用nstimer实现倒计时功能，废话不多说，直接上代码，详细解释请参照注释

// [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(timerFireMethod:) userInfo:nil repeats:YES]; // - (void)timerFireMethod:(NSTimer *)theTimer { BOOL timeStart = YES; NSCalendar *cal = [NSCalendar currentCalendar];//定义一个NSCalendar对象 NSDateComponents *endTime = [[NSDateComponents alloc] init]; //初始化目标时间... NSDate *today = [NSDate date]; //得到当前时间 NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init]; [dateFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"]; NSDate *dateString = [dateFormatter dateFromString:todate]; NSString *overdate = [dateFormatter stringFromDate:dateString]; // NSLog(@"overdate=%@",overdate); static int year; static int month; static int day; static int hour; static int minute; static int second; if(timeStart) {//从NSDate中取出年月日，时分秒，但是只能取一次 year = [[overdate substringWithRange:NSMakeRange(0, 4)] intValue]; month = [[overdate substringWithRange:NSMakeRange(5, 2)] intValue]; day = [[overdate substringWithRange:NSMakeRange(8, 2)] intValue]; hour = [[overdate substringWithRange:NSMakeRange(11, 2)] intValue]; minute = [[overdate substringWithRange:NSMakeRange(14, 2)] intValue]; second = [[overdate substringWithRange:NSMakeRange(17, 2)] intValue]; timeStart= NO; } [endTime setYear:year]; [endTime setMonth:month]; [endTime setDay:day]; [endTime setHour:hour]; [endTime setMinute:minute]; [endTime setSecond:second]; NSDate *overTime = [cal dateFromComponents:endTime]; //把目标时间装载入date //用来得到具体的时差，是为了统一成北京时间 unsigned int unitFlags = NSYearCalendarUnit| NSMonthCalendarUnit| NSDayCalendarUnit| NSHourCalendarUnit| NSMinuteCalendarUnit| NSSecondCalendarUnit; NSDateComponents *d = [cal components:unitFlags fromDate:today toDate:overTime options:0]; NSString *t = [NSString stringWithFormat:@"%d", [d day]]; NSString *h = [NSString stringWithFormat:@"%d", [d hour]]; NSString *fen = [NSString stringWithFormat:@"%d", [d minute]]; if([d minute] < 10) { fen = [NSString stringWithFormat:@"0%d",[d minute]]; } NSString *miao = [NSString stringWithFormat:@"%d", [d second]]; if([d second] < 10) { miao = [NSString stringWithFormat:@"0%d",[d second]]; } // NSLog(@"===%@天 %@:%@:%@",t,h,fen,miao); [_longtime setText:[NSString stringWithFormat:@"%@天 %@:%@:%@",t,h,fen,miao]]; if([d second] > 0) { //计时尚未结束，do_something // [_longtime setText:[NSString stringWithFormat:@"%@:%@:%@",d,fen,miao]]; } else if([d second] == 0) { //计时结束 do_something } else{ //计时器失效 [theTimer invalidate]; } }

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