实现类似促销商品或活动倒计时功能，当前时间距某个指定时间还剩：12：06：43(时：分：秒) 。

QQ20180205-0.png

1、创建定时器（可以使用GCD定时器，也可以使用NSTimer，本文使用NSTimer）

 NSTimer *countDownTimer = [NSTimer scheduledTimerWithTimeInterval:1 target:self selector:@selector(countDownAction) userInfo:nil repeats:YES];
self.countDownTimer = countDownTimer;
 [[NSRunLoop mainRunLoop] addTimer:countDownTimer forMode:NSRunLoopCommonModes];

2、实现定时调用的方法

pragma mark 倒计时时间

 -(void)countDownAction{
      // getTomorrowDay 为自定义NSDate分类方法：实现获取明天凌晨整点时间
      NSString *tomorrowStr = [NSDate getTomorrowDay:[NSDate date]];
      // compareNowDateToEndDateForSecound ：实现当前时间距离指定时间间隔秒数
      int timeout = [Common compareNowDateToEndDateForSecound:tomorrowStr].intValue;
      // 计算时分秒
      int days = (int)(timeout/(3600*24));
      int hours = (int)((timeout-days*24*3600)/3600);
      int minute = (int)(timeout-days*24*3600-hours*3600)/60;
      int second = timeout-days*24*3600-hours*3600-minute*60;
      // 更新UI显示
      if (hours<10) {
          self.hourLabel.text = [NSString stringWithFormat:@"0%d",hours];
      }else{
          self.hourLabel.text = [NSString stringWithFormat:@"%d",hours];
      }
      if (minute<10) {
          self.minuteLabel.text = [NSString stringWithFormat:@"0%d",minute];
      }else{
          self.minuteLabel.text = [NSString stringWithFormat:@"%d",minute];
      }
      if (second<10) {
          self.secondLabel.text =  [NSString stringWithFormat:@"0%d",second];
      }else{
          self.secondLabel.text =  [NSString stringWithFormat:@"%d",second];
      }
}

3、传入今天的时间，返回明天的时间（整点时间）

 + (NSString *)getTomorrowDay:(NSDate *)aDate {
    NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSCalendarIdentifierGregorian];
    NSDateComponents *components = [gregorian components:NSCalendarUnitWeekday | NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay fromDate:aDate];
    [components setDay:([components day]+1)];
    NSDate *beginningOfWeek = [gregorian dateFromComponents:components];
    NSDateFormatter *dateday = [[NSDateFormatter alloc] init];
    [dateday setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
    return [dateday stringFromDate:beginningOfWeek];
}

4、比较当前日期和指定日期的时间间隔秒数

 +(NSString *)compareNowDateToEndDateForSecound:(NSString *)endDate{
    NSString * nowDateStr =[Common dateToString:[NSDate date] format:@"yyyy-MM-dd HH:mm:ss"];
    NSDate *startDate =[Common dateFromString:nowDateStr format:@"yyyy-MM-dd HH:mm:ss"];
    NSDate *toDate =[Common dateFromString:endDate format:@"yyyy-MM-dd HH:mm:ss"];
    //利用NSCalendar比较日期的差异
    NSCalendar *calendar = [NSCalendar currentCalendar];
    //比较的结果是NSDateComponents类对象
    NSDateComponents *delta = [calendar components:NSCalendarUnitSecond fromDate:startDate toDate:toDate options:0];
    return [NSString stringWithFormat:@"%ld",delta.second] ;
}

方法中用到的Common类是自定义的工具类，dateToString和dateFromString为自定义工具类的方法，实现日期格式转换格式化功能，比较简单，在此就不贴出了，本文实现的倒计时功能中用到的方法只是自己想到的实现方式，如果有哪位朋友有更便捷的实现方式可以留言给我，很乐意向大家学习