需求要做个椭圆形的镂空的蒙版，因为设计不在，只好自己绘制。

//自定义的一个View,添加到window上。self为自定义的View
 //贝塞尔曲线 整个屏幕
        UIBezierPath *bpath = [UIBezierPath bezierPathWithRoundedRect:CGRectMake(0, 0, ScreenWidth, ScreenHeight) cornerRadius:0];
        //贝塞尔曲线 画一个椭圆
        [bpath appendPath:[UIBezierPath bezierPathWithOvalInRect:CGRectMake(0, self.topOffset, self.maskWidth, self.maskHeight)].bezierPathByReversingPath];
        
        //创建一个CAShapeLayer 图层
        CAShapeLayer *shapeLayer = [CAShapeLayer layer];
        shapeLayer.path = bpath.CGPath;
        
        //添加图层蒙板
        self.layer.mask = shapeLayer;