有时候需要让view显示某一侧的边框线，这时设置layer的border是达不到效果的。在网上查阅资料发现有一个投机取巧的办法，原理是给view的layer再添加一个layer，让这个layer充当边框线的角色。根据这个方法我写了一个函数，有需要的朋友可以拿去试下。

- (void)setBorderWithView:(UIView *)view top:(BOOL)top left:(BOOL)left bottom:(BOOL)bottom right:(BOOL)right borderColor:(UIColor *)color borderWidth:(CGFloat)width
{
    if (top) {
        CALayer *layer = [CALayer layer];
        layer.frame = CGRectMake(0, 0, view.frame.size.width, width);
        layer.backgroundColor = color.CGColor;
        [view.layer addSublayer:layer];
    }
    if (left) {
        CALayer *layer = [CALayer layer];
        layer.frame = CGRectMake(0, 0, width, view.frame.size.height);
        layer.backgroundColor = color.CGColor;
        [view.layer addSublayer:layer];
    }
    if (bottom) {
        CALayer *layer = [CALayer layer];
        layer.frame = CGRectMake(0, view.frame.size.height - width, view.frame.size.width, width);
        layer.backgroundColor = color.CGColor;
        [view.layer addSublayer:layer];
    }
    if (right) {
        CALayer *layer = [CALayer layer];
        layer.frame = CGRectMake(view.frame.size.width - width, 0, width, view.frame.size.height);
        layer.backgroundColor = color.CGColor;
        [view.layer addSublayer:layer];
    }
}