viewcontroller.swift

import UIKit
import PencilKit

class ViewController: UIViewController {
    
    @IBOutlet var canvasView: PKCanvasView!

    override func viewDidLoad() {
        super.viewDidLoad()
        
        //Set the canvas view to opaque and clear
        canvasView.backgroundColor = UIColor(white: 0, alpha: 0.05)
        canvasView.isOpaque = false
    }

    override func willTransition(to newCollection: UITraitCollection, with coordinator: UIViewControllerTransitionCoordinator) {
        let targetTransform = coordinator.targetTransform
        self.canvasView.drawing.transform(using: targetTransform)
    }
    
    var oldCanvasSize: CGSize!
    var newCanvasSize: CGSize!
    
    override func viewWillLayoutSubviews() {
        oldCanvasSize = canvasView.bounds.size
    }
    
    override func viewDidLayoutSubviews() {
        newCanvasSize = canvasView.bounds.size
        
        let targetTransform = CGAffineTransform(scaleX: newCanvasSize.width / oldCanvasSize.width, y: newCanvasSize.height / oldCanvasSize.height)
        self.canvasView.drawing.transform(using: targetTransform)
    }
}