SwiftUI StackView for Decompose integration

StackView.swift

import Shared
import SwiftUI

/*
 A custom implementation of the stack view wrapper with:
 - no indices for `onBack` action
 - no `getTitle`
 - no `NavigationStack`
 - custom UINavigationController
 
 https://github.com/arkivanov/Decompose/blob/11a66f0ec46e42259af5510fc5bb28250e027924/sample/app-ios/app-ios/DecomposeHelpers/StackView.swift
*/

struct StackView<T: AnyObject, Content: View>: View {
  @ObservedObject var stackValue: ObservableValue<ChildStack<AnyObject, T>>
  var onBack: () -> Void
  @ViewBuilder var childContent: (T) -> Content
  
  var stack: [Child<AnyObject, T>] { stackValue.value.items }
  
  var body: some View {
    StackInteropView(
      components: stack.map { $0.instance! },
      onBack: onBack,
      childContent: childContent
    )
    .ignoresSafeArea(.container)
  }
}

private struct StackInteropView<T: AnyObject, Content: View>: UIViewControllerRepresentable {
  var components: [T]
  var onBack: () -> Void
  var childContent: (T) -> Content
  
  func makeCoordinator() -> Coordinator {
    Coordinator(self)
  }
  
  func makeUIViewController(context: Context) -> UINavigationController {
    context.coordinator.syncChanges(self)
    let navigationController = CustomNavigationController(
      rootViewController: context.coordinator.viewControllers.first!
    )
    
    return navigationController
  }
  
  func updateUIViewController(
    _ navigationController: UINavigationController,
    context: Context
  ) {
    context.coordinator.syncChanges(self)
    navigationController.setViewControllers(
      context.coordinator.viewControllers,
      animated: true
    )
  }
  
  private func createViewController(
    _ component: T,
    _ coordinator: Coordinator
  ) -> NavigationItemHostingController {
    let controller = NavigationItemHostingController(
      rootView: childContent(component)
    )
    controller.coordinator = coordinator
    controller.component = component
    controller.onBack = onBack
    
    return controller
  }
  
  private final class CustomNavigationController: UINavigationController, UIGestureRecognizerDelegate {
    override func viewDidLoad() {
      super.viewDidLoad()
      navigationBar.isHidden = true
      interactivePopGestureRecognizer?.delegate = self
    }
    
    // fixes swipes back, when parent stack view intercepts child's gestures
    func gestureRecognizerShouldBegin(_ gestureRecognizer: UIGestureRecognizer) -> Bool {
      viewControllers.count > 1
    }
  }
  
  fileprivate final class Coordinator: NSObject {
    var parent: StackInteropView<T, Content>
    var viewControllers = [NavigationItemHostingController]()
    var preservedComponents = [T]()
    
    init(_ parent: StackInteropView<T, Content>) {
      self.parent = parent
    }
    
    func syncChanges(_ parent: StackInteropView<T, Content>) {
      self.parent = parent
      let count = max(preservedComponents.count, parent.components.count)
      
      for i in 0..<count {
        if (i >= parent.components.count) {
          viewControllers.removeLast()
        } else if (i >= preservedComponents.count) {
          viewControllers.append(parent.createViewController(parent.components[i], self))
        } else if (parent.components[i] !== preservedComponents[i]) {
          viewControllers[i] = parent.createViewController(parent.components[i], self)
        }
      }
      
      preservedComponents = parent.components
    }
  }
  
  fileprivate final class NavigationItemHostingController: UIHostingController<Content> {
    fileprivate(set) weak var coordinator: Coordinator?
    fileprivate(set) var component: T?
    fileprivate(set) var onBack: (() -> Void)?
    
    override func viewDidDisappear(_ animated: Bool) {
      super.viewDidDisappear(animated)
      
      if isMovingFromParent && coordinator?.preservedComponents.last === component {
        onBack?()
      }
    }
  }
}