Display measurements on PDF pages or spreads in Swift for iOS

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//
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//  Copyright © 2020-2026 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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/// Swift code from https://www.nutrient.io/guides/ios/customizing-pdf-pages/adding-auxiliary-or-decorative-views/
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///
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/// For an in-depth explanation of the classes and structs, please read this article, as well as its companion guide
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/// “Customizing Interactions with an Annotation Type”.
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///
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/// This is older sample code that uses measurements as an example. If you actually want measurements don’t use this code.
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/// Instead see our Measurement Tools component, which is highlighted in `MeasurementsExample.swift`.
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class SpreadMeasurementsExample: Example {
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    override init() {
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        super.init()
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        title = "Auxiliary views on pages"
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        contentDescription = "Shows how to add auxiliary views to spreads/pages"
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        category = .viewCustomization
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    }
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    private var manager: MeasuringPDFControllerManager?
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    override func invoke(with delegate: ExampleRunnerDelegate) -> UIViewController? {
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        manager = .init()
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        manager?.document = AssetLoader.document(for: .welcome)
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        return manager?.documentViewController
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    }
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}
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// MARK: - Measurement and Datasource:
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protocol SpreadMeasurement {
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    var pageRange: NSRange { get }
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    var path: CGPath { get }
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    var value: Measurement<Dimension> { get }
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}
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protocol DocumentMeasurementDatasource: AnyObject {
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    func measurements(at pageIndex: Int) -> [SpreadMeasurement]
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}
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// MARK: - View and Extension:
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private extension SpreadMeasurement {
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    var isArea: Bool {
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        value.unit is UnitArea
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    }
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}
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private class SpreadMeasurementView: UIView, AnnotationPresenting {
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    var pdfScale: CGFloat {
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        didSet {
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            if oldValue != pdfScale, let measurement {
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                // The transform for PDF to page view coordinates just changed, so we have to adapt
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                updateFrameAndLayer(measurement: measurement, scale: pdfScale)
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            }
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        }
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    }
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    var zoomScale: CGFloat {
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        didSet {
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            // make sure the label is always crisp
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            let viewScale = window?.traitCollection.displayScale ?? 1
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            dimensionLabel.contentScaleFactor = zoomScale * viewScale
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        }
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    }
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    func prepareForReuse() {
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        measurement = nil
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    }
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    var measurement: SpreadMeasurement? {
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        didSet {
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            guard let measurement else {
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                dimensionLabel.isHidden = true
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                shapeLayer.path = nil
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                return
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            }
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            dimensionLabel.isHidden = false
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            dimensionLabel.text = formatter.string(from: measurement.value)
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            updateFrameAndLayer(measurement: measurement, scale: pdfScale)
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            if measurement.isArea {
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                shapeLayer.fillColor = UIColor(white: 0.2, alpha: 0.4).cgColor
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                shapeLayer.lineDashPattern = nil
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            } else {
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                shapeLayer.fillColor = nil
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                shapeLayer.lineDashPattern = [5, 3, 2, 3]
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            }
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        }
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    }
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    private func updateFrameAndLayer(measurement: SpreadMeasurement, scale: CGFloat) {
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        guard scale > 0 else {
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            return
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        }
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        let path = measurement.path
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        var transform = CGAffineTransform(scaleX: scale, y: scale)
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        let boundingBox = path.boundingBox.applying(transform)
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        frame = boundingBox
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        // The layer is in coordinates of the bounds so we need to account for the offset, too
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        transform.tx = -boundingBox.origin.x
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        transform.ty = -boundingBox.origin.y
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        shapeLayer.path = path.copy(using: &transform)
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        setNeedsLayout()
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    }
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    override init(frame: CGRect) {
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        pdfScale = 0
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        zoomScale = 0
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        shapeLayer = .init()
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        shapeLayer.lineWidth = 1
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        shapeLayer.bounds.size = frame.size
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        shapeLayer.strokeColor = UIColor.systemRed.cgColor
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        dimensionLabel = UILabel()
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        dimensionLabel.translatesAutoresizingMaskIntoConstraints = false
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        dimensionLabel.backgroundColor = UIColor.systemRed.withAlphaComponent(0.6)
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        dimensionLabel.textColor = .white
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        formatter = .init()
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        formatter.unitOptions = .providedUnit
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        formatter.unitStyle = .short
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        formatter.numberFormatter.maximumFractionDigits = 2
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        super.init(frame: frame)
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        clipsToBounds = false
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        layer.addSublayer(shapeLayer)
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        addSubview(dimensionLabel)
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        NSLayoutConstraint.activate([
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            dimensionLabel.topAnchor.constraint(equalToSystemSpacingBelow: bottomAnchor, multiplier: 1),
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            dimensionLabel.centerXAnchor.constraint(equalTo: centerXAnchor),
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        ])
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    }
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    required init?(coder: NSCoder) {
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        fatalError("NSCoding is not supported")
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    }
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    private let shapeLayer: CAShapeLayer
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    private let dimensionLabel: UILabel
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    private let formatter: MeasurementFormatter
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}
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// MARK: - Page View:
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private class MeasurementDisplayingPageView: PDFPageView {
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    private var measureViewReusePool = [SpreadMeasurementView]()
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    private var visibleMeasureViews = [SpreadMeasurementView]()
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    override func prepareForReuse() {
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        visibleMeasureViews.forEach { view in
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            view.isHidden = true
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            view.prepareForReuse()
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        }
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        measureViewReusePool.append(contentsOf: visibleMeasureViews)
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        visibleMeasureViews.removeAll(keepingCapacity: true)
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        super.prepareForReuse()
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    }
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    func dequeueMeasureView() -> SpreadMeasurementView {
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        let view = measureViewReusePool.popLast() ?? SpreadMeasurementView()
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        view.isHidden = false
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        // Ensure the measure view is added to the annotation container view - PDFPageView.prepareForReuse()
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        // removes it from the view hierarchy. Also, make sure the view has the correct scales set, so it
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        // displays correctly.
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        annotationContainerView.addSubview(view)
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        visibleMeasureViews.append(view)
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        view.pdfScale = scaleForPageView
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        view.zoomScale = zoomView?.zoomScale ?? 1
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        return view
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    }
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    func markForReuse(measureView: SpreadMeasurementView) {
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        measureView.prepareForReuse()
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        measureView.isHidden = true
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        visibleMeasureViews.removeAll {
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            $0 === measureView
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        }
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        measureViewReusePool.append(measureView)
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    }
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}
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// MARK: - Integration:
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private class MeasuringPDFControllerManager: NSObject, PDFViewControllerDelegate {
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    var measurementsSource: DocumentMeasurementDatasource?
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    let documentViewController: PDFViewController
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    var document: Document? {
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        get { documentViewController.document }
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        set {
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            if newValue == nil {
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                measurementsSource = nil
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                documentViewController.document = nil
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            } else if documentViewController.document !== newValue {
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                // <# create a new measurements datasource for this document here! #>
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                measurementsSource = newValue.map(MeasurementStore.init(document:)) // this line is removed from guide
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                // then:
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                documentViewController.document = newValue
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            }
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        }
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    }
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    override init() {
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        documentViewController = PDFViewController { builder in
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            builder.pageTransition = .scrollContinuous
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            builder.pageMode = .double
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            builder.scrollDirection = .vertical
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            builder.overrideClass(PDFPageView.self, with: MeasurementDisplayingPageView.self)
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        }
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        super.init()
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        documentViewController.delegate = self
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    }
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    func pdfViewController(_ pdfController: PDFViewController, didConfigurePageView pageView: PDFPageView, forPageAt pageIndex: Int) {
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        guard
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            let page = pageView as? MeasurementDisplayingPageView,
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            let allMeasurements = measurementsSource?.measurements(at: pageIndex)
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        else {
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            return
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        }
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        for measurement in allMeasurements
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        // a measurement can span multiple pages => make sure we don’t add one to more than one page at once
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        where measurement.pageRange.location == pageIndex {
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            let view = page.dequeueMeasureView()
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            view.measurement = measurement
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        }
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        /*
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         Ensure the second page in a spread is always below the first one in the hierarchy, to allow measurements to
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         reach across the page binding.
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         */
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        if pdfController.configuration.pageMode == .double && pageIndex % 2 == 0 {
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            page.superview?.sendSubviewToBack(page)
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        }
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    }
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}