用CAKeyframeAnimation构建动画路径

复杂路径的动画，我们可以借助关键关键帧动画（CAKeyframeAnimation）来实现，给其的path属性设置相应的路径信息即可。

以下为一个红色的小球按照指定的路径运动的动画。

此动画关键在于如何把路径画出来（如两个圆弧）

//创建一个可变路径
let circleKeyframePath = CGPathCreateMutable()
//创建用于转移坐标的Transform，这样我们不用按照实际显示做坐标计算，以这个坐标做基准点。坐标为下半个弧的中心点
var circleKeyframeTransform:CGAffineTransform = CGAffineTransformMakeTranslation(self.view.frame.size.width / 2, 260)
        
CGPathMoveToPoint(circleKeyframePath, &circleKeyframeTransform, 0, 0)
//CGPathAddLineToPoint(circleKeyframePath, &circleKeyframeTransform, -100, 0)
//创建一个1/4弧（圆的左下角弧）
CGPathAddArc(circleKeyframePath, &circleKeyframeTransform, 0, -100, 100, CGFloat(0.5 * M_PI), CGFloat(M_PI), false)
CGPathAddLineToPoint(circleKeyframePath, &circleKeyframeTransform, -100, -100)
CGPathAddLineToPoint(circleKeyframePath, &circleKeyframeTransform, -50, -100)
//创建一个以半径为50的两条切线的内切圆弧
CGPathAddArcToPoint(circleKeyframePath, &circleKeyframeTransform, 0, -200, 50, -100, 50)
CGPathAddLineToPoint(circleKeyframePath, &circleKeyframeTransform, 50, -100)
        
CGPathAddLineToPoint(circleKeyframePath, &circleKeyframeTransform, 100, -100)
//CGPathAddLineToPoint(circleKeyframePath, &circleKeyframeTransform, 100, 0)
//创建一个1/4弧(圆的右下角弧)
CGPathAddArc(circleKeyframePath, &circleKeyframeTransform, 0, -100, 100, 0, CGFloat(0.5 * M_PI), false)
//关闭路径
CGPathCloseSubpath(circleKeyframePath)
let backgroundLayer:CAShapeLayer = CAShapeLayer()
backgroundLayer.path = circleKeyframePath
backgroundLayer.strokeColor = UIColor.yellowColor().CGColor
backgroundLayer.lineWidth = 3
backgroundLayer.fillColor = UIColor.clearColor().CGColor
self.view.layer.addSublayer(backgroundLayer)

 此时在模拟器上运行后的效果如下：

看起来还不错哦。像个元宝，呵，接下来就创建一个UIView对象让它成圆形，并按此路径做运动即可。

let circleView:UIView = UIView(frame: CGRect(x: 0, y: 0,  20, height: 20))
let redCircleLayer:CAShapeLayer = CAShapeLayer()
let redCirclePath:UIBezierPath = UIBezierPath(ovalInRect: CGRect(x: 0, y: 0,  20, height: 20))
redCircleLayer.path = redCirclePath.CGPath
redCircleLayer.fillColor = UIColor.redColor().CGColor
circleView.layer.addSublayer(redCircleLayer)


self.view.addSubview(circleView)
//创建关键帧动画对象
let circleKeyframeAnimation:CAKeyframeAnimation = CAKeyframeAnimation(keyPath: "position")
circleKeyframeAnimation.path = circleKeyframePath
circleKeyframeAnimation.duration = 5
//让 Core Animation 向被驱动的对象施加一个恒定速度，不管路径的各个线段有多长。
circleKeyframeAnimation.calculationMode = kCAAnimationPaced
circleKeyframeAnimation.repeatCount = HUGE
//让它自身也做旋转，不过是圆的看不出效果
circleKeyframeAnimation.rotationMode = kCAAnimationRotateAutoReverse
//print(circleView.layer.anchorPoint)
circleView.layer.addAnimation(circleKeyframeAnimation, forKey: nil)

 到此，就完成了，比较重要的要区分CGPathAddArc以及CGPathAddLineToPoint的不同，不同可以参考StackOverflow

 CGPathAddArc方法工作方式类似于，(x,y)为圆心所在的坐标，radius为圆的半径，startAngle路径开始的角度按弧度算，endAngle路径结束的角度按弧度算,

clockwise方向（与实际的方向相反）

CGPathAddLineToPoint方法工作如下图,x1,y1,x2,y2为方法的四个位置参数，r为半径。