来源：https://www.jianshu.com/p/739408a7aae1

给section添加圆角和阴影效果，效果如下：

image.png

全部实现都在UITableView的willDisplayCell代理方法里：
给每个section第一个和最后一个cell分别添加顶部和底部圆角

- (void)tableView:(UITableView *)tableView willDisplayCell:(UITableViewCell *)cell forRowAtIndexPath:(NSIndexPath *)indexPath {
    // 圆角角度
    CGFloat radius = 10.f;
    // 设置cell 背景色为透明
    cell.backgroundColor = UIColor.clearColor;
    // 创建两个layer
    CAShapeLayer *normalLayer = [[CAShapeLayer alloc] init];
    CAShapeLayer *selectLayer = [[CAShapeLayer alloc] init];
    // 获取显示区域大小
    CGRect bounds = CGRectInset(cell.bounds, kScaleX * 20, 0);
    // cell的backgroundView
    UIView *normalBgView = [[UIView alloc] initWithFrame:bounds];
    // 获取每组行数
    NSInteger rowNum = [tableView numberOfRowsInSection:indexPath.section];
    // 贝塞尔曲线
    UIBezierPath *bezierPath = nil;
    
    if (rowNum == 1) {
        // 一组只有一行（四个角全部为圆角）
        bezierPath = [UIBezierPath bezierPathWithRoundedRect:bounds byRoundingCorners:UIRectCornerAllCorners cornerRadii:CGSizeMake(radius, radius)];
        normalBgView.clipsToBounds = NO;
    }else {
        normalBgView.clipsToBounds = YES;
        if (indexPath.row == 0) {
            normalBgView.frame = UIEdgeInsetsInsetRect(bounds, UIEdgeInsetsMake(-5, 0, 0, 0));
            CGRect rect = UIEdgeInsetsInsetRect(bounds, UIEdgeInsetsMake(5, 0, 0, 0));
            // 每组第一行（添加左上和右上的圆角）
            bezierPath = [UIBezierPath bezierPathWithRoundedRect:rect byRoundingCorners:(UIRectCornerTopLeft|UIRectCornerTopRight) cornerRadii:CGSizeMake(radius, radius)];
        }else if (indexPath.row == rowNum - 1) {
            normalBgView.frame = UIEdgeInsetsInsetRect(bounds, UIEdgeInsetsMake(0, 0, -5, 0));
            CGRect rect = UIEdgeInsetsInsetRect(bounds, UIEdgeInsetsMake(0, 0, 5, 0));
            // 每组最后一行（添加左下和右下的圆角）
            bezierPath = [UIBezierPath bezierPathWithRoundedRect:rect byRoundingCorners:(UIRectCornerBottomLeft|UIRectCornerBottomRight) cornerRadii:CGSizeMake(radius, radius)];
        }else {
            // 每组不是首位的行不设置圆角
            bezierPath = [UIBezierPath bezierPathWithRect:bounds];
        }
    }
    
    // 阴影
    normalLayer.shadowColor = [UIColor blackColor].CGColor;
    normalLayer.shadowOpacity = 0.2;
    normalLayer.shadowOffset = CGSizeMake(0, 0);
    normalLayer.path = bezierPath.CGPath;
    normalLayer.shadowPath = bezierPath.CGPath;
    
    // 把已经绘制好的贝塞尔曲线路径赋值给图层，然后图层根据path进行图像渲染render
    normalLayer.path = bezierPath.CGPath;
    selectLayer.path = bezierPath.CGPath;
    
    // 设置填充颜色
    normalLayer.fillColor = [UIColor whiteColor].CGColor;
    // 添加图层到nomarBgView中
    [normalBgView.layer insertSublayer:normalLayer atIndex:0];
    normalBgView.backgroundColor = UIColor.clearColor;
    cell.backgroundView = normalBgView;
    
    // 替换cell点击效果
    UIView *selectBgView = [[UIView alloc] initWithFrame:bounds];
    selectLayer.fillColor = [UIColor colorWithWhite:0.95 alpha:1.0].CGColor;
    [selectBgView.layer insertSublayer:selectLayer atIndex:0];
    selectBgView.backgroundColor = UIColor.clearColor;
    cell.selectedBackgroundView = selectBgView;
}