Update list items in PagingLibrary w/o using Room (Network only)

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);

P.S: If there is a better way to update list items without using Room please share.

Upcast

Are you able to implement the below solution? Is it working ? can you please update with sample code for reference. – Commissioner

Have you implemented this, share it here if you got a solution – Howard

Answer 1

All you need to do is to invalidate current DataSource each time you update your data.

What I would do:

• move networking logic from MentionKeyedDataSource to new class that extends PagedList.BoundaryCallback and set it when building your PagedList.

• move all you data to some DataProvider that holds all your downloaded data and has reference to DataSource. Each time data is updated in DataProvider invalidate current DataSource

Something like that maybe

    val dataProvider = PagedDataProvider()
    val dataSourceFactory = InMemoryDataSourceFactory(dataProvider = dataProvider)

where

class PagedDataProvider : DataProvider<Int, DataRow> {

    private val dataCache = mutableMapOf<Int, List<DataRow>>()
    override val sourceLiveData = MutableLiveData<DataSource<Int, DataRow>>()

    //implement data add/remove/update here
    //and after each modification call
    //sourceLiveData.value?.invalidate()
}

and

class InMemoryDataSourceFactory<Key, Value>(
    private val dataProvider: DataProvider<Key, Value>
) : DataSource.Factory<Key, Value>() {

    override fun create(): DataSource<Key, Value> {
        val source = InMemoryDataSource(dataProvider = dataProvider)
        dataProvider.sourceLiveData.postValue(source)
        return source
    }
}

This approach is very similar to what Room does - every time table row is updated - current DataSource is invalidated and DataSourceFactory creates new data source.

Nutter

Do you have any samples for the above-suggested solution for reference? I was looking for a solution to the same problem and found this one. The above solution makes sense. What is a DataProvider class in the sample code? What is the purpose of it? Is it a custom class? InMemoryDataSource will extend any of the data sources - PageKeyedDataSource, ItemKeyedDataSource, PositionalDataSource. Is that understanding correct? It will be great if you can write some blogs with a detailed solution. I couldn't find any online other than this, which give some reference to the solution. – Commissioner

I experimented this approach to see how to design my data layer, there are couple of issues come across, eg. Observability, Propagating error, Invalidating data. I realized that implementing all these on my own leads to reinventing the wheel, so I'd prefer to use Room instead – Howard

Answer 2

You can modify directly in your adapter if you called currentList like that

class ItemsAdapter(): PagedListAdapter<Item, ItemsAdapter.ViewHolder(ITEMS_COMPARATOR) {
    ...
    fun changeItem(position: Int,newData:String) {
        currentList?.get(position)?.data = newData
        notifyItemChanged(position)
    }
   
}

Rope