Navigation & presentation — push vs present

How it works

• Push needs a container: navigationController is nil in a bare modal — present a UINavigationController wrapping it.
• Who presents? UIKit walks up to the context-defining VC: from a child, the modal’s presentingViewController is the nav/tab controller, not the child.
• Dismiss rule (Apple): the presenting VC is responsible. Calling dismiss on the presented VC forwards to its presenter; calling it lower in the chain dismisses its presented child and everything above.
• One presented VC per presenter, presenter in the window — else a warning and nothing shows. visibleViewController includes a modal.
• Sheets (iOS 15+): sheetPresentationController · .detents = [.medium(), .large()] (.custom iOS 16) · prefersGrabberVisible;
  largestUndimmedDetentIdentifier keeps the presenter usable.
• Swipe-down: isModalInPresentation = true blocks it → presentationControllerDidAttemptToDismiss. presentationControllerDidDismiss fires only for user (swipe) dismissal, never for your dismiss().

modalPresentationStyle

Example — push, present, delegate back

protocol EditDelegate: AnyObject { func didSave(_ i: Item) }
class EditVC: UIViewController { weak var delegate: EditDelegate? }
class ListVC: UIViewController, EditDelegate {
  func open(_ i: Item) {                   // push: same stack
    navigationController?
      .pushViewController(DetailVC(i), animated: true) }
  func edit(_ i: Item) {                   // present: new chain
    let e = EditVC(i); e.delegate = self
    let nav = UINavigationController(rootViewController: e)
    nav.sheetPresentationController?.detents = [.medium()]
    nav.isModalInPresentation = true       // no swipe-close
    present(nav, animated: true) }
  func didSave(_ i: Item) {                // presenter closes it
    dismiss(animated: true); reload() } }  // no viewWillAppear!

SwiftUI

• NavigationStack(path: $path) (iOS 16, replaces NavigationView): NavigationLink(value:) pushes a value; .navigationDestination(for: T.self) maps it — once per type, on a persistent ancestor (not inside a lazy List row).
• NavigationPath = type-erased Hashable list (mixed types); [Route] if one type. Push append, pop-to-root removeLast(path.count), deep link = assign the whole path. Codable routes → path.codable → @SceneStorage.
• Modals: .sheet(item:) (prefer over isPresented + separate state — stale item race), .fullScreenCover, .presentationDetents. @Environment(\.dismiss) closes the nearest context: inside a pushed view it pops, it does not close the sheet. One NavigationStack per tab; never nest stacks.

Deep links

URL scheme → scene(_:openURLContexts:); universal link → scene(_:continue:); on a cold launch both arrive in willConnectTo’s connectionOptions instead. Parse to a route enum, then setViewControllers / assign path (SwiftUI: .onOpenURL).

Interview traps

• iOS 13 .pageSheet default: the list under a sheet gets no viewWillAppear on dismiss — refresh via delegate/closure.
• push from a VC presented without a nav → nil optional chain, silent no-op.
• Delegate: weak var + protocol: AnyObject (weak needs a class type, and is Optional because it auto-nils). Closures: [weak self].
• Coordinator must drop its child coordinator on the back-swipe pop too — detect it in UINavigationControllerDelegate.didShow.

Likely questions

1. dismiss inside the modal? — forwarded to its presenter.
2. A→B→C, A.dismiss? — B and C go; only the top animates.
3. Block swipe-close? — isModalInPresentation = true.
4. Deep link 3 levels? — setViewControllers / set path.