Prepare PDF to Capture Digital Signature in Swift for iOS

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//
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//  Copyright © 2019-2025 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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class PrepareDocumentForContainedSignaturesExample: Example {
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    override init() {
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        super.init()
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        title = "Prepare a document to embed a digital signature afterwards"
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        category = .forms
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        priority = 20
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    }
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    override func invoke(with delegate: ExampleRunnerDelegate) -> UIViewController {
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        // Load a sample document with an already created signature form field.
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        let document = AssetLoader.writableDocument(for: "Form.pdf", overrideIfExists: true)
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        // We know this document must have a file URL because we are loading it from a file.
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        let newURL = document.fileURL!
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        // Now we get the signature form widget.
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        let signatureFormElement = document.annotations(at: 0, type: SignatureFormElement.self).first!
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        // During document preparation, we are able to configure a custom digital signature appearance.
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        // We use an instance of a `PSPDFSignatureAppearance` to do that.
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        let signatureAppearance = PDFSignatureAppearance {
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            $0.appearanceMode = PDFSignatureAppearance.Mode.signatureAndDescription
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            $0.showSigningDate = false
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        }
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        // Configure a data sink as destination.
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        let dataSink = try! FileDataSink(fileURL: newURL, options: [])
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        // Now configure the signer that will prepare the signature field. Configuring the "signersName" property
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        // will customize the signer's name that will appear on the visual signature.
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        let signer = PDFSigner()
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        signer.signersName = "PSPDFKit GmbH"
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        var preparedDocument: Document?
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        // Typically, when preparing a document, we are not interested in what is inside the contents of the signature form field.
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        // Passing an instance of `BlankSignatureContents` will fill the contents with zeroes.
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        let signatureContents = BlankSignatureContents()
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        signer.prepare(signatureFormElement, toBeSignedWith: signatureAppearance, contents: signatureContents, writingTo: dataSink) { (_ success: Bool, _ document: DataSink?, _ err: Error?) in
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            let fileDataProvider = FileDataProvider(fileURL: newURL)
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            do {
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                try fileDataProvider.replaceContents(with: dataSink)
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            } catch {
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                print(error)
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            }
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            preparedDocument = Document(dataProviders: [fileDataProvider])
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        }
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        return PDFViewController(document: preparedDocument!)
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    }
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}
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/// A sample `PSPDFSignatureContents` implementation that constructs a signature container from a binary file (.bin).
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/// A real implementation would make use of the document byte range covered by a signature, hash it, encrypt it,
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/// and return an hex-encoded PKCS7 container.
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class BinaryFileSignatureContents: NSObject, PDFSignatureContents {
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    func sign(_ dataToSign: Data) -> Data {
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        return try! Data(contentsOf: self.signatureContentsPath)
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    }
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    init(signatureContentsPath: URL) {
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        self.signatureContentsPath = signatureContentsPath
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        super.init()
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    }
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    private let signatureContentsPath: URL
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}
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class EmbedContainedSignatureExample: Example {
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    override init() {
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        super.init()
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        title = "Embed a digital signature in an already prepared PDF document"
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        category = .forms
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        priority = 21
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    }
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    override func invoke(with delegate: ExampleRunnerDelegate) -> UIViewController {
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        // Load a sample document with a signature form field that was already prepared for signing.
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        // See `PrepareDocumentForContainedSignaturesExample` to learn how to do that.
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        let document = AssetLoader.writableDocument(for: "Form for Signing.pdf", overrideIfExists: true)
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        let newURL = document.fileURL!
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        // Now we get the signature form widget.
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        let signatureFormElement = document.annotations(at: 0, type: SignatureFormElement.self).first!
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        // The prepared signature is a .bin file that contains a PKCS7 signature for this document.
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        let preparedSignatureSampleURL = AssetLoader.assetURL(for: "John Appleseed Signature.bin")
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        let signatureContents = BinaryFileSignatureContents(signatureContentsPath: preparedSignatureSampleURL)
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        let dataSink = try! FileDataSink(fileURL: newURL, options: [])
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        // Finally, create the signer and embed the PKCS7 signature in this document.
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        let signer = PDFSigner()
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        var preparedDocument: Document?
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        signer.embedSignature(in: signatureFormElement, with: signatureContents, writingTo: dataSink) { (_ success: Bool, _ document: DataSink?, _ err: Error?) in
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            let fileDataProvider = FileDataProvider(fileURL: newURL)
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            do {
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                try fileDataProvider.replaceContents(with: dataSink)
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            } catch {
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                print(error)
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            }
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            // Optionally, here you could make sure that the signed document is valid.
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            // Read the `PDFSignatureValidator` documentation to learn how to do that.
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            preparedDocument = Document(dataProviders: [fileDataProvider])
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        }
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        return PDFViewController(document: preparedDocument!)
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    }
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}