Ios UIPageViewControllerDatasource同时调用左扫和右扫方法

如果

viewControllerBeforeViewController

viewControllerAfterViewController

任一方法返回nil，计数器部分也被调用

这种行为是预期行为吗？不管怎样我都能阻止这一切的发生。这就是我试图做的

- (UIViewController *)pageViewController:(UIPageViewController *)pageViewController viewControllerBeforeViewController:(UIViewController *)viewController {

    if ([viewController isKindOfClass:[QuestionViewController class]]){
        QuestionViewController* qvc = (QuestionViewController*)viewController;

        CVPQuestion* prevQuestion = [_surveyContext previousQuestion:qvc.question];
        return [self questionControllerWithQuestion:prevQuestion];
    }
    if ([viewController isKindOfClass:[SurveyEndedViewController class]]){
        return [self questionControllerWithQuestion:[self.questionList lastObject]];
    }
    return nil;
}

- (UIViewController *)pageViewController:(UIPageViewController *)pageViewController viewControllerAfterViewController:(QuestionViewController *)viewController {

    if ([viewController respondsToSelector:@selector(question)]){
        CVPQuestion* nextQuestion = [_surveyContext previousQuestion:viewController.question];
        if (nextQuestion == nil && ![_surveyContext isAnswered:viewController.question]){
            [viewController  showToasterWithText:@"Please answer the question to continue"];
        }
        QuestionViewController* qc = [self questionControllerWithQuestion:[_surveyContext nextQuestion:viewController.question]];

        if (!qc){return [self surveyEndedController:viewController.question];}
        return qc;
    }
    return nil;
}

---

如果

viewControllerAfterViewController

将在

viewControllerAfterViewController

后调用通知

[viewController showToasterWithText:@“请回答问题以继续”]也将被发送。
看起来“UIPageviewcontroller”实际上没有使用“UIScrollViewDelegate”。因此，对“UIPageViewController”进行子类化并使用“UIScrollViewDelegate”似乎可以找出其向前滑动还是向后滑动

//从页面视图控制器获取滚动视图滚动视图

 - (void)viewDidLoad
     {
        [super viewDidLoad];
        // Do any additional setup after loading the view.
        for (UIView* v in [self.view subviews]){
            if ([v isKindOfClass:[UIScrollView class]]){
                _scrollView = (UIScrollView*)v;
            }
        }

        if (_scrollView){
            _scrollView.delegate = self;
        }
    }

//滚动视图开始拖动时记录内容偏移量

-(void)scrollViewWillBeginDragging:(UIScrollView *)scrollView{
    _startContentOffset = _scrollView.contentOffset;
    _transitionState = SurveyPageviewCtlrTransitionInitiated;
    [_transitionStateDelegate transitionInitiated:self];
}

//基于位移的方向

 -(void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate{
    if (_scrollView.contentOffset.x > _startContentOffset.x){
       [self.delegate forwardDrag];
     }
     else{
       [self.delegate reverseDrag];
 }