方法一

iOS UITableView section圆角阴影
如果自定义的cell底部有线，则不适合此方法。
距底部有个5pt的偏移。UIEdgeInsetsMake(0, 0, 5, 0)

- (void)tableView:(UITableView *)tableView willDisplayCell:(UITableViewCell *)cell forRowAtIndexPath:(NSIndexPath *)indexPath {
    // 圆角角度
    CGFloat radius = 4.f;
    // 设置cell 背景色为透明
    cell.backgroundColor = UIColor.clearColor;
    cell.contentView.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, 0, 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;
}
 

方法二

https://www.jianshu.com/p/92662061c0dc

- (void)tableView:(UITableView *)tableView willDisplayCell:(UITableViewCell *)cell forRowAtIndexPath:(NSIndexPath *)indexPath{
    [self shearTableViewSection:cell tableView:tableView IndexPath:indexPath cornerRadius:4 width:0];
}
#pragma mark TableView Section 切圆角
- (void)shearTableViewSection:(UITableViewCell *)cell tableView:(UITableView *)tableView IndexPath:(NSIndexPath *)indexPath cornerRadius:(CGFloat)radius width:(CGFloat)width
{
    // 圆角弧度半径
    CGFloat cornerRadius = 0.f;
    if (radius == 0) {
        cornerRadius = 10.f;
    }else{
        cornerRadius = radius;
    }
    
    // 设置cell的背景色为透明，如果不设置这个的话，则原来的背景色不会被覆盖
    cell.backgroundColor = UIColor.clearColor;
    cell.contentView.backgroundColor = UIColor.clearColor;
    
    // 创建一个shapeLayer
    CAShapeLayer *layer = [[CAShapeLayer alloc] init];
    CAShapeLayer *backgroundLayer = [[CAShapeLayer alloc] init]; //显示选中
    // 创建一个可变的图像Path句柄，该路径用于保存绘图信息
    CGMutablePathRef pathRef = CGPathCreateMutable();
    // 获取cell的size
    // 第一个参数,是整个 cell 的 bounds, 第二个参数是距左右两端的距离,第三个参数是距上下两端的距离
    CGRect bounds;
    bounds = CGRectInset(cell.bounds, width, 0);
    
    // CGRectGetMinY：返回对象顶点坐标
    // CGRectGetMaxY：返回对象底点坐标
    // CGRectGetMinX：返回对象左边缘坐标
    // CGRectGetMaxX：返回对象右边缘坐标
    // CGRectGetMidX: 返回对象中心点的X坐标
    // CGRectGetMidY: 返回对象中心点的Y坐标
    
    // 这里要判断分组列表中的第一行，每组section的第一行，每组section的中间行
    
    // CGPathAddRoundedRect(pathRef, nil, bounds, cornerRadius, cornerRadius);
    
    if ([tableView numberOfRowsInSection:indexPath.section]-1 == 0) {
        CGPathMoveToPoint(pathRef, nil, CGRectGetMinX(bounds), CGRectGetMaxY(bounds));
        CGPathAddArcToPoint(pathRef, nil, CGRectGetMinX(bounds), CGRectGetMinY(bounds), CGRectGetMidX(bounds), CGRectGetMinY(bounds), cornerRadius);
        CGPathAddArcToPoint(pathRef, nil, CGRectGetMaxX(bounds), CGRectGetMinY(bounds), CGRectGetMaxX(bounds), CGRectGetMidY(bounds), cornerRadius);
        CGPathAddArcToPoint(pathRef, nil, CGRectGetMaxX(bounds), CGRectGetMaxY(bounds), CGRectGetMidX(bounds), CGRectGetMaxY(bounds), cornerRadius);
        CGPathAddArcToPoint(pathRef, nil, CGRectGetMinX(bounds), CGRectGetMaxY(bounds), CGRectGetMinX(bounds), CGRectGetMidY(bounds), cornerRadius);
        CGPathAddLineToPoint(pathRef, nil, CGRectGetMinX(bounds), CGRectGetMaxY(bounds));
        
        
    }else if (indexPath.row == 0) {
        // 初始起点为cell的左下角坐标
        CGPathMoveToPoint(pathRef, nil, CGRectGetMinX(bounds), CGRectGetMaxY(bounds));
        // 起始坐标为左下角，设为p，（CGRectGetMinX(bounds), CGRectGetMinY(bounds)）为左上角的点，设为p1(x1,y1)，(CGRectGetMidX(bounds), CGRectGetMinY(bounds))为顶部中点的点，设为p2(x2,y2)。然后连接p1和p2为一条直线l1，连接初始点p到p1成一条直线l，则在两条直线相交处绘制弧度为r的圆角。
        CGPathAddArcToPoint(pathRef, nil, CGRectGetMinX(bounds), CGRectGetMinY(bounds), CGRectGetMidX(bounds), CGRectGetMinY(bounds), cornerRadius);
        CGPathAddArcToPoint(pathRef, nil, CGRectGetMaxX(bounds), CGRectGetMinY(bounds), CGRectGetMaxX(bounds), CGRectGetMidY(bounds), cornerRadius);
        // 终点坐标为右下角坐标点，把绘图信息都放到路径中去,根据这些路径就构成了一块区域了
        CGPathAddLineToPoint(pathRef, nil, CGRectGetMaxX(bounds), CGRectGetMaxY(bounds));
        
    } else if (indexPath.row == [tableView numberOfRowsInSection:indexPath.section]-1) {
        
        // 初始起点为cell的左上角坐标
        CGPathMoveToPoint(pathRef, nil, CGRectGetMinX(bounds), CGRectGetMinY(bounds));
        CGPathAddArcToPoint(pathRef, nil, CGRectGetMinX(bounds), CGRectGetMaxY(bounds), CGRectGetMidX(bounds), CGRectGetMaxY(bounds), cornerRadius);
        CGPathAddArcToPoint(pathRef, nil, CGRectGetMaxX(bounds), CGRectGetMaxY(bounds), CGRectGetMaxX(bounds), CGRectGetMidY(bounds), cornerRadius);
        // 添加一条直线，终点坐标为右下角坐标点并放到路径中去
        CGPathAddLineToPoint(pathRef, nil, CGRectGetMaxX(bounds), CGRectGetMinY(bounds));
    } else {
        
        // 添加cell的rectangle信息到path中（不包括圆角）
        CGPathAddRect(pathRef, nil, bounds);
    }
    // 把已经绘制好的可变图像路径赋值给图层，然后图层根据这图像path进行图像渲染render
    layer.path = pathRef;
    backgroundLayer.path = pathRef;
    // 注意：但凡通过Quartz2D中带有creat/copy/retain方法创建出来的值都必须要释放
    CFRelease(pathRef);
    // 按照shape layer的path填充颜色，类似于渲染render
    // layer.fillColor = [UIColor colorWithWhite:1.f alpha:0.8f].CGColor;
    layer.fillColor = [UIColor whiteColor].CGColor;
    
    
    // view大小与cell一致
    UIView *roundView = [[UIView alloc] initWithFrame:bounds];
    // 添加自定义圆角后的图层到roundView中
    [roundView.layer insertSublayer:layer atIndex:0];
    roundView.backgroundColor = UIColor.clearColor;
    // cell的背景view
    cell.backgroundView = roundView;
}