UIPageViewController viewControllerAfterViewController not called after once returned nil

Question

I'm trying to implement UIPageViewController with validation on every step. My code was working fine without validation, so everything with pages seems to be done right. This is my code:

- (UIViewController *)pageViewController:(UIPageViewController *)pageViewController viewControllerBeforeViewController:(UIViewController *)viewController {
    int index = [(MyViewController *) viewController index];
    index--;
    return [_viewModel viewControllerAtIndex:index];
}

- (UIViewController *)pageViewController:(UIPageViewController *)pageViewController viewControllerAfterViewController:(UIViewController *)viewController {
    BOOL isDataValid = [(MyViewController *) viewController validateData];
    if (isDataValid) {
        int index = [(MyViewController *) viewController index];
        index++;
        return [_viewModel viewControllerAtIndex:index];
    }
    return nil;
}

My problem is that when step 1 is shown, viewControllerAfterViewController is called to prepare step 2, but because step 2 cannot be valid (user didn't have chance to input data) so it is returning nil. I would like UIPageViewController to call viewControllerAfterViewController every time user tries to swipe to the next screen and not give up after receiving a nil. Is it doable?

Answer 1

UIPageViewController has an underlying cache. You can reset it with

pageViewController.dataSource = nil;
pageViewController.dataSource = self;

You can call this once the page you are on becomes valid.