倒计时

@property (nonatomic, strong) NSTimer *timer;
@property (nonatomic, assign) NSInteger seconds;

- (void)countDown{
  [self.timer fire];
}

- (NSTimer *)timer{
    if (!_timer) {
        _timer = [NSTimer timerWithTimeInterval:1 target:self selector:@selector(countDown) userInfo:nil repeats:YES];
        [[NSRunLoop mainRunLoop] addTimer:_timer forMode:NSRunLoopCommonModes];
    }
    return _timer;
}

- (void)countDown{
    NSLog(@"countDown");
    
    if (self.seconds == 0) {
        self.text = LS(@"3");
    }else if (self.seconds == 1){
        self.text = LS(@"2");
    }
    else if (self.seconds == 2){
        self.text = LS(@"1");
    }

    if (self.seconds == 3) {
        self.alpha = 0;
        [self.timer invalidate];
        self.timer = nil;
        return;
    }
    self.seconds ++;
}

- (void)dealloc{
    [self.timer invalidate];
    self.timer = nil;
}

 __block NSInteger timeOut = 3;

    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
    dispatch_source_set_timer(timer, DISPATCH_TIME_NOW, 1 * NSEC_PER_SEC, 0 * NSEC_PER_SEC);
    dispatch_source_set_event_handler(timer, ^{
        if (timeOut <= 0) {
            dispatch_cancel(timer);
            dispatch_async(dispatch_get_main_queue(), ^{
                self.alpha = 0;
            });
        }else{
            dispatch_async(dispatch_get_main_queue(), ^{
                self.text = [NSString stringWithFormat:@"%ld",(long)timeOut];
                timeOut --;
            });
        }
    });
    dispatch_resume(timer);