Removing a view controller from UIPageViewController

While the answers here are all informative, there is an alternate way of handling the problem, given here:

UIPageViewController navigates to wrong page with Scroll transition style

When I first searched for an answer to this problem, the way I was wording my search wound me up at this question, and not the one I’ve just linked to, so I felt obligated to post an answer linking to this other question, now that I’ve found it, and also elaborating a little bit.

The problem is described pretty well by matt here:

This is actually a bug in UIPageViewController. It occurs only with the scroll style (UIPageViewControllerTransitionStyleScroll) and only
after calling setViewControllers:direction:animated:completion: with
animated:YES. Thus there are two workarounds:

Don’t use UIPageViewControllerTransitionStyleScroll.

Or, if you call setViewControllers:direction:animated:completion:, use
only animated:NO.

To see the bug clearly, call
setViewControllers:direction:animated:completion: and then, in the
interface (as user), navigate left (back) to the preceding page
manually. You will navigate back to the wrong page: not the preceding
page at all, but the page you were on when
setViewControllers:direction:animated:completion: was called.

The reason for the bug appears to be that, when using the scroll
style, UIPageViewController does some sort of internal caching. Thus,
after the call to setViewControllers:direction:animated:completion:,
it fails to clear its internal cache. It thinks it knows what the
preceding page is. Thus, when the user navigates leftward to the
preceding page, UIPageViewController fails to call the dataSource
method pageViewController:viewControllerBeforeViewController:, or
calls it with the wrong current view controller.

This is a good description, not quite the problem noted in this question but very close. Note the line about if you do setViewControllers with animated:NO you will force the UIPageViewController to re-query its data source next time the user pans with a gesture, as it no longer “knows where it is” or what view controllers are next to its current view controller.

However, this didn’t work for me because there were times when I need to programmatically move the PageView around with an animation.

So, my first thought was to call setViewControllers with an animation, and then in the completion block call the method again with whatever view controller was now showing, but with no animation. So the user can pan, fine, but then we call the method again to get the page view to reset.

Unfortunately when I tried that I started getting strange “assertion errors” from the page view controller. They look something like this:

*** Assertion failure in -[UIPageViewController queuingScrollView: …

Not knowing exactly why this was happening, I backtracked and eventually started using Jai’s answer as a solution, creating an entirely new UIPageViewController, pushing it onto a UINavigationController, then popping out the old one. Gross, but it works–mostly. I have been finding I’m still getting occasional Assertion Failures from the UIPageViewController, like this one:

*** Assertion failure in -[UIPageViewController queuingScrollView:didEndManualScroll:toRevealView:direction:animated:didFinish:didComplete:],
/SourceCache/UIKit_Sim/UIKit-2380.17/UIPageViewController.m:1820 $1 =
154507824 No view controller managing visible view >

And the app crashes. Why? Well, searching, I found this other question that I mentioned up top, and particularly the accepted answer which advocates my original idea, of simply calling setViewControllers: animated:YES and then as soon as it completes calling setViewControllers: animated:NO with the same view controllers to reset the UIPageViewController, but it had the missing element: calling that code back on the main queue! Here’s the code:

__weak YourSelfClass *blocksafeSelf = self;     
[self.pageViewController setViewControllers:viewControllers direction:UIPageViewControllerNavigationDirectionForward animated:YES completion:^(BOOL finished){
                dispatch_async(dispatch_get_main_queue(), ^{
                    [blocksafeSelf.pageViewController setViewControllers:viewControllers direction:UIPageViewControllerNavigationDirectionForward animated:NO completion:NULL];// bug fix for uipageview controller

Wow! The only reason this actually made sense to me is because I have watched the the WWDC 2012 Session 211, Building Concurrent User Interfaces on iOS (available here with a dev account). I recall now that attempting to modify data source objects that UIKit objects (like UIPageViewController) depend on, and doing it on a secondary queue, can cause some nasty crashes.

What I have never seen particularly documented, but must now assume to be the case and read up on, is that the completion block for an animation is performed on a secondary queue, not the main one. So the reason why UIPageViewController was squawking and giving assertion failures, both when I originally attempted to call setViewControllers animated:NO in the completion block of setViewControllers animated:YES and also now that I am simply using a UINavigationController to push on a new UIPageViewController (but doing it, again, in the completion block of setViewControllers animated:YES) is because it’s all happening on that secondary queue.

That’s why that piece of code up there works perfectly, because you come from the animation completion block and send it back over to the main queue so you don’t cross the streams with UIKit. Brilliant.

Anyway, wanted to share this journey, in case anyone runs across this problem.

EDIT: Swift version here, if anyone’s interested.

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