PDFViewController controller state in Swift for iOS

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//
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//  Copyright © 2015-2026 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 enum ControllerState: String {
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    case normal
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    case empty
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    case error
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    case locked
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    func nextState() -> ControllerState {
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        switch self {
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        case .normal: return .empty
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        case .empty: return .error
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        case .error: return .locked
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        case .locked: return .normal
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        }
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    }
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}
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// MARK: CustomStringConvertible
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extension ControllerState: CustomStringConvertible {
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    var description: String {
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        return "State: \(rawValue)"
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    }
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}
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class ControllerStateExample: Example {
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    // MARK: Properties
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    private weak var pdfController: PDFViewController?
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    private var toggleButton: UIBarButtonItem!
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    private var displayState: ControllerState = .normal {
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        didSet {
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            guard let pdfController else { return }
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            switch displayState {
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            case .normal:
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                pdfController.document = AssetLoader.document(for: .welcome)
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            case .empty:
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                pdfController.document = nil
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            case .error:
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                pdfController.document = Document(dataProviders: [DataContainerProvider(data: Data())])
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            case .locked:
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                pdfController.document = AssetLoader.document(for: "Password-Protected.pdf")
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            }
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            toggleButton.title = String(describing: displayState)
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        }
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    }
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    override init() {
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        super.init()
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        title = "Controller States"
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        contentDescription = "Shows default, empty, error, and locked states."
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        category = .controllerCustomization
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        toggleButton = UIBarButtonItem(title: "", style: .plain, target: self, action: #selector(ControllerStateExample.toggleButtonPressed(_:)))
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    }
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    @objc
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    private func toggleButtonPressed(_ sender: UIBarButtonItem) {
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        // If pressed, toggle to the next state.
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        displayState = displayState.nextState()
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    }
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    override func invoke(with delegate: ExampleRunnerDelegate) -> UIViewController {
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        let items = [toggleButton!]
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        let document = AssetLoader.document(for: .welcome)
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        pdfController = {
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            let pdfController = PDFViewController(document: document)
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            for viewMode: ViewMode in [.document, .documentEditor, .thumbnails] {
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                pdfController.navigationItem.setRightBarButtonItems(items, for: viewMode, animated: false)
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            }
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            pdfController.barButtonItemsAlwaysEnabled = items
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            return pdfController
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        }()
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        // Set displayState to trigger toggleButton title change
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        displayState = .normal
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        return pdfController!
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    }
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}