Cycle through PDF documents using Swift for iOS

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//
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//  Copyright © 2017-2026 PSPDFKit GmbH. All rights reserved.
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//
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//  The Nutrient sample applications are licensed with a modified BSD license.
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//  Please see License for details. This notice may not be removed from this file.
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//
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import PSPDFKit
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import PSPDFKitUI
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private class PageViewController: UIPageViewController {
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    override func viewDidLayoutSubviews() {
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        super.viewDidLayoutSubviews()
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        // This is somewhat of a hack to work around the inflexible UIPageViewController
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        // This will extend the content to include the page control indicator.
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        for view in self.view.subviews {
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            if view is UIScrollView {
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                view.frame = self.view.frame
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            } else if view is UIPageControl {
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                self.view.bringSubviewToFront(view)
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                view.backgroundColor = UIColor.clear
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            }
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        }
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    }
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}
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class PageViewControllerExample: Example {
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    var controller: [UIViewController] = []
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    var pageViewController: UIPageViewController?
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    var pageIndex: Int {
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        didSet {
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            updateTitle()
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        }
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    }
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    override init() {
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        pageIndex = 0
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        super.init()
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        title = "UIPageViewController Example"
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        contentDescription = "Use UIPageViewController to cycle through PDFs"
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        category = .viewCustomization
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        priority = 11
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        prefersLargeTitles = false
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    }
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    override func invoke(with delegate: ExampleRunnerDelegate) -> UIViewController {
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        // prepare list of documents you want to present
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        let assetNames: [AssetName] = [.welcome, .about, .web, .annualReport]
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        let documents = assetNames.map { assetName in
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            return AssetLoader.document(for: assetName)
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        }
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        // allocating many PDFViewController instances is memory intensive
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        // if you have many documents, you should switch to a on-demand solution and re-use controllers.
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        controller = documents.map { document in
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            let pdfController = PDFViewController(document: document) {
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                $0.pageTransition = .scrollContinuous
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                $0.scrollDirection = .vertical
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                $0.useParentNavigationBar = true
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                $0.userInterfaceViewMode = .automaticNoFirstLastPage
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                // the scrubber wouldn't work well when we expect horizontal movements to scroll content
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                $0.thumbnailBarMode = .none
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                // background is defined by the page view controller already
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                $0.backgroundColor = UIColor.clear
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            }
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            pdfController.navigationItem.rightBarButtonItems = [pdfController.thumbnailsButtonItem, pdfController.outlineButtonItem, pdfController.searchButtonItem, pdfController.annotationButtonItem]
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            return pdfController
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        }
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        // set up the page view controller
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        let pageController = PageViewController(transitionStyle: .scroll, navigationOrientation: .horizontal, options: [.interPageSpacing: 10])
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        pageController.dataSource = self
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        pageController.delegate = self
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        pageController.setViewControllers([controller[0]], direction: .forward, animated: false, completion: nil)
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        pageController.view.backgroundColor = #colorLiteral(red: 0.05882352963, green: 0.180392161, blue: 0.2470588237, alpha: 1)
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        pageViewController = pageController
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        pageIndex = 0
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        // styling as we customize the page indicator to overlay
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        let pageControl = UIPageControl.appearance(whenContainedInInstancesOf: ([PageViewController.self]))
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        pageControl.pageIndicatorTintColor = UIColor.lightGray
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        pageControl.currentPageIndicatorTintColor = UIColor.darkGray
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        UserInterfaceView.appearance(whenContainedInInstancesOf: ([PageViewController.self])).pageLabelInsets = UIEdgeInsets(top: 0, left: 0, bottom: 40, right: 0)
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        return pageController
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    }
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    func updateTitle() {
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        // Title is set a bit later in PDFViewController initialization, so we use it from the
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        // document directly. Aternatively, title could be KVO ovserved.
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        guard let pdfController = controller[pageIndex] as? PDFViewController else { return }
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        pageViewController?.title = pdfController.document?.title
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        // Since we forward the navigationitem from the hosted view controller, we need to explicly update
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        pdfController.updateToolbar(animated: false)
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    }
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}
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extension PageViewControllerExample: UIPageViewControllerDelegate {
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    public func pageViewController(_ pageViewController: UIPageViewController, didFinishAnimating finished: Bool, previousViewControllers: [UIViewController], transitionCompleted completed: Bool) {
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        guard completed,
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            let currentVisibleController = pageViewController.viewControllers?.first,
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            let index = controller.firstIndex(of: currentVisibleController) else { return }
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        // update page index after it updated successfully
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        pageIndex = index
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    }
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}
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extension PageViewControllerExample: UIPageViewControllerDataSource {
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    public func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
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        guard let index = controller.firstIndex(of: viewController), index > 0 else { return nil }
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        return controller[index - 1]
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    }
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    public func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {
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        guard let index = controller.firstIndex(of: viewController), index + 1 < controller.count else { return nil }
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        return controller[index + 1]
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    }
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    // The number of items reflected in the page indicator.
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    public func presentationCount(for pageViewController: UIPageViewController) -> Int {
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        return controller.count
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    }
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    // The selected item reflected in the page indicator.
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    public func presentationIndex(for pageViewController: UIPageViewController) -> Int {
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        return pageIndex
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    }
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}