设置固定圆角，iOS11之后有更简单的方法（适配如下）

特别注意，在iOS10版本下，UITableview通过Masonry约束后直接用下面的方法设置圆角，结果UITableview不显示，可以直接frame赋值

/**
 圆角
 @param radius 圆角尺寸
 @param maskCorner 圆角位置列如：kCALayerMinXMinYCorner|kCALayerMaxXMinYCorner
 @param corner 圆角位置列如：UIRectCornerTopRight|UIRectCornerTopLeft
 */
- (void)acs_radiusWithRadius:(CGFloat)radius maskedCorner:(CACornerMask)maskCorner corner:(UIRectCorner)corner {
    if (@available(iOS 11.0, *)) {
        self.layer.cornerRadius = radius;
        self.layer.maskedCorners = maskCorner;
    } else {
        UIBezierPath * path = [UIBezierPath bezierPathWithRoundedRect:self.bounds byRoundingCorners:corner cornerRadii:CGSizeMake(radius, radius)];
        CAShapeLayer *maskLayer = [[CAShapeLayer alloc] init];
        maskLayer.frame = self.bounds;
        maskLayer.path = path.CGPath;
        self.layer.mask = maskLayer;
    }
}

UIRectCorner的定义如下：

typedef NS_OPTIONS(NSUInteger, UIRectCorner) {
    UIRectCornerTopLeft     = 1 <&lt; 0,
    UIRectCornerTopRight    = 1 <&lt; 1,
    UIRectCornerBottomLeft  = 1 <&lt; 2,
    UIRectCornerBottomRight = 1 <&lt; 3,
    UIRectCornerAllCorners  = ~0UL
};

CACornerMask的定义如下：

typedef NS_OPTIONS (NSUInteger, CACornerMask)
{
  kCALayerMinXMinYCorner = 1U << 0, // 左上
  kCALayerMaxXMinYCorner = 1U << 1, 右上
  kCALayerMinXMaxYCorner = 1U << 2, // 左下
  kCALayerMaxXMaxYCorner = 1U << 3, // 右下
};