layer阴影动画问题

CALayer的动画比较繁杂，有几种方式：

CATransaction
CAAnimation
本质上是一样的。先看CATransaction

[CATransaction begin];
[CATransaction setAnimationDuration:3.0];
layer.shadowOffset = CGSizeMake(10, 10);
[CATransaction commit];
然后是CAAnimation，这个比较简单，可以用CABasicAnimation

CABasicAnimation *anim = [CABasicAnimation animationWithKeyPath:@"shadowOffset"];
anim.toValue = [NSValue valueWithCGSize:CGSizeMake(10, 10)];
anim.duration = 3.0;
anim.beginTime = 0;

[self.layer addAnimation:anim forKey:@"animName"];