Before revision:  In this paper, for the first time, a comprehensive literature review is provided to summarize the ten-year development in DG . First, we cover the background by giving the problem definitions and discussing how DG is related to other fields like domain adaptation and transfer learning.
After revision:  In this paper, for the first time, a comprehensive literature review is provided to summarize the developments in DG in the past decade. Specifically, we first cover the background by giving the problem definitions and discussing how DG is related to other fields like domain adaptation and transfer learning.