inserted, changed or removed. Everything outside the initial load is treated as being removed - this shouldn't be noticeable in the UI.
5. When scrolling, the PagedListAdapter code can spot that new items need to be loaded, and call loadBefore or loadAfter.
6. When these complete, an entire new PagedList is provided to the PagedListAdapter, who runs the diff checking code to see what's actually changed. Usually - just an insertion.

I'm not sure how that corresponds to what you're trying to do, but maybe that helps? Whenever you provide a new PagedList, it will be diffed against the previous one, and you want to make sure that there's no spurious insertions or deletions, or it can get really confused.

Other ideas

I've also seen issues where PAGE_SIZE is not big enough. The docs recommend several times the maximum number of elements that can be visible at a time.