按钮字体添加下划线

NSMutableAttributedString * string = [[NSMutableAttributedString alloc] initWithString:@"立即注册"];

NSRange strRange = {0,[string length]};

[string addAttribute:NSUnderlineStyleAttributeName value:[NSNumber numberWithInteger:NSUnderlineStyleSingle] range:strRange];

[string addAttribute:NSForegroundColorAttributeName value:[UIColorlightGrayColor] range:strRange];

[signButton setAttributedTitle:string forState:UIControlStateNormal];