android RecyclerView获得单个Item的ViewHolder

int firstItemPosition = layoutManager.findFirstVisibleItemPosition();
        if (position - firstItemPosition >= 0) {
            //得到要更新的item的view
            View view = mRecyclerView.getChildAt(position - firstItemPosition+1);
            if (null != mRecyclerView.getChildViewHolder(view)){
                ProductsViewHolder viewHolder = (ProductsViewHolder)mRecyclerView.getChildViewHolder(view);

//do something
           }