ios ~ 设置UIView的外边框

一般设置内边框：

    self.headImg.layer.borderColor = [UIColor redColor].CGColor;
    self.headImg.layer.borderWidth = [UIScreen mainScreen].bounds.size.width/375*2;

而设置外边框：(注意：backView 是headImg的父视图，这个layer是沿着headImg边框路径，添加到backView的layer)

        CGFloat borderWidth = [UIScreen mainScreen].bounds.size.width/375*2;
        CALayer *layer = [CALayer layer];
        layer.frame = CGRectMake(self.headImg.frame.origin.x - borderWidth, self.headImg.frame.origin.y - borderWidth, self.headImg.frame.size.width + 2 * borderWidth, self.headImg.frame.size.height + 2 * borderWidth);
        
        layer.masksToBounds = YES;
        layer.cornerRadius = [UIScreen mainScreen].bounds.size.width/375*(50 + 2 * borderWidth)/2;
//        layer.borderColor = RGBA(255, 196, 15, 0.3).CGColor;
        layer.borderColor = [UIColor redColor].CGColor;
        layer.borderWidth = [UIScreen mainScreen].bounds.size.width/375*2;
        [self.backView.layer addSublayer:layer];