不会造成线程阻塞的定时器

@property(nonatomic,assign)int startMinute;//开始倒计时值

-(void)startTimer
{
   _startMinute = 60;

   NSTimer *timer = [NSTimer scheduledTimerWithTimeInterval:1.0f target:self selector:@selector(updateUI) userInfo:nil repeats:YES];
 
   //界面的其他操作不会导致定时器暂停
   [[NSRunLoop currentRunLoop] addTimer:timer forMode:UITrackingRunLoopMode];
}

-(void)updateUI
{
    _startMinute--;
    _minuteLabel.text = [NSString stringWithFormat:@"%d",_startMinute];
}