facing problems in Implementing pagination in firestore

Question

I am trying to implement pagination in my app I have a lot of data in my Firestore and I want to implement pagination with a progress bar. I have read many documents but all are confusing and most of in Kotlin, I need them in java. Please guide me.

I found the following guide by Alex Memo Sir How to paginate Firestore with Android?

but there are some doubts, one is what is the limit in it I am using it and my data is not showing after 15 items it is stopping??

COMPLETE CODE FOR IT

   firebaseFirestore.collectionGroup("room_details").orderBy("title", Query.Direction.ASCENDING).limit(limit)
            .get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
        @Override
        public void onComplete(@NonNull Task<QuerySnapshot> task) {


            progressBar.setVisibility(View.VISIBLE);
            if (task.isSuccessful()) {
                for (DocumentSnapshot d : task.getResult()) {
                    RoomsDetails obj = d.toObject(RoomsDetails.class);
                    roomsDetails.add(obj);
                }

                roomsAdapter.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;
                        progressBar.setVisibility(View.GONE);
                    }
                }

                @Override
                public void onScrolled(@NonNull RecyclerView recyclerView, int dx, int dy) {
                    super.onScrolled(recyclerView, dx, dy);
                    LinearLayoutManager linearLayoutManager = ((LinearLayoutManager) recyclerView.getLayoutManager());
                    int firstVisibleItemPosition = linearLayoutManager.findFirstVisibleItemPosition();
                    int visibleItemCount = linearLayoutManager.getChildCount();
                    int totalItemCount = linearLayoutManager.getItemCount();

                    if (isScrolling && (firstVisibleItemPosition + visibleItemCount == totalItemCount) && !isLastItemReached) {
                        isScrolling = false;
                        Query nextQuery = firebaseFirestore.collectionGroup("room_details").orderBy("title", Query.Direction.ASCENDING).startAfter(lastVisible).limit(limit);
                        nextQuery.get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
                            @Override
                            public void onComplete(@NonNull Task<QuerySnapshot> t) {
                                if (t.isSuccessful()) {
                                    for (DocumentSnapshot d : t.getResult()) {
                                        RoomsDetails productModel = d.toObject(RoomsDetails.class);
                                        roomsDetails.add(productModel);
                                    }
                                    roomsAdapter.notifyDataSetChanged();
                                    lastVisible = t.getResult().getDocuments().get(t.getResult().size() - 1);

                                    if (t.getResult().size() < limit) {
                                        isLastItemReached = true;
                                    }
                                }
                            }
                        });
                    }
                }
            };

            recyclerViewRooms.addOnScrollListener(onScrollListener);


        }
    });
}

I have two doubts sir one is I want to add a Progress bar like this sir how can I implement it?? and second is what is limit() means here(on the first line)

IT IS THROWING AN ERROR SIR HERE

    firebaseFirestore = FirebaseFirestore.getInstance();
       //HERE SIR COLLECTION REFENCE IS GIVING ME ERROR SAYING ClassCastException: com.google.firebase.firestore.Query cannot be cast to com.google.firebase.firestore.CollectionReference
        CollectionReference productsRef =  firebaseFirestore.collectionGroup("room_details");
    Query query = productsRef.orderBy("title", Query.Direction.ASCENDING).limit(limit);

Answer 1

According to your last edit, you are getting the following error:

ClassCastException: com.google.firebase.firestore.Query cannot be cast to com.google.firebase.firestore.CollectionReference

Because you are trying to save an object of type Query into a variable of type CollectionReference at the following line of code:

CollectionReference productsRef =  firebaseFirestore.collectionGroup("room_details");

Which is actually not possible in Java. Why? Because the Query class, doesn't extend the CollectionReference, it's exactly vice versa, hence the error. Please note FirebaseFirestore#collectionGroup(String collectionId) method, returns an object of type Query and not CollectionReference. So to solve this, please change the above line of code to:

Query productsRef =  firebaseFirestore.collectionGroup("room_details");
//👆

And your error will go away.