A SwiftUI modifier for inserting the parent UIViewController in the environment

View+ParentViewController.swift

import Foundation
import SwiftUI

extension View {
    /// Adds introspection to find the parent view controller in the view hierarchy and
    /// makes that view controller available to downstream views in the view hierarchy.
    public func addParentViewControllerIntrospection() -> some View {
        modifier(ParentViewControllerEnvironmentModifier())
    }
}

extension EnvironmentValues {
    /// The parent view controller hosting this SwiftUI hierarhy, if there is one available.
    public var parentViewController: UIViewController? {
        parentViewControllerBox.value
    }
}

// MARK: - Implementation

extension EnvironmentValues {
    @Entry fileprivate var parentViewControllerBox = Weak<UIViewController>(nil)
}

private struct Weak<T: AnyObject> {
    weak var value: T?
    init(_ value: T?) {
        self.value = value
    }
}

private final class IntrospectionView: UIView {
    var parentViewControllerBinding: Binding<Weak<UIViewController>>? {
        didSet {
            updateBinding()
        }
    }

    override func didMoveToWindow() {
        super.didMoveToWindow()
        updateBinding()
    }

    func updateBinding() {
        DispatchQueue.main.async {
            self.parentViewControllerBinding?.wrappedValue.value = self.parentViewController
        }
    }

    var parentViewController: UIViewController? {
        for responder in sequence(first: self, next: \.next) {
            if let viewController = responder as? UIViewController {
                return viewController
            }
        }
        return nil
    }
}

private struct IntrospectionViewRepresentable: UIViewRepresentable {
    @Binding var parentViewController: Weak<UIViewController>

    func makeUIView(context: Context) -> IntrospectionView {
        IntrospectionView()
    }

    func updateUIView(_ uiView: IntrospectionView, context: Context) {
        uiView.parentViewControllerBinding = $parentViewController
    }
}

private struct ParentViewControllerEnvironmentModifier: ViewModifier {
    @State var viewController = Weak<UIViewController>(nil)

    func body(content: Content) -> some View {
        content
            .environment(\.parentViewControllerBox, viewController)
            .background(alignment: .bottomTrailing) {
                IntrospectionViewRepresentable(parentViewController: $viewController)
                    .frame(width: 0, height: 0)
                    .allowsHitTesting(false)
            }
    }
}