在使用CABasicAnimation旋转一个CALayer之后，层跳回到它的非旋转位置

I am trying to create a falling coin. The coin image is a CALayer with 2 CABasicAnimations on it - a falling down and a rotation one. When the falling down animation gets to its end, it stays there. The rotation animation though, which is supposed to be random and end up in a different angle every time, just pops back to the original CALAyer image.I am trying to create a falling coin. The coin