Show PDF download progress in 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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/// This example showing asynchronous document loading.
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/// The downloaded document can be modified, such as adding annotations.
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/// If you don’t need such fine-grained control and the document can be read-only, see the simpler `RemoteDocumentURLExample` instead.
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class DocumentProgressExample: Example {
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    let destinationFileURL = FileHelper.temporaryPDFFileURL(prefix: "document")
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    let remoteURL = URL(string: "https://github.com/PSPDFKit/pspdfkit-ios-catalog/raw/master/Samples/Magazine.pdf")!
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    var downloader: Downloader?
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    override init() {
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        super.init()
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        title = "Download and annotate a PDF from a remote URL"
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        contentDescription = "Downloads a PDF using URLSession. Shows progress using the data provider progress property."
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        category = .documentDataProvider
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        priority = 3
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    }
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    override func invoke(with delegate: ExampleRunnerDelegate) -> UIViewController {
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        let cancelButton = UIBarButtonItem(title: localizedString("Cancel"), style: .plain, target: self, action: #selector(cancelButtonPressed))
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        downloader?.cleanup()
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        downloader = Downloader(remoteURL: remoteURL, destinationFileURL: destinationFileURL)
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        downloader?.didFinishDownloading = { _ in
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            cancelButton.isEnabled = false
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        }
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        // Configure the data provider with our NSProgress instance.
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        let provider = CoordinatedFileDataProvider(fileURL: destinationFileURL, progress: downloader?.progress)
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        let document = Document(dataProviders: [provider])
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        let controller = PDFViewController(document: document)
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        controller.navigationItem.setRightBarButtonItems([cancelButton] + controller.navigationItem.rightBarButtonItems!, animated: false)
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        return controller
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    }
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    @objc func cancelButtonPressed(sender: UIBarButtonItem?) {
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        sender?.isEnabled = false
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        guard let downloader = self.downloader else { return }
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        downloader.progress.cancel()
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        downloader.cleanup()
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    }
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}
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class Downloader: NSObject {
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    let progress = Progress(totalUnitCount: 100)
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    let downloadProgress = Progress(totalUnitCount: 1)
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    let moveProgress = Progress(totalUnitCount: 1)
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    let destinationFileURL: URL
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    let remoteURL: URL
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    var session: URLSession?
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    var task: URLSessionDownloadTask?
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    var didFinishDownloading: ((_ location: URL) -> Void)?
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    init(remoteURL: URL, destinationFileURL: URL) {
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        self.remoteURL = remoteURL
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        self.destinationFileURL = destinationFileURL
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        super.init()
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        // Download the file using URLSession API.
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        let session = URLSession(configuration: URLSessionConfiguration.default, delegate: self, delegateQueue: OperationQueue.main)
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        self.session = session
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        let task = session.downloadTask(with: remoteURL)
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        self.task = task
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        task.resume()
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        // Setup progress hierarchy. The download should take much longer than the final move.
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        progress.addChild(downloadProgress, withPendingUnitCount: 99)
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        progress.addChild(moveProgress, withPendingUnitCount: 1)
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    }
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    func cleanup() {
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        do {
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            task?.cancel()
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            session?.invalidateAndCancel()
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            try FileManager().removeItem(at: destinationFileURL)
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        } catch {
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            print(error)
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        }
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    }
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}
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extension Downloader: URLSessionDelegate, URLSessionDownloadDelegate {
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    func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) {
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        do {
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            try FileManager().moveItem(at: location, to: destinationFileURL)
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            // We must ensure that the complete progress (`progress`) only completes when the file is already at its final location.
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            moveProgress.completedUnitCount = moveProgress.totalUnitCount
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            didFinishDownloading?(location)
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        } catch {
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            print(error)
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        }
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    }
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    func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didWriteData bytesWritten: Int64, totalBytesWritten: Int64, totalBytesExpectedToWrite: Int64) {
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        downloadProgress.totalUnitCount = totalBytesExpectedToWrite
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        downloadProgress.completedUnitCount = totalBytesWritten
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    }
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    func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) {
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        if let error {
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            print(error.localizedDescription)
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            // Complete the progress.
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            progress.completedUnitCount = progress.totalUnitCount
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        }
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    }
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}