UIPageViewController shows blank view controller when setting programmatically - ios

Question

I have a UIPageViewController in which I've implemented a caching mechanism. Practically I have a pool of view controllers that I try to reuse as much as possible when the next or previous view controller is requested in the UIPageViewControllerDataSource. So when the data source requests the previous or the next page I first check if that page has already been shown and return the appropriate view controller.
It all works fine with the default swipe gesture, and the internal method to change page. Now I need to disable the swipe gesture, so I'm not setting the data source, and instead, I have two buttons to go to the previous and next page, and using setViewControllers(_:direction:animated:completion:) to change page programmatically. It works fine when going always in the same direction, but as soon as I change it, and so I'm using a cached view controller, it shows a blank page.
The code is exactly the same as before, if not for setting the view controller programmatically. I've tried both with or without animation and the result is the same and if I remove caching it also works well. My question is, am I forgetting something? Is the UIPageViewController doing something internally that I'm not doing when the data source is assigned?
EDIT
Here's a simplified version of the code. It's a Xamarin Project, but I suppose it's understandable
public class MyPageViewController : AbstractPageViewController
{
const int CacheCapacity = 5;
public int initialItemId { get; set; }
public List<int> ItemIds { get; set; }
readonly List<ChildViewController> viewControllerCache = new List<ChildViewController>(CacheCapacity + 1);
public override void ViewDidLoad()
{
base.ViewDidLoad();
DataSource = null;
var vc = GetDocumentViewController(initialItemId);
SetViewControllers(new[] { vc }, UIPageViewControllerNavigationDirection.Forward, false, null);
var nextButtonItem = new UIBarButtonItem
{
Enabled = true
};
var previousButtonItem = new UIBarButtonItem
{
Enabled = true
};
nextButtonItem.Clicked += NextDocumentButton_Clicked;
previousButtonItem.Clicked += PreviousDocumentButton_Clicked;
var rightButtons = new UIBarButtonItem[2];
rightButtons[0] = nextButtonItem;
rightButtons[1] = previousButtonItem;
NavigationItem.SetRightBarButtonItems(rightButtons, false);
}
private void PreviousDocumentButton_Clicked(object sender, EventArgs e)
{
var referenceVc = (ChildViewController)ViewControllers.FirstOrDefault();
var referenceId = referenceVc.ItemId;
var index = ItemIds.FindIndex(dp => dp.Id == referenceId);
if (index < 1)
return;
var previousDocumentId = ItemIds[index - 1];
var vc = GetDocumentViewController(previousDocumentId);
SetViewControllers(new[] { vc }, UIPageViewControllerNavigationDirection.Reverse, false, null);
}
private void NextDocumentButton_Clicked(object sender, EventArgs e)
{
var referenceVc = (ChildViewController)ViewControllers.FirstOrDefault();
var referenceId = referenceVc.ItemId;
var index = ItemIds.FindIndex(dp => dp.Id == referenceId);
if (index < 0 || index >= ItemIds.Count)
return;
var nextDocumentId = ItemIds[index + 1];
var vc = GetDocumentViewController(nextDocumentId);
SetViewControllers(new[] { vc }, UIPageViewControllerNavigationDirection.Forward, false, null);
}
ChildViewController GetDocumentViewController(int ItemId)
{
var cachedViewController = viewControllerCache.FirstOrDefault(dvc => dvc.ItemId == ItemId);
if (cachedViewController != null)
return cachedViewController;
var vc = new ChildViewController();
vc.SetData(itemId);
viewControllerCache.Add(vc);
if (viewControllerCache.Count > CacheCapacity)
{
viewControllerCache[0].RecycleIfNeeded();
viewControllerCache.RemoveAt(0);
}
return vc;
}
}

Answer 1

It turned out that it was due to a recycling mechanism that was implemented in my code.
In particular, there's a forced recycling of the content of the view controller on ViewDidDisappear. I've disabled it in this case and then it works as it should. This also means that when the UIPageViewController has a data source, and is managing the child view controller directly, ViewDidDisappear is not called when changing page through swiping.