iOS 一天之内只提示一次方法

NSUserDefaults *userDefault = [NSUserDefaults standardUserDefaults];
//拿到当前时间
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:@"yyyy-MM-dd"];
//当前时间
NSDate *nowDate = [NSDate date];
NSString *nowDateStr= [dateFormatter stringFromDate:nowDate];
//之前时间
NSString *agoDateStr = [userDefault objectForKey:@"saveNowDate"];
//判断时间差
if ([agoDateStr isEqualToString:nowDateStr]) {
    //是当天时间
}else{
    //不是当天，需要提示
    //这里写提示相关代码
    //保存当前时间
    [userDefault setObject:nowDateStr forKey:@"saveNowDate"];
    [userDefault synchronize];
}