code: public int notifyAdapterSubItemsChanged(int position, int previousCount) {
          Item item = mFastAdapter.getItem(position);
          if (item != null && item instanceof IExpandable) {
              IExpandable expandable = (IExpandable) item;
              IAdapter adapter = mFastAdapter.getAdapter(position);
              if (adapter != null && adapter instanceof IItemAdapter) {
                  ((IItemAdapter) adapter).removeRange(position + 1, previousCount);
                  ((IItemAdapter) adapter).add(position + 1, expandable.getSubItems());
              }
              return expandable.getSubItems().size();
          }
          return 0;
      }
docs: notifies the fastAdapter about new / removed items within a sub hierarchy
  NOTE this currently only works for sub items with only 1 level
  
  @param position      the global position of the parent item
  @param previousCount the previous count of sub items
  @return the new count of subItems