UIPageViewController: return the current visible v

Question

How do you know what is the current page/view displayed inside an UIPageViewController?

I have overridden the viewDidAppear method of my child views, so that they send an id to the parent view in their viewDidAppear method.

However, the problem is this: i cannot reliably use that id as id for the displayed page. because if the user turns the page but halfway through decides to stop the turning and put the page back, viewDidAppear will already have been called. (the view is visible behind the curled page).

Maybe i should only switch to a new id if the current view disappears. But I wonder if there is not a more simple way to return the view that is currently visible?

Answer 1

Unfortunately nothing above works for me.

I have two view controllers and when I slightly (around 20px) scroll the last view backwards it triggers the delegate:

pageViewController:didFinishAnimating:previousViewControllers:transitionCompleted:

and saying that the current page (index) is 0 which is wrong.

Using delegate inside child viewController something like:

- (void)ViewController:(id)VC didShowWithIndex:(long)page;

// and a property

@property (nonatomic) NSInteger index;

that is triggered inside viewDidAppear like:

- (void)viewDidAppear:(BOOL)animated
{
    ...

    [self.delegate ViewController:self didShowWithIndex:self.index];
}

Worked for me.

Answer 2

This is the solution I came up with:

class DefaultUIPageViewControllerDelegate: NSObject, UIPageViewControllerDelegate {

    // MARK: Public values
    var didTransitionToViewControllerCallback: ((UIViewController) -> Void)?

    // MARK: Private values
    private var viewControllerToTransitionTo: UIViewController!

    // MARK: Methods
    func pageViewController(
        _ pageViewController: UIPageViewController,
        willTransitionTo pendingViewControllers: [UIViewController]
    ) {
        viewControllerToTransitionTo = pendingViewControllers.last!
    }

    func pageViewController(
        _ pageViewController: UIPageViewController,
        didFinishAnimating finished: Bool,
        previousViewControllers: [UIViewController],
        transitionCompleted completed: Bool
    ) {
        didTransitionToViewControllerCallback?(viewControllerToTransitionTo)
    }
}

Usage:

 let pageViewController = UIPageViewController()
 let delegate = DefaultUIPageViewControllerDelegate()

 delegate.didTransitionToViewControllerCallback = {
    pageViewController.title = $0.title
 }

 pageViewController.title = viewControllers.first?.title
 pageViewController.delegate = delegate

Make sure to set the initial title