How can we create a UIKit-based navigationDestination operator?

[bobspryn]

On the project we're building we're stuck with UIKit based navigation for the moment, but since the navigationDestination API doesn't play nice with a UINavigationController, we're still using deprecated NavigationLinks. So I'm wanting to build an operator that mimics the navigationDestination behavior, but uses UIKit under the hood. @jshier had the same need, and gave me a good head start with most of the code below. The problem is that onChange seems to only be fired when the body re-renders.

I could get this API working by making the API require a Store instead of a Binding, and use the state keypath to pull off a publisher that I observe. I'm going to play with that, but ideally it would be the same exact API as navigationDestination.

import Foundation
import SwiftUI
import ComposableArchitecture

private struct UINavigationControllerKey: EnvironmentKey {
	static let defaultValue: UINavigationController? = nil
}

extension EnvironmentValues {
	var navigationController: UINavigationController? {
		get { self[UINavigationControllerKey.self] }
		set { self[UINavigationControllerKey.self] = newValue }
	}
}

extension View {
	func uiNavigationDestination<DestinationState, DestinationAction>(
		store: Binding<ComposableArchitecture.Store<DestinationState, DestinationAction>?>,
		@ViewBuilder destination: @escaping (ComposableArchitecture.Store<DestinationState, DestinationAction>) -> some View
	) -> some View where DestinationState: ObservableState {
		modifier(
			UIKitNavigationDestinationModifier(store: store, destination: destination)
		)
	}
}

private var pushedControllers: [UUID: UIViewController] = [:]

private struct UIKitNavigationDestinationModifier<Destination, DestinationState, DestinationAction>: ViewModifier where Destination: View, DestinationState: ObservableState {
	@Environment(\.navigationController) private var navigationController

	@Binding var store: Store<DestinationState, DestinationAction>?
	let destination: (Store<DestinationState, DestinationAction>) -> Destination

	private let id = UUID()

	private var pushedViewController: UIViewController? {
		pushedControllers[id]
	}

	func body(content: Content) -> some View {
		Group {
			content
			Text(store == nil ? "N" : "Y")
		}
			.onChange(of: store) { newStore in
				guard let navigationController else {
					fatalError("""
					You must use .environment() to attach a UINavigationController to the root SwiftUI view pushed \
					into a UIKit navigation flow.
					""")
				}

				guard let newStore else {
					pushedViewController?.dismiss(animated: true)
					pushedControllers[id] = nil
					return
				}

				let controller = OnDisappearHostingViewController(content: self.destination(newStore)) {
					pushedControllers[id] = nil
					store = nil
				}

				pushedControllers[id] = controller
				navigationController.pushViewController(controller, animated: !navigationController.viewControllers.isEmpty)
			}
	}
}

private final class OnDisappearHostingViewController<Content: View>: SwiftUIViewController<Content> {
	private let onDisappear: () -> Void

	init(content: Content, onDisappear: @escaping () -> Void) {
		self.onDisappear = onDisappear

		super.init(content: content)
	}

	override func willMove(toParent parent: UIViewController?) {
		super.willMove(toParent: parent)

		guard parent == nil else { return }

		onDisappear()
	}

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

[bobspryn]

Tried to get cute and pull a publisher off the binding to see if that would fire, but it did not:

private extension Binding {
	func withPublisher() -> (Binding<Value>, AnyPublisher<Value, Never>) {
		let subject = CurrentValueSubject<Value, Never>(self.wrappedValue)
		let binding = Binding<Value>(
			get: {
				subject.send(wrappedValue)
				return wrappedValue
			},
			set: { value in
				self.wrappedValue = value
			}
		)
		return (binding, subject.eraseToAnyPublisher())
	}
}

private struct UINavigationControllerKey: EnvironmentKey {
	static let defaultValue: UINavigationController? = nil
}

extension EnvironmentValues {
	var navigationController: UINavigationController? {
		get { self[UINavigationControllerKey.self] }
		set { self[UINavigationControllerKey.self] = newValue }
	}
}

extension View {
	func uiNavigationDestination<Item>(
		item: Binding<Item?>,
		@ViewBuilder destination: @escaping (Item) -> some View
	) -> some View {
		modifier(
			UIKitNavigationDestinationModifier(item: item, destination: destination)
		)
	}
}

private var pushedControllers: [UUID: UIViewController] = [:]

private struct UIKitNavigationDestinationModifier<Destination, Item>: ViewModifier where Destination: View {
	internal init(item: Binding<Item?>, destination: @escaping (Item) -> Destination) {
		let (myBinding, publisher) = item.withPublisher()
		self._item = myBinding
		self.itemPublisher = publisher
		self.destination = destination
	}
	
	@Environment(\.navigationController) private var navigationController

	@Binding var item: Item?
	var itemPublisher: AnyPublisher<Item?, Never>
	let destination: (Item) -> Destination

	private let id = UUID()

	private var pushedViewController: UIViewController? {
		pushedControllers[id]
	}

	func body(content: Content) -> some View {
		content
			.onReceive( itemPublisher) { newItem in
				guard let navigationController else {
					fatalError("""
					You must use .environment() to attach a UINavigationController to the root SwiftUI view pushed \
					into a UIKit navigation flow.
					""")
				}
				
				if let newItem, pushedControllers[id] == nil {
					let controller = OnDisappearHostingViewController(content: self.destination(newItem)) {
						pushedControllers[id] = nil
						item = nil
					}

					pushedControllers[id] = controller
					navigationController.pushViewController(controller, animated: !navigationController.viewControllers.isEmpty)
				} else if newItem == nil, pushedControllers[id] != nil {
					pushedViewController?.dismiss(animated: true)
					pushedControllers[id] = nil
				}
			}
	}
}

private final class OnDisappearHostingViewController<Content: View>: SwiftUIViewController<Content> {
	private let onDisappear: () -> Void

	init(content: Content, onDisappear: @escaping () -> Void) {
		self.onDisappear = onDisappear

		super.init(content: content)
	}

	override func willMove(toParent parent: UIViewController?) {
		super.willMove(toParent: parent)

		guard parent == nil else { return }

		onDisappear()
	}

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