iOS 按钮上的文字添加下划线的方法

[MyTools createMyImageview:topEditView frame:CGRectMake(widthAll-90, 12, 17, 16) imageName:@"离线课程_下载更多.png"];
UIButton *downMoreButton = [MyTools createMyBtn:@"下载更多" frame:CGRectMake(widthAll-70, 10, 60, 20) uiview:topEditView uifont:14 color:[UIColor colorWithRed:92/255.0 green:218/255.0 blue:231/255.0 alpha:1]];
NSMutableAttributedString *title = [[NSMutableAttributedString alloc] initWithString:@"下载更多"];
NSRange titleRange = {0,[title length]};
[title addAttribute:NSUnderlineStyleAttributeName value:[NSNumber numberWithInteger:NSUnderlineStyleSingle] range:titleRange];
[downMoreButton setAttributedTitle:title
forState:UIControlStateNormal];
[downMoreButton.titleLabel setFont:[UIFont systemFontOfSize:14]];
[downMoreButton addTarget:self action:@selector(downMore) forControlEvents:UIControlEventTouchUpInside];