Firebase android pagination

Below is the code I'm using for pagination which shows the latest node first.

      public void getImages() {
            Query imagesQuery = FirebaseDatabase.getInstance().getReference().child("englishDps").child(mChildName).orderByKey().limitToLast(21);

            ChildEventListener childEventListener = new ChildEventListener() {
                @Override
                public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                    Image image = dataSnapshot.getValue(Image.class);
                    image.setNodeKey(dataSnapshot.getKey());

                    mTempImages.add(image);
                    if (mTempImages.size() == 21) {
                        mLastKey = mTempImages.get(0).getNodeKey();
                        Collections.reverse(mTempImages);
                        mTempImages.remove(mTempImages.size() - 1);
                        mImages.addAll(mTempImages);
                        setAdapter();
                    }
                }

                @Override
                public void onChildChanged(DataSnapshot dataSnapshot, String s) {

                }

                @Override
                public void onChildRemoved(DataSnapshot dataSnapshot) {

                }

                @Override
                public void onChildMoved(DataSnapshot dataSnapshot, String s) {

                }

                @Override
                public void onCancelled(DatabaseError databaseError) {
                    if (isAdded()) {
                        Toast.makeText(getActivity(), "Problem loading more images...", Toast.LENGTH_LONG).show();
                    }
                }
            };

            imagesQuery.addChildEventListener(childEventListener);
        }


  @Override
    public void getMoreImages() {
        if (!mGettingMoreImages) {
            mGettingMoreImages = true;
            Query imagesQuery = FirebaseDatabase.getInstance().getReference("englishDps").child(mChildName).orderByKey().endAt(mLastKey).limitToLast(21);

            ChildEventListener childEventListener = new ChildEventListener() {
                @Override
                public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                    Image image = dataSnapshot.getValue(Image.class);
                    image.setNodeKey(dataSnapshot.getKey());
                    mMoreImages.add(image);
                    if (mMoreImages.size() == 21) {
                        mLastKey = mMoreImages.get(0).getNodeKey();
                        Collections.reverse(mMoreImages);
                        mMoreImages.remove(mMoreImages.size() - 1);
                        mImages.addAll(mMoreImages);
                        mMoreImages.clear();
                        mGettingMoreImages = false;
                        mImagesAdapter.notifyDataSetChanged();
                        return;
                    }

                    if (mLastKey.equalsIgnoreCase(image.getNodeKey())) {
                        Collections.reverse(mMoreImages);
                        mImages.addAll(mMoreImages);
                        mMoreImages.clear();
                        mGettingMoreImages = false;
                        mImagesAdapter.onNoMoreImages();
                        ;
                        mImagesAdapter.notifyDataSetChanged();
                    }
                }

                @Override
                public void onChildChanged(DataSnapshot dataSnapshot, String s) {

                }

                @Override
                public void onChildRemoved(DataSnapshot dataSnapshot) {

                }

                @Override
                public void onChildMoved(DataSnapshot dataSnapshot, String s) {

                }

                @Override
                public void onCancelled(DatabaseError databaseError) {
                    if (isAdded()) {
                        Toast.makeText(getActivity(), "Problem loading more images...", Toast.LENGTH_LONG).show();
                    }
                }
            };

            imagesQuery.addChildEventListener(childEventListener);
        }
    }

You want to be using the limitToFirst/limitToLast methods to retrieve a limited number of results.

videosQuery.orderByKey().limitToFirst(20)

https://www.firebase.com/docs/web/api/query/limittofirst.html

You should really consider changing the naming convention of your videos to include leading 0s (i.e. video01, video02... video10, video11) because the above code will display them exactly as you have them above (which I assume is out of order?)

Alternatively, if you're adding the videos via Java, you could just let firebase create the uniqueids via push(). The uniqueid are generated in a way that they'll sort chronilogically, which sounds like it'll suit your need to grab the most recent(ly added?) videos.

https://www.firebase.com/docs/web/api/firebase/push.html

I have following method to paginate through a firebase realtime database node:

private void getUsers(String nodeId) {
        Query query;

        if (nodeId == null)
            query = FirebaseDatabase.getInstance().getReference()
                    .child(Consts.FIREBASE_DATABASE_LOCATION_USERS)
                    .orderByKey()
                    .limitToFirst(mPostsPerPage);
        else
            query = FirebaseDatabase.getInstance().getReference()
                    .child(Consts.FIREBASE_DATABASE_LOCATION_USERS)
                    .orderByKey()
                    .startAt(nodeId)
                    .limitToFirst(mPostsPerPage);

        query.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                UserModel user;
                List<UserModel> userModels = new ArrayList<>();
                for (DataSnapshot userSnapshot : dataSnapshot.getChildren()) {
                    userModels.add(userSnapshot.getValue(UserModel.class));
                }

                mAdapter.addAll(userModels);
                mIsLoading = false;
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {
                mIsLoading = false;
            }
        });
    }

Every time I reach the bottom of the paginated data, I call the getUsers(mAdapter.getLastItemId()); and then it brings the next set of records.

I have written a complete guide with open source sample app on this that you can check at https://blog.shajeelafzal.com/2017/12/13/firebase-realtime-database-pagination-guide-using-recyclerview/