CustomUndoManagerView.swift

import SwiftUI

private let associationKey = UnsafeMutablePointer<UInt8>.allocate(capacity: 1)

@MainActor
struct CustomUndoManagerView<Content: View>: UIViewControllerRepresentable {
    private let undoManager: UndoManager
    private let content: Content

    init(undoManager: UndoManager, @ViewBuilder builder: () -> Content) {
        self.undoManager = undoManager
        self.content = builder()
    }

    func makeUIViewController(context: Context) -> UIViewController {
        CustomUndoManagerViewController(undoManager: undoManager)
    }

    func updateUIViewController(_ uiViewController: UIViewController, context: Context) {
        guard let viewController = uiViewController as? CustomUndoManagerViewController else {
            return
        }

        viewController._undoManager = undoManager

        weak var existingHostingController = objc_getAssociatedObject(viewController, associationKey) as? UIViewController
        if let existingHostingController {
            existingHostingController.willMove(toParent: nil)
            existingHostingController.view.removeFromSuperview()
            existingHostingController.removeFromParent()
        }

        let hostingController = UIHostingController(rootView: content)
        objc_setAssociatedObject(viewController, associationKey, hostingController, .OBJC_ASSOCIATION_ASSIGN)
        viewController.addChild(hostingController)
        hostingController.view.translatesAutoresizingMaskIntoConstraints = false
        viewController.view.addSubview(hostingController.view)
        NSLayoutConstraint.activate([
            hostingController.view.topAnchor.constraint(equalTo: viewController.view.topAnchor),
            hostingController.view.leadingAnchor.constraint(equalTo: viewController.view.leadingAnchor),
            hostingController.view.trailingAnchor.constraint(equalTo: viewController.view.trailingAnchor),
            hostingController.view.bottomAnchor.constraint(equalTo: viewController.view.bottomAnchor)
        ])
        hostingController.didMove(toParent: viewController)
    }
}

@MainActor
private final class CustomUndoManagerViewController: UIViewController {
    var _undoManager: UndoManager?

    override var undoManager: UndoManager? {
        _undoManager
    }

    init(undoManager: UndoManager) {
        _undoManager = undoManager
        super.init(nibName: nil, bundle: nil)
    }

    @available(*, unavailable)
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

CustomUndoManagerView(undoManager: UndoManager()) {
    ContentView()
}