UIView設置指定的border

轉自：http://stackoverflow.com/questions/17355280/how-to-add-a-border-just-on-the-top-side-of-a-uiview

其實也是往view.layer上面addSublayer，但是挺好用的

代碼：

- (CALayer *)prefix_addUpperBorder:(UIRectEdge)edge color:(UIColor *)color thickness:(CGFloat)thickness
{
    CALayer *border = [CALayer layer];

    switch (edge) {
        case UIRectEdgeTop:
            border.frame = CGRectMake(0, 0, CGRectGetWidth(self.frame), thickness);
            break;
        case UIRectEdgeBottom:
            border.frame = CGRectMake(0, CGRectGetHeight(self.frame) - thickness, CGRectGetWidth(self.frame), thickness);
            break;
        case UIRectEdgeLeft:
            border.frame = CGRectMake(0, 0, thickness, CGRectGetHeight(self.frame));
            break;
        case UIRectEdgeRight:
            border.frame = CGRectMake(CGRectGetWidth(self.frame) - thickness, 0, thickness, CGRectGetHeight(self.frame));
            break;
        default:
            break;
    }

    border.backgroundColor = color.CGColor;

    [self.layer addSublayer:border];

    return border;
}