用nstimer实现倒计时

用nstimer实现倒计时
//
[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];
    } 

}