CAAnimation 之屬性 isRemovedOnCompletion 要設(shè)置為 false ，才能不停地循環(huán)轉(zhuǎn)動(dòng)

    /* When true, the animation is removed from the render tree once its
     * active duration has passed. Defaults to YES. */
    
    open var isRemovedOnCompletion: Bool

    func startAnimation() {
        let anim = CABasicAnimation(keyPath: "transform.rotation")
        
        anim.toValue = 2 * Double.pi
        anim.repeatCount = Float.greatestFiniteMagnitude
        anim.duration = 10
        anim.isRemovedOnCompletion = false
        
        someView.layer.add(anim, forKey: nil)
    }