code: func (wr *protoWatchResp) GetPrevValue(msg proto.Message) (prevValueExist bool, err error) {
  	prevVal := wr.BytesWatchResp.GetPrevValue()
  	if prevVal == nil {
  		return false, nil
  	}
  	err = wr.serializer.Unmarshal(prevVal, msg)
  	if err != nil {
  		return true, err
  	}
  	return true, nil
  }
docs: // GetPrevValue returns the previous value after the change.