How to sync scrolling first-positions of 2 RecyclerViews?

Question

Background

I have 2 RecyclerView instances. One is horizontal, and the second is vertical.

They both show the same data and have the same amount of items,

Answer 1

Building on Burak's TopLinearLayoutManager, but correcting the logic of the OnScrollListener we finally get working smoothscrolling and correct snapping (of the horizontal RecyclerView).

public class MainActivity extends AppCompatActivity {
    View masterView = null;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity);
        final LayoutInflater inflater = LayoutInflater.from(this);
        final RecyclerView topRecyclerView = findViewById(R.id.topReccyclerView);
        RecyclerView.Adapter adapterTop = new RecyclerView.Adapter() {
            @Override
            public RecyclerView.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
                return new ViewHolder(inflater.inflate(R.layout.horizontal_cell, parent, false));
            }

            @Override
            public void onBindViewHolder(RecyclerView.ViewHolder holder, int position) {
                ((TextView) holder.itemView).setText(String.valueOf(position));
                holder.itemView.setBackgroundColor(position % 2 == 0 ? Integer.valueOf(0xffff0000) : Integer.valueOf(0xff00ff00));
            }

            @Override
            public int getItemCount() {
                return 100;
            }

            class ViewHolder extends RecyclerView.ViewHolder {
                final TextView textView;

                ViewHolder(View itemView) {
                    super(itemView);
                    textView = itemView.findViewById(R.id.textView);
                }
            }
        };
        topRecyclerView.setAdapter(adapterTop);

        final RecyclerView bottomRecyclerView = findViewById(R.id.bottomRecyclerView);
        RecyclerView.Adapter adapterBottom = new RecyclerView.Adapter() {
            int baseHeight = (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, 50f, getResources().getDisplayMetrics());

            @Override
            public RecyclerView.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
                return new ViewHolder(inflater.inflate(R.layout.vertical_cell, parent, false));
            }

            @Override
            public void onBindViewHolder(RecyclerView.ViewHolder holder, int position) {
                ((TextView) holder.itemView).setText(String.valueOf(position));
                holder.itemView.setBackgroundColor((position % 2 == 0) ? Integer.valueOf(0xffff0000) : Integer.valueOf(0xff00ff00));
                holder.itemView.getLayoutParams().height = baseHeight + (position % 3 == 0 ? 0 : baseHeight / (position % 3));
            }

            @Override
            public int getItemCount() {
                return 100;
            }

            class ViewHolder extends RecyclerView.ViewHolder {
                final TextView textView;

                ViewHolder(View itemView) {
                    super(itemView);
                    textView = itemView.findViewById(R.id.textView);
                }
            }
        };
        bottomRecyclerView.setAdapter(adapterBottom);

        TopLinearLayoutManager topLayoutManager = new TopLinearLayoutManager(this, LinearLayoutManager.HORIZONTAL);
        topRecyclerView.setLayoutManager(topLayoutManager);
        TopLinearLayoutManager bottomLayoutManager = new TopLinearLayoutManager(this, LinearLayoutManager.VERTICAL);
        bottomRecyclerView.setLayoutManager(bottomLayoutManager);

        final OnScrollListener topOnScrollListener = new OnScrollListener(topRecyclerView, bottomRecyclerView);
        final OnScrollListener bottomOnScrollListener = new OnScrollListener(bottomRecyclerView, topRecyclerView);
        topRecyclerView.addOnScrollListener(topOnScrollListener);
        bottomRecyclerView.addOnScrollListener(bottomOnScrollListener);

        GravitySnapHelper snapHelperTop = new GravitySnapHelper(Gravity.START);
        snapHelperTop.attachToRecyclerView(topRecyclerView);
    }

    class OnScrollListener extends RecyclerView.OnScrollListener {
        private RecyclerView thisRecyclerView;
        private RecyclerView otherRecyclerView;
        int lastItemPos = Integer.MIN_VALUE;

        OnScrollListener(RecyclerView thisRecyclerView, RecyclerView otherRecyclerView) {
            this.thisRecyclerView = thisRecyclerView;
            this.otherRecyclerView = otherRecyclerView;
        }

        @Override
        public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
            super.onScrollStateChanged(recyclerView, newState);
            if (newState == RecyclerView.SCROLL_STATE_DRAGGING) {
                masterView = thisRecyclerView;
            } else if (newState == RecyclerView.SCROLL_STATE_IDLE && masterView == thisRecyclerView) {
                masterView = null;
                lastItemPos = Integer.MIN_VALUE;
            }
        }

        @Override
        public void onScrolled(RecyclerView recyclerview, int dx, int dy) {
            super.onScrolled(recyclerview, dx, dy);
            if ((dx == 0 && dy == 0) || (masterView != thisRecyclerView)) {
                return;
            }
            int currentItem = ((TopLinearLayoutManager) thisRecyclerView.getLayoutManager()).findFirstCompletelyVisibleItemPosition();
            if (lastItemPos == currentItem) {
                return;
            }
            lastItemPos = currentItem;
            otherRecyclerView.getLayoutManager().smoothScrollToPosition(otherRecyclerView, null, currentItem);
        }
    }
}