It's not really a trade in the traditional way, since DeRozan was a free agent. It just helps everyone move salary and players around for salary cap purposes.
I actually hate this for Keegan, and not sure how much it really improves the Kings? DeRozan scores best in isolation and with ball in hand, Fox also wants the ball in hand. So means less touches for Keegan. Kings couldn't guard anybody last year and they picked up a player who is just as bad a defender as Barnes is, in DeRozan.
That said, when Kings are cooking they are going to be fun to watch cause they will score in bunches.