UIPageViewController转换无法正常工作

Question

我有一个带页面卷曲的UIPageViewController（并不是转换类型不重要），其中第二页在动画完成之前不会显示。代码用于工作，但不再是，任何想法都会很棒

代码如下：

- (void)viewDidLoad
{
    self.pageViewController = [self.storyboard instantiateViewControllerWithIdentifier:@"PageViewController"];
    self.pageViewController.dataSource = self;
    self.pageViewController.delegate = self;
    // other code deleted, for easy reading
}


- (PsalmViewController *)viewControllerAtIndex:(NSUInteger)index
{
    if (([self.mDelegate.mPsalms count] == 0) || (index >= [self.mDelegate.mPsalms count]))
        return nil;

    // Create a new view controller and pass suitable data.
    PsalmViewController *controller = [self.storyboard instantiateViewControllerWithIdentifier:@"PsalmViewController"];
    PsalmData *data = [self.mDelegate.mPsalms objectAtIndex:index];
    [controller setupView:index psalm:data];
    self.mNextPsalm = controller;

    return controller;
}

#pragma mark - Page View Controller Data Source

- (UIViewController *)pageViewController:(UIPageViewController *)pageViewController viewControllerBeforeViewController:(UIViewController *)viewController
{
    if (self.mNextIndex == 0 || self.mNextIndex == NSNotFound)
        return nil;

    self.mNextIndex = self.mPageIndex - 1;
    self.mAnimating = true;
    NSLog(@"Prev %lud", (unsigned long)self.mNextIndex);
    return [self viewControllerAtIndex:self.mNextIndex];
}

- (UIViewController *)pageViewController:(UIPageViewController *)pageViewController viewControllerAfterViewController:(UIViewController *)viewController
{
    if (self.mNextIndex == NSNotFound)
        return nil;

    if (self.mNextIndex + 1 == [self.mDelegate.mPsalms count])
        return nil;

    self.mNextIndex = self.mPageIndex + 1;
    self.mAnimating = true;
    NSLog(@"Next %lu", (unsigned long)self.mNextIndex);
    return [self viewControllerAtIndex:self.mNextIndex];
}

- (NSInteger)presentationCountForPageViewController:(UIPageViewController *)pageViewController
{
    return [self.mDelegate.mPsalms count];
}

- (NSInteger)presentationIndexForPageViewController:(UIPageViewController *)pageViewController
{
    return 0;
}

- (void)pageViewController:(UIPageViewController *)pageViewController didFinishAnimating:(BOOL)finished previousViewControllers:(NSArray<UIViewController *> *)previousViewControllers transitionCompleted:(BOOL)completed
{
    if (completed)
    {
        self.mPageIndex = self.mNextIndex;
        [self setCurrentPsalm:self.mNextPsalm];
    }
    else if (finished)
    {
        self.mNextIndex = self.mPageIndex;
    }

    if (completed || finished)
        self.mAnimating = false;
    NSLog(@"C %d F %d", completed, finished);
}

再次感谢任何帮助，我不知道是什么导致了这个问题。