關(guān)鍵函數(shù)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)
}

關(guān)鍵代碼解釋：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其實畫的是一個點，ovalPathLarge才是變大后的這個原（直徑95），圓心其實就位于ovalPathSmall所畫的這個點上

expandAnimation.fillMode = kCAFillModeForwards
expandAnimation.removedOnCompletion = false

確保動畫完成以后確保圓為變化后的大圓圖形。
簡單來說，動畫就是給定變化前后的圖形，變化交給系統(tǒng)來實現(xiàn)。