通过CGPath绘制出空心的圆形 

func drawCircular() {
    let centerX = view.bounds.size.width/2
    let boundingRect = CGRect(x:centerX-100, y:50, width:200, height:200)
    var orbitPath: CGPath!
    //通过CGPath的ellipseIn方法，创建一个圆形的CGPath
    orbitPath = CGPath(ellipseIn: boundingRect, transform: nil)
    //绘制路线的图层
    drawPath(path: orbitPath)
 }
    
///绘制路线的图层
func drawPath(path: CGPath) {
        
    let pathLayer = CAShapeLayer()
    pathLayer.frame = self.view.bounds
    
    pathLayer.path = path
    pathLayer.fillColor = nil
    pathLayer.lineWidth = 3
    pathLayer.strokeColor = UIColor.white.cgColor
        
    self.view.layer.addSublayer(pathLayer)
}