NSTimer依赖于RunLoop，如果RunLoop的任务过于繁重，可能会导致NSTimer不准时。

而GCD的定时器会更加准时。

- (void)viewDidLoad {

    [super viewDidLoad];

    self.view.backgroundColor = [UIColor whiteColor];

    _timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, dispatch_queue_create("001", DISPATCH_QUEUE_CONCURRENT));

    dispatch_source_set_timer(_timer, DISPATCH_TIME_NOW, 1.0 * NSEC_PER_SEC, 1.0 * NSEC_PER_SEC);

    dispatch_source_set_event_handler(_timer, ^{

        [self test];

    });

    dispatch_resume(_timer);

}

- (void)viewWillDisappear:(BOOL)animated{

    _timer = nil;

}

- (void)test{

    NSLog(@"%s",__func__);

}

- (void)dealloc{

    NSLog(@"%s",__func__);

}