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In last post, we provided a guide on how to create UIPageViewController in storyboard.

But we leave out quite a few things about UIPageViewController. This guide will cover some of these things.

The Pages & ViewControllers

In our UIPageViewController, we declared pages to hold all our view controllers.

This pages is different from the provided property viewControllers.

• pages are all your view controllers
• viewControllers are the current views being displayed in the UIPageViewController. In iBook, 2 pages could be displayed together, and this property will hold the 2 view controllers.

Currently Selected Index

Oddly, there is no property or event to let you know of the currently selected index.

As such, you have ride on UIPageViewControllerDelegate and manually track it.

// Track the current index
var currentIndex: Int?
private var pendingIndex: Int?

func pageViewController(pageViewController: UIPageViewController, willTransitionToViewControllers pendingViewControllers: [UIViewController]) {
    pendingIndex = pages.indexOf(pendingViewControllers.first!)
}

func pageViewController(pageViewController: UIPageViewController, didFinishAnimating finished: Bool, previousViewControllers: [UIViewController], transitionCompleted completed: Bool) {
    if completed {
        currentIndex = pendingIndex
    }
}

Page Indicator

Page indicator is automatically provided if you use “scroll” transition, and you implement the 2 UIPageViewControllerDataSource methods below:

func presentationCountForPageViewController(pageViewController: UIPageViewController) -> Int {
    return pages.count
}

func presentationIndexForPageViewController(pageViewController: UIPageViewController) -> Int {
    return 0
}

You could be wondering why return 0 for the index?

As the doc says:

After gesture-driven navigation, these methods are not called. The index is updated automatically and the number of view controllers is expected to remain constant.

The method is never called when you swipe to scroll. It is called only when you call setViewControllers:direction:animated:completion: on the first time.