PencilKit을 먼저 써보도록 하겠습니다.

StoryBoard에서 PKCanvasView 구성

ViewController.swift

import UIKit
import PencilKit

class ViewController: UIViewController, PKToolPickerObserver {

    @IBOutlet weak var pkCanvasView: PKCanvasView!

    var toolPicker: PKToolPicker!

    override func viewDidLoad() {
        super.viewDidLoad()
        pkCanvasView.allowsFingerDrawing = false
    }

    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
    }

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        if let window = self.pkCanvasView.window {
            let toolPicker = PKToolPicker.shared(for: window)
            toolPicker?.addObserver(pkCanvasView)
            toolPicker?.setVisible(true, forFirstResponder: pkCanvasView)
            pkCanvasView.becomeFirstResponder()
            print("PKToolPicker Set")
        }
    }
}

이렇게 하면 여기까지 할 수 있어요.