UIPageViewController can return the wrong index

Question

As the title states, there is a way it can return the wrong index. This messes up the index presentation dots at the bottom of the page. The way this is done is by skipping a page without releasing a finger from the screen. If this happens, it messes of the rest of the presentation dots.

here is what the bug it looks like in action.

And here is the code that was used for the UIPageViewController.

    import UIKit

class PageViewController: UIPageViewController {

    override func preferredStatusBarStyle() -> UIStatusBarStyle {
        return UIStatusBarStyle.LightContent
    }

    private(set) lazy var orderedViewControllers: [UIViewController] = {
        return [self.newColoredViewController("Green"),
            self.newColoredViewController("Red"),
            self.newColoredViewController("Blue")]

    }()

    private func newColoredViewController(color: String) -> UIViewController {
        return UIStoryboard(name: "Main", bundle: nil) .
            instantiateViewControllerWithIdentifier("\(color)ViewController")
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        dataSource = self

        if let firstViewController = orderedViewControllers.first {
            setViewControllers([firstViewController],
                direction: .Forward,
                animated: true,
                completion: nil)
        }

        // Do any additional setup after loading the view.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }


    /*
    // MARK: - Navigation

    // In a storyboard-based application, you will often want to do a little preparation before navigation
    override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
        // Get the new view controller using segue.destinationViewController.
        // Pass the selected object to the new view controller.
    }
    */

}

//MARK: UIPageViewControllerDataSource

extension PageViewController: UIPageViewControllerDataSource {

    func pageViewController(pageViewController: UIPageViewController, viewControllerBeforeViewController viewController: UIViewController) -> UIViewController? {
        guard let viewControllerIndex = orderedViewControllers.indexOf(viewController) else {
            return nil
        }

        let previousIndex = viewControllerIndex - 1

        guard previousIndex >= 0 else {
            return nil
        }

        guard orderedViewControllers.count > previousIndex else {
            return nil
        }

        return orderedViewControllers[previousIndex]
    }

    func pageViewController(pageViewController: UIPageViewController, viewControllerAfterViewController viewController: UIViewController) -> UIViewController? {
        guard let viewControllerIndex = orderedViewControllers.indexOf(viewController) else {
            return nil
        }

        let nextIndex = viewControllerIndex + 1

        let orderedViewControllersCount = orderedViewControllers.count

        guard orderedViewControllersCount != nextIndex else {
            return nil
        }

        guard orderedViewControllersCount > nextIndex else {
            return nil
        }

        return orderedViewControllers[nextIndex]
    }

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

    func presentationIndexForPageViewController(pageViewController: UIPageViewController) -> Int {
        guard let firstViewController = viewControllers?.first,
            firstViewControllerIndex = orderedViewControllers.indexOf(firstViewController) else {
                return 0
        }

        return firstViewControllerIndex
    }

}

Any help with this would be greatly appreciated, thanks.

Answer 1

I made a workaround because this is still not fixed!! It has been more than 4 years darn it.

Class to subclass instead of UIPageViewController:

import UIKit

class PageController: UIPageViewController {
    lazy var pages: [UIViewController] = []

    var newOffset: CGFloat = 20

    var showReal = true
    var pageControlX: CGFloat = 0 // 0 is the middle of the screen
    var pageControlY: CGFloat = 200

    private var scrollView: UIScrollView?
    private var scrollPoints: [UIView] = []
    private var oldScrollPoints: [UIView] = []

    private var indexKeeper = IndexKeeper()

    private var selectedColor = UIColor(white: 1, alpha: 1)
    private var unselectedColor = UIColor(white: 1, alpha: 0.2)

    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()

        // whole screen scroll
        scrollView = view.subviews.filter{ $0 is UIScrollView }.first! as? UIScrollView
        scrollView!.frame = UIScreen.main.bounds

        // get pageControl and scroll view from view's subviews
        let pageControl = view.subviews.filter{ $0 is UIPageControl }.first! as! UIPageControl
        oldScrollPoints = pageControl.subviews

        // remove all constraint from view that are tied to pageControl
        let const = view.constraints.filter { $0.firstItem as? NSObject == pageControl || $0.secondItem as? NSObject == pageControl }
        view.removeConstraints(const)

        // customize pageControl
        pageControl.translatesAutoresizingMaskIntoConstraints = false
        pageControl.frame = CGRect(x: pageControlX, y: pageControlY,
                                   width: view.frame.width, height: 0)

        DispatchQueue.main.asyncAfter(deadline: .now() + 0.01) {
            self.replacePoints()
            self.updateIndex()
        }
    }

    private func replacePoints() {
        for point in scrollPoints {
            point.removeFromSuperview()
        }
        scrollPoints = []
        for oldPoint in oldScrollPoints {
            let newPoint = UIView(frame: oldPoint.frame)
            // make rounded
            newPoint.layer.borderWidth = 0
            newPoint.layer.masksToBounds = false
            newPoint.layer.cornerRadius = newPoint.frame.height / 2
            newPoint.clipsToBounds = true

            newPoint.center = CGPoint(x: oldPoint.center.x, y: newOffset)
            newPoint.backgroundColor = oldPoint.backgroundColor
            oldPoint.superview?.addSubview(newPoint)
            scrollPoints.append(newPoint)
            oldPoint.alpha = showReal ? 1 : 0
        }
    }

    private func offsetFromFirstPage() -> CGFloat {
        var coordinates: [CGFloat] = []
        for (index, page) in pages.enumerated() {
            let offset = scrollView!.convert(scrollView!.bounds.origin, to: page.view!)
            coordinates.append(offset.x + CGFloat(index) * view.frame.width)
        }

        let duplicates: [CGFloat] = coordinates.enumerated().map
        { current in
            if coordinates.firstIndex(of: current.element) != coordinates.lastIndex(of: current.element) {
                return current.element
            } else {
                return .nan
            }
        }

        return duplicates.first(where: { !$0.isNaN }) ?? 0
    }

    private var lastIndex: Int = 0
    private func pageIndexFrom(offset: CGFloat, visibleRatio: CGFloat=0.6) -> Int {
        let adjustedOffset = (offset / view.frame.width)

        if adjustedOffset > CGFloat(lastIndex) + visibleRatio {
            if lastIndex + 1 < pages.count {
                lastIndex += 1
            }
        } else if adjustedOffset < CGFloat(lastIndex) - visibleRatio {
            if lastIndex - 1 >= 0 {
                lastIndex -= 1
            }
        }
        return lastIndex
    }

    private func updateIndex() {
        let fastIndex = pages.firstIndex(of: viewControllers!.first!)!
        indexKeeper.fastIndexUpdate(index: fastIndex)

        let offset = offsetFromFirstPage()
        let slowIndex = pageIndexFrom(offset: offset)
        indexKeeper.slowIndexUpdate(index: slowIndex)

        for (index, point) in scrollPoints.enumerated() {
            if index == indexKeeper.finalIndex {
                point.backgroundColor = selectedColor
            } else {
                point.backgroundColor = unselectedColor
            }
        }

        DispatchQueue.main.asyncAfter(deadline: .now() + 0.01) {
            self.updateIndex()
        }
    }
}

class IndexKeeper {
    var finalIndex = 0
    private var slowIndex = 0
    private var fastIndex = 0

    func slowIndexUpdate(index: Int) {
        if index != slowIndex {
            slowIndex = index
            finalIndex = slowIndex
        }
    }

    func fastIndexUpdate(index: Int) {
        if index != fastIndex {
            fastIndex = index
            finalIndex = fastIndex
        }
    }
}

Example Use:

import UIKit

class PageViewController: PageController, UIPageViewControllerDataSource {

    private func pageInstance(name:String) -> UIViewController {
        return UIStoryboard(name: "Main", bundle: nil).instantiateViewController(withIdentifier: name)
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        pages = [
            pageInstance(name: "FirstPage"),
            pageInstance(name: "SecondPage"),
            pageInstance(name: "ThirdPage"),
            pageInstance(name: "FourthPage")
        ]

        dataSource = self

        setViewControllers([pages.first!], direction: .forward, animated: false, completion: nil)
    }

    // get page before current page
    func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
        guard let index = pages.firstIndex(of: viewController), index > 0 else {
            return nil
        }

        return pages[index - 1]
    }

    // get page after current page
    func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {
        guard let index = pages.firstIndex(of: viewController), index + 1 < pages.count else {
            return nil
        }

        return pages[index + 1]
    }

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

    func presentationIndex(for pageViewController: UIPageViewController) -> Int {
        if let vc = viewControllers?.first {
            return pages.firstIndex(of: vc)!
        }
        return 0
    }
}