Custom Thumbnail PDF View Filter in Swift for iOS

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//
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//  Copyright © 2018-2025 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 extension ThumbnailViewFilter {
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    static let inkAnnotations = ThumbnailViewFilter("Ink Annotations")
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}
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private class CustomThumbnailViewController: ThumbnailViewController {
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    override func pages(forFilter filter: ThumbnailViewFilter, groupingResultsBy groupSize: UInt, result resultHandler: @escaping (IndexSet) -> Void, completion: @escaping (Bool) -> Void) -> PSPDFKit.Progress? {
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        // Only shows pages with ink annotations.
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        if filter == .inkAnnotations {
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            guard let pagesWithInkAnnotations = document?.allAnnotations(of: .ink).map({ $0.key.intValue }) else { return nil }
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            var annotationIndexes: IndexSet = []
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            pagesWithInkAnnotations.forEach { annotationIndexes.insert($0) }
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            resultHandler(annotationIndexes)
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            completion(true)
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            return nil
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        }
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        return super.pages(forFilter: filter, groupingResultsBy: groupSize, result: resultHandler, completion: completion)
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    }
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}
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class CustomThumbnailViewControllerFilterExample: Example {
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    override init() {
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        super.init()
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        title = "Custom Thumbnail View Controller Filter"
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        contentDescription = "Shows how to add a custom filter by subclassing ThumbnailViewController"
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        category = .subclassing
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        priority = 400
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    }
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    override func invoke(with delegate: ExampleRunnerDelegate) -> UIViewController {
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        // Playground is convenient for testing
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        let document = AssetLoader.writableDocument(for: .welcome, overrideIfExists: false)
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        let controller = PDFViewController(document: document) {
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            // Register the override to use a custom search thumbnail view controller subclass.
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            $0.overrideClass(ThumbnailViewController.self, with: CustomThumbnailViewController.self)
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        }
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        controller.navigationItem.rightBarButtonItems = [controller.thumbnailsButtonItem]
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        // Add the custom filter option.
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        controller.thumbnailController.filterOptions = [.showAll, .bookmarks, .inkAnnotations]
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        return controller
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    }
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}