1.如果需要将UIView的4个角全部都为圆角，添加边框，做法相当简单，只需设置其Layer的cornerRadius、borderWidth、borderColor就可以了。

    UIView *view = [[UIView alloc]initWithFrame:CGRectMake(100, 100, 100, 100)];
    view.layer.cornerRadius = 25;
    view.layer.borderWidth = 1;
    view.layer.borderColor = [UIColor redColor].CGColor;
    [self.view addSubview:view];

圆角边框.png

2.如果要指定某几个角为圆角而别的不变时，最简单优雅的方案，就是使用UIBezierPath。

    UIView *view = [[UIView alloc]initWithFrame:CGRectMake(100, 100, 100, 100)];
    [self.view addSubview:view];
    view.backgroundColor = [UIColor lightGrayColor];
    UIBezierPath *path = [UIBezierPath bezierPathWithRoundedRect:view.bounds byRoundingCorners:UIRectCornerTopRight|UIRectCornerBottomLeft cornerRadii:CGSizeMake(25, 25)];
    CAShapeLayer *mask = [CAShapeLayer layer];
    mask.path = path.CGPath;
    view.layer.mask = mask;

任意圆角.png

其中，参数：(UIRectCorner)corners指定了需要成为圆角的角,使用“|”来组合。

typedef NS_OPTIONS(NSUInteger, UIRectCorner) {
    UIRectCornerTopLeft     = 1 << 0,
    UIRectCornerTopRight    = 1 << 1,
    UIRectCornerBottomLeft  = 1 << 2,
    UIRectCornerBottomRight = 1 << 3,
    UIRectCornerAllCorners  = ~0UL
};

3.给上面切了两个圆角的view增加边框。
如果直接设置borderWidth和borderColor，设置圆角的部分边线被贝塞尔曲线切掉。

圆角边线被切掉.png

解决思路：用UIBezierPath画好边框，在切圆角。
给view加了一个Category

- (void)applyRoundCorners:(UIRectCorner)corners radius:(CGFloat)radius
{
    UIBezierPath *path = [UIBezierPath bezierPathWithRoundedRect:self.bounds byRoundingCorners:corners cornerRadii:CGSizeMake(radius, radius)];
    
    CAShapeLayer *temp = [CAShapeLayer layer];
    temp.lineWidth = 2.f;
    temp.fillColor = [UIColor clearColor].CGColor;
    temp.strokeColor = [UIColor redColor].CGColor;
    temp.frame = self.bounds;
    temp.path = path.CGPath;
    [self.layer addSublayer:temp];
    
    CAShapeLayer *mask = [[CAShapeLayer alloc]initWithLayer:temp];
    mask.path = path.CGPath;
    self.layer.mask = mask;
}

使用示例：

    UIView *view = [[UIView alloc]initWithFrame:CGRectMake(100, 100, 100, 100)];
    view.backgroundColor = [UIColor lightGrayColor];
    [self.view addSubview:view];
    [view applyRoundCorners:UIRectCornerTopRight|UIRectCornerBottomLeft radius:25];

最终效果.png