关键函数如下

func expand() {
  var expandAnimation: CABasicAnimation =     CABasicAnimation(keyPath: "path")
  expandAnimation.fromValue = ovalPathSmall.CGPath
  expandAnimation.toValue = ovalPathLarge.CGPath
  expandAnimation.duration = animationDuration
  expandAnimation.fillMode = kCAFillModeForwards
  expandAnimation.removedOnCompletion = false
  addAnimation(expandAnimation, forKey: nil)
}

关键代码解释：ovalPathSmall和ovalPathLarge如下：

 var ovalPathSmall: UIBezierPath {
    return UIBezierPath(ovalInRect: CGRect(x: 50.0, y: 50.0, width: 0.0, height: 0.0))
}

var ovalPathLarge: UIBezierPath {
    return UIBezierPath(ovalInRect: CGRect(x: 2.5, y: 2.5, width: 95.0, height: 95.0))
}

ovalPathSmall其实画的是一个点，ovalPathLarge才是变大后的这个原（直径95），圆心其实就位于ovalPathSmall所画的这个点上

expandAnimation.fillMode = kCAFillModeForwards
expandAnimation.removedOnCompletion = false

确保动画完成以后确保圆为变化后的大圆图形。
简单来说，动画就是给定变化前后的图形，变化交给系统来实现。