Cannot load more data pagination using Recycler view android

Question

I have added pagination feature to my application, but data cannot load more. When i set a limit of 20 data, then i add 23 data and load more, 20 data is invisible and 3 data is visible. Can anyone help me to fix this error?

Screenshot of the first 20 data sets :

Screenshot of when i scroll :

Code :

@Override
protected void onStart() {
    super.onStart();

    collectionReference.limit(20).get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
        @Override
        public void onComplete(@NonNull Task<QuerySnapshot> task) {
            if (task.isSuccessful()) {
                if(!task.getResult().isEmpty()) {
                    documentsList.clear();

                    for (DocumentSnapshot documentSnapshot : task.getResult()) {
                        Documents documents = documentSnapshot.toObject(Documents.class);
                        documentsList.add(documents);
                        documentListAdapter.notifyDataSetChanged();
                        lastVisible = task.getResult().getDocuments().get(task.getResult().size() - 1);
                    }

                    RecyclerView.OnScrollListener onScrollListener = new RecyclerView.OnScrollListener() {
                        @Override
                        public void onScrollStateChanged(@NonNull RecyclerView recyclerView, int newState) {
                            super.onScrollStateChanged(recyclerView, newState);

                            if (newState == AbsListView.OnScrollListener.SCROLL_STATE_TOUCH_SCROLL) {
                                isScrolling = true;
                            }
                        }

                        @Override
                        public void onScrolled(@NonNull RecyclerView recyclerView, int dx, int dy) {
                            super.onScrolled(recyclerView, dx, dy);

                            int firstVisibleDocument = layoutManager.findFirstVisibleItemPosition();
                            int visibleDocumentCount = layoutManager.getChildCount();
                            int totalDocumentCount = layoutManager.getItemCount();

                            if (isScrolling && (firstVisibleDocument + visibleDocumentCount == totalDocumentCount) && !isLastItemReached) {
                                isScrolling = false;

                                collectionReferences.startAfter(lastVisible).limit(20).get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
                                    @Override
                                    public void onComplete(@NonNull Task<QuerySnapshot> task) {
                                        if (task.isSuccessful()) {
                                            documentsList.clear();

                                            for (DocumentSnapshot documentSnapshot : task.getResult()) {
                                                Documents documents = documentSnapshot.toObject(Documents.class);
                                                documentsList.add(documents);
                                                documentListAdapter.notifyDataSetChanged();
                                                lastVisible = task.getResult().getDocuments().get(task.getResult().size() - 1);
                                            }

                                            if (task.getResult().size() < 20) {
                                                isLastItemReached = true;
                                            }
                                        }
                                    }
                                });
                            }
                        }
                    };

                    recyclerViewDocument.addOnScrollListener(onScrollListener);
                }
            }
        }
    });
}

Answer 1

You have that behaviour because when you perform the second query to get those three new items, you clear your documentsList using the following line of code:

documentsList.clear();

To solve this, simply remove it from the second query because you need to add more data and to the list not to remove the initial one. I have answered a similar question here.