最终效果图：

近日在项目中遇到一个问题，UIButton需要设置border，同时设置其中三个角为圆角，第四个角为直角，于是第一次我的代码是这样的

[self.btnContent setBackgroundColor:[UIColor clearColor]];
self.btnContent.layer.borderWidth = 1;
self.btnContent.layer.borderColor =RGBCOLOR(123, 184, 255).CGColor;
UIBezierPath *maskPath = [UIBezierPath bezierPathWithRoundedRect:rect  byRoundingCorners:(UIRectCornerTopLeft|UIRectCornerTopRight|UIRectCornerBottomLeft)  cornerRadii:CGSizeMake(5, 5)];
CAShapeLayer *maskLayer = [CAShapeLayer layer];
maskLayer.path = maskPath.CGPath;
self.btnContent.layer.mask = maskLayer;

结果就是直线部分有边框，但是设置圆角的部分边线被贝塞尔曲线切掉，后来在网上找个一个方法，在button上覆盖一个有边框的UIView：

UIView *strokeView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, self.btnContent.width, self.btnContent.height)];
[self.btnContent addSubview:strokeView];

CAShapeLayer *strokeLayer = [CAShapeLayer layer];
strokeLayer.path = maskPath.CGPath;
strokeLayer.fillColor = [UIColor clearColor].CGColor; //内容填充的颜色设置为clear
strokeLayer.strokeColor = RGBCOLOR(123, 184, 255).CGColor; //边色
strokeLayer.lineWidth = 1; // 边宽
[strokeView.layer addSublayer:strokeLayer];

这样的确可以解决问题，但是代码太过复杂，后来研究发现，在button的layer上添加一个layer，同样可以解决问题

[self.btnContent setBackgroundColor:[UIColor clearColor]];
CAShapeLayer *maskLayer = [CAShapeLayer layer];
CGRect rect = CGRectMake(1, 1, self.btnContent.width-2, self.btnContent.height-2);

UIBezierPath *maskPath = [UIBezierPath bezierPathWithRoundedRect:rect  byRoundingCorners:(UIRectCornerTopLeft|UIRectCornerTopRight|UIRectCornerBottomLeft)  cornerRadii:CGSizeMake(5, 5)];
maskLayer.path = maskPath.CGPath;
maskLayer.fillColor = [UIColor clearColor].CGColor; //内容填充的颜色设置为clear
maskLayer.strokeColor = RGBCOLOR(123, 184, 255).CGColor; //边色
maskLayer.lineWidth = 1.0f;
maskLayer.frame = CGRectMake(0, 0, CGRectGetWidth(rect), CGRectGetHeight(rect));
[self.btnContent.layer addSublayer:maskLayer];
//＊这里注意是lineWidth，区别于borderWidth