我有一个绘制圆弧的自定义CALayer子类。它看起来像:classArcLayer:CALayer{varstrokeColor:UIColor=UIColor.blackColor(){didSet{self.setNeedsDisplay()}}varstrokeWidth:CGFloat=1.0{didSet{self.setNeedsDisplay()}}varstrokeCap:CGLineCap=.Butt{didSet{self.setNeedsDisplay()}}varstartRadians:CGFloat=0.0{didSet{self.setNeedsDispl