GCD倒计时

• 倒计时
  if (_timer) {
  dispatch_source_cancel(_timer);
  }
  dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
  _timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);

  dispatch_source_set_event_handler(_timer, ^{

  NSCalendar *calendar = [NSCalendar currentCalendar];NSDate *fireDate = [NSDate dateWithTimeIntervalSinceNow:_timeInterval];NSDate *today = [NSDate date];unsigned int unitFlag = NSCalendarUnitYear|NSCalendarUnitMonth|NSCalendarUnitDay|NSCalendarUnitHour|NSCalendarUnitMinute|NSCalendarUnitSecond;/** !计算时差*/NSDateComponents *d = [calendar components:unitFlag fromDate:today toDate:fireDate options:0];if ([d hour] <= 0 && [d minute] <= 0 && [d second] <= 0) {    dispatch_async(dispatch_get_main_queue(), ^{       //todo 倒计时结束    });    if (_timer) {        dispatch_source_cancel(_timer);    }}else{    _timeInterval --;    NSString *time = [NSString stringWithFormat:@"%.2li小时%.2li分%.2li秒",(long)[d hour],(long)[d minute],(long)[d second]];    dispatch_async(dispatch_get_main_queue(), ^{       //todo 正在计时    });}

  });
  dispatch_resume(_timer);