Meet PaperKit

// Adopt PaperKit in iOS

override func viewDidLoad() {
    super.viewDidLoad()
    
    let markupModel = PaperMarkup(bounds: view.bounds)
    let paperViewController = PaperMarkupViewController(markup: markupModel, supportedFeatureSet: .latest)
    view.addSubview(paperViewController.view)
    addChild(paperViewController)
    paperViewController.didMove(toParent: self)
    becomeFirstResponder()    

    let toolPicker = PKToolPicker()
    toolPicker.addObserver(paperViewController)
    
    pencilKitResponderState.activeToolPicker = toolPicker
    pencilKitResponderState.toolPickerVisibility = .visible
    
    toolPicker.accessoryItem = UIBarButtonItem(barButtonSystemItem: .add, target: self, action: #selector(plusButtonPressed(_:)))
}

@objc func plusButtonPressed(_ button: UIBarButtonItem) {
    let markupEditViewController = MarkupEditViewController(supportedFeatureSet: .latest)    
    markupEditViewController.delegate = paperViewController
    markupEditViewController.modalPresentationStyle = .popover
    markupEditViewController.popoverPresentationController?.barButtonItem = button
    present(markupEditViewController, animated: true)
}

// Adopt PaperKit in macOS

override func viewDidLoad() {
    super.viewDidLoad()
    
    let markupModel = PaperMarkup(bounds: view.bounds)
    let paperViewController = PaperMarkupViewController(markup: markupModel, supportedFeatureSet: .latest)
    view.addSubview(paperViewController.view)
    addChild(paperViewController)

    // Create toolbar for macOS
    let toolbarViewController = MarkupToolbarViewController(supportedFeatureSet: .latest)
    toolbarViewController.delegate = paperViewController
    view.addSubview(toolbarViewController.view)
    
    // Set layout
    setupLayoutConstraints()
}

// Auto-save markup changes
    
func paperMarkupViewControllerDidChangeMarkup(_ paperMarkupViewController: PaperMarkupViewController) {
    let markupModel = paperMarkupViewController.markup
    Task {
        // Create a data blob and save it
        let data = try await markupModel.dataRepresentation()
        try data.write(toFile: paperKitDataURL)
    }
}

// Thumbnail for forward compatibility

func updateThumbnail(_ markupModel: PaperMarkup) async throws {
    // Set up CGContext to render thumbnail in
    let thumbnailSize = CGSize(width: 200, height: 200)
    let context = makeCGContext(size: thumbnailSize)
    context.setFillColor(gray: 1, alpha: 1)
    context.fill(renderer.format.bounds)            

    // Render the PaperKit markup
    await markupModel.draw(in: context, frame: CGRect(origin: .zero, size: thumbnailSize))
    
    thumbnail = context.makeImage()
}

// Customized markup FeatureSet
    
var featureSet: FeatureSet = .latest

featureSet.remove(.text)
featureSet.insert(.stickers)

// HDR support
featureSet.colorMaximumLinearExposure = 4
toolPicker.colorMaximumLinearExposure = 4

let paperViewController = PaperMarkupViewController(supportedFeatureSet: featureSet)
let markupEditViewController = MarkupEditViewController(supportedFeatureSet: featureSet)

// Custom background on markup controller

let template = UIImage(named: "MyTemplate.jpg")
let templateView = UIImageView(image: template)
paperViewController.contentView = templateView