如何随意设置View（上下左右）的圆角

UIView *redView = [[UIView alloc]initWithFrame:CGRectMake(0, 0, 200, 200)];

redView.center = self.view.center;

redView.backgroundColor = [UIColor redColor];

UIRectCorner rectCorner = UIRectCornerTopLeft|UIRectCornerTopRight;

UIBezierPath *path = [UIBezierPath bezierPathWithRoundedRect:redView.bounds byRoundingCorners:rectCorner cornerRadii:CGSizeMake(40, 40)];

CAShapeLayer *shaperLayer = [CAShapeLayer layer];

shaperLayer.path = path.CGPath;

redView.layer.mask = shaperLayer;

[self.view addSubview:redView];

关于mask的理解，可以看之前我的一篇文章 ：关于CAGradientLayer的几个小动画