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

1-) Layout manager

The current smoothScrollToPosition does not take the element to the top. So let's write a new layout manager. And let's override this layout manager's smoothScrollToPosition.

public class TopLinearLayoutManager extends LinearLayoutManager
{
    public TopLinearLayoutManager(Context context, int orientation)
    {
        //orientation : vertical or horizontal
        super(context, orientation, false);
    }

    @Override
    public void smoothScrollToPosition(RecyclerView recyclerView, RecyclerView.State state, int position)
    {
        RecyclerView.SmoothScroller smoothScroller = new TopSmoothScroller(recyclerView.getContext());
        smoothScroller.setTargetPosition(position);
        startSmoothScroll(smoothScroller);
    }

    private class TopSmoothScroller extends LinearSmoothScroller
    {
        TopSmoothScroller(Context context)
        {
            super(context);
        }

        @Override
        public int calculateDtToFit(int viewStart, int viewEnd, int boxStart, int boxEnd, int snapPreference)
        {
            return (boxStart - viewStart);
        }
    }
}

2-) Setup

    //horizontal one
    RecyclerView rvMario = (RecyclerView) findViewById(R.id.rvMario);

    //vertical one
    RecyclerView rvLuigi = (RecyclerView) findViewById(R.id.rvLuigi);

    final LinearLayoutManager managerMario = new LinearLayoutManager(MainActivity.this, LinearLayoutManager.HORIZONTAL, false);
    rvMario.setLayoutManager(managerMario);
    ItemMarioAdapter adapterMario = new ItemMarioAdapter(itemList);
    rvMario.setAdapter(adapterMario);

     //Snap to start by using Ruben Sousa's RecyclerViewSnap
    SnapHelper snapHelper = new GravitySnapHelper(Gravity.START);
    snapHelper.attachToRecyclerView(rvMario);

    final TopLinearLayoutManager managerLuigi = new TopLinearLayoutManager(MainActivity.this, LinearLayoutManager.VERTICAL);
    rvLuigi.setLayoutManager(managerLuigi);
    ItemLuigiAdapter adapterLuigi = new ItemLuigiAdapter(itemList);
    rvLuigi.setAdapter(adapterLuigi);

3-) Scroll listener

rvMario.addOnScrollListener(new RecyclerView.OnScrollListener()
{
    @Override
    public void onScrolled(RecyclerView recyclerView, int dx, int dy)
    {
     super.onScrolled(recyclerView, dx, dy);

     //get firstCompleteleyVisibleItemPosition
     int firstCompleteleyVisibleItemPosition = managerMario.findFirstCompletelyVisibleItemPosition();

     if (firstCompleteleyVisibleItemPosition >= 0)
     {  
      //vertical one, smooth scroll to position
      rvLuigi.smoothScrollToPosition(firstCompleteleyVisibleItemPosition);
     }
    }

    @Override
    public void onScrollStateChanged(RecyclerView recyclerView, int newState)
    {
     super.onScrollStateChanged(recyclerView, newState);
    }
});

4-) Output