ios-在视图上挖个洞

view背景为黑色半透明，右下角挖一个矩形显示底部图层。

		_tapView = [[UIView alloc] initWithFrame:self.view.bounds];
        _tapView.backgroundColor = [UIColor colorWithWhite:0 alpha:.5];
        // 贝塞尔绘制区域
        UIBezierPath *path = [UIBezierPath bezierPathWithRect:self.view.bounds];
        // 扣出右下角矩形区域
        [path appendPath:[[UIBezierPath bezierPathWithRoundedRect:smallRect cornerRadius:5] bezierPathByReversingPath]];
        
        CAShapeLayer *shapeLayer = [CAShapeLayer layer];
        shapeLayer.path = path.CGPath;
        // 约束layer
        [_tapView.layer setMask:shapeLayer];

二维码扫描，用户指引等功能界面会用到。