想做一个IOS倒计时器，00:30:00，点击开始 就倒计时，切换界面，时间还在走，再次打开，时间继续。

1. //////每隔一秒执行一次
   
2. NSTimer *timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(timerFire:) userInfo:nil repeats:YES];
   
3. 
   
4. - (void)timerFire:(NSTimer *)timer
   
5. {
   
6.     NSCalendar *calendar = [NSCalendar currentCalendar];
   
7.     NSDateComponents *components = [[NSDateComponents alloc] init];
   
8.     [components setYear:2014];
   
9.     [components setMonth:12];
   
10.     [components setDay:12];
    
11.     [components setHour:12];
    
12.     [components setMinute:0];
    
13.     [components setSecond:0];
    
14.     NSDate *fireDate = [calendar dateFromComponents:components];//目标时间
    
15.     NSDate *today = [NSDate date];//当前时间
    
16.     unsigned int unitFlags = NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit | NSHourCalendarUnit | NSMinuteCalendarUnit | NSSecondCalendarUnit;
    
17.      NSDateComponents *d = [calendar components:unitFlags fromDate:today toDate:fireDate options:0];//计算时间差
    
18.     auctionTime.text = [NSString stringWithFormat:@"%d天%d小时%d%分%d秒", [d day], [d hour], [d minute], [d second]];//倒计时显示
    
19. }

复制代码

真是不错哦

没学IOS所以看不懂