UIKitにPencilKitを組み込む最小構成

ViewController.swift

import PencilKit
import UIKit

class ViewController: UIViewController {
    private lazy var canvasView = PKCanvasView(frame: view.frame)
    private lazy var toolPicker = PKToolPicker()

    override func viewDidLoad() {
        super.viewDidLoad()

        view.addSubview(canvasView)

        toolPicker.addObserver(canvasView)
        toolPicker.setVisible(true, forFirstResponder: canvasView)

        canvasView.becomeFirstResponder()
    }
}