UIPageViewController second and third page flickers to white

Question

I have a pageviewcontroller, but when I swipe to the second page, it shows the right controller for a split second and then the screen goes white, if I then swipe again it will show up as it should be. It also has 4 pages instead of my intended 3 pages.

Here is a video to demonstrate the problem: https://streamable.com/i1inq

The indexing is as following:

 profileController.index = 0
 discoverController.index = 1
 matchController.index = 2

func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController?
{
    if index == 0
    {
        index = discoverController.index
        return discoverController
    }
    else if index == 1
    {
        index = matchController.index
        return matchController
    }

    return nil
}

EDIT: I should also mention that the initial controller is the profileController, and so the index starts at 0 when the app is run.

Answer 1

Updated

You can simply modify your code customized viewcontroller with index property:

func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController?
{
    let index = (viewController as? MyCustomVCWithIndexForPageViewController).index
    if index == 0
    {
        index = discoverController.index
        return discoverController
    }
    else if index == 1
    {
        index = matchController.index
        return matchController
    }

    return nil
}

Original

You should not cache parameter related to pageviewcontroller. UIPageViewController sometimes preload your page to have a better performance.

Ex:

You have 3 pages to display.

You are now displaying first page initial.

Then you scroll to second view. viewControllerAfter called with viewController of first viewcontroller for requesting second viewcontroller.

And viewControllerAfter called again with viewController of second viewcontrollerfor requesting third viewcontroller (preload).

In this case if you cache index, it will lead to unpredictable results.

Answer 2

you didnt provide enough code to detect what is causing the problem. but this is how i usually use UIPageController

class ViewController: UIViewController {

    var pages = [UIViewController]()
    override func viewDidLoad() {
        super.viewDidLoad()
        // add pages to the array
        // ...
    }
}

extension ViewController: UIPageViewControllerDataSource {

    func pageViewController(_ pageViewController: UIPageViewController,
                            viewControllerAfter viewController: UIViewController) -> UIViewController? {
        // find index of viewcontroller in array
        if let index = pages.index(of: viewController) {
            switch index {
                // if index is last item you should return first item
            case pages.count - 1:
                return pages[0]
            default:
                // by default return next item in array
                return pages[index + 1]
            }
        }
        return nil
    }

    func pageViewController(_ pageViewController: UIPageViewController,
                            viewControllerBefore viewController: UIViewController) -> UIViewController? {
        // find index of viewcontroller in array
        if let index = pages.index(of: viewController) {
            switch index {
                // if index is 0 you should return last item in array
            case 0:
                return pages[pages.count - 1]
            default:
                // by default return previous item in array
                return pages[index - 1]
            }
        }
        return nil
    }
}