Button单边倒角

// 创建一个Button
   UIButton *button = [UIButton buttonWithType:UIButtonTypeCustom];
   button.frame = CGRectMake(100, 200, 100, 40);
   [button setTitle:@"按钮" forState:UIControlStateNormal];
   [button setTitleColor:UIColor.blackColor forState:UIControlStateNormal];
   [button setBackgroundColor:UIColor.blueColor];
   [self.view addSubview:button];
   
   // 按bounds来画贝塞尔曲线路径
   CGRect rect = button.bounds;
   CGSize radii = CGSizeMake(rect.size.height * 0.5, rect.size.height * 0.5);
   // 贝塞尔曲线倒角
   UIBezierPath *path = [UIBezierPath bezierPathWithRoundedRect:rect byRoundingCorners:UIRectCornerTopLeft | UIRectCornerBottomLeft cornerRadii:radii];
   // 创建并按曲线绘制图层
   CAShapeLayer *layer = [CAShapeLayer layer];
   layer.path = path.CGPath;
   // 将图层设置为button的遮罩层
   button.layer.mask = layer;


Swift:
// CACornerMask.layerMaxXMinYCorner.rawValue 右上，
// CACornerMask.layerMaxXMaxYCorner.rawValue 右下
if #available(iOS 11.0, *) {
          // iOS11:只需要带用这个系统方法就可以随意设置View的圆角了，是不是很方便，赶快试一下吧
           userInfoBackgroundView.layer.cornerRadius = 21
           userInfoBackgroundView.layer.maskedCorners = CACornerMask(rawValue: CACornerMask.layerMaxXMinYCorner.rawValue | CACornerMask.layerMaxXMaxYCorner.rawValue)
       } else {
           let rect = CGRect(x: 0, y: 0, width: 180, height: 42)
           let size = CGSize(width: rect.width * 0.5, height: rect.height * 0.5);
           // 贝塞尔曲线倒角
           let path = UIBezierPath(roundedRect: rect, byRoundingCorners: [.topRight, .bottomRight], cornerRadii: size);
           // 创建并按曲线绘制图层
           let layer = CAShapeLayer();
           layer.path = path.cgPath;
           // 将图层设置为button的遮罩层
           userInfoBackgroundView.layer.mask = layer;
       }