根据字符串长度，计算字符串的高度

CGSize rect1;

    rect1 = [string boundingRectWithSize:CGSizeMake(kDeviceWidth-40, 20000.f) options:NSStringDrawingUsesLineFragmentOrigin |NSStringDrawingUsesFontLeading attributes:[NSDictionary dictionaryWithObjectsAndKeys:[UIFont systemFontOfSize:15.f], NSFontAttributeName,nil] context:nil].size;