code: @Override
      public void registerAdapterDataObserver(RecyclerView.AdapterDataObserver observer) {
          super.registerAdapterDataObserver(observer);
          if (mFastAdapter != null) {
              mFastAdapter.registerAdapterDataObserver(observer);
          }
      }
docs: overwrite the registerAdapterDataObserver to correctly forward all events to the FastAdapter
  
  @param observer