可以直接使用UIBezierPath指定一个或多个角路径，图形上下文切圆角
UIView * view = [[UIView alloc]initWithFrame:CGRectMake(50, 50, 100, 100)];
view.backgroundColor = [UIColor redColor];
[self.view addSubview:view];
UIBezierPath *maskPath = [UIBezierPath bezierPathWithRoundedRect:view.bounds byRoundingCorners: UIRectCornerTopLeft|UIRectCornerBottomRight cornerRadii:CGSizeMake(50,50)];
CAShapeLayer *maskLayer = [[CAShapeLayer alloc] init];
maskLayer.frame = view.bounds;
maskLayer.path = maskPath.CGPath;
view.layer.mask = maskLayer;
