code: @Override
      @UiThread
      public void onAttachedToRecyclerView(@NonNull RecyclerView recyclerView) {
          super.onAttachedToRecyclerView(recyclerView);
          mAttachedRecyclerViewPool.add(recyclerView);
      }
docs: Implementation of Adapter#onAttachedToRecyclerView(RecyclerView).
  <p>
  Called when this {@link ExpandableRecyclerAdapter} is attached to a RecyclerView.
  
  @param recyclerView The {@code RecyclerView} this {@code ExpandableRecyclerAdapter}
  is being attached to