Migrate UIKit navigation to SwiftUI NavigationStack

[g-enius]

Summary

Replaces the UIKit navigation layer (UINavigationController + Coordinator hierarchy) with pure SwiftUI NavigationStack, while keeping Combine for reactive state.

• 17 files deleted (coordinators, view controllers, UIKit extensions)
• 6 files added (AppCoordinator, AppRootView, MainTabView + SwiftUI equivalents)
• Net reduction: ~880 lines
• Minimum deployment target raised from iOS 15 → iOS 16

Next step: PR #6 — Replace Combine with @Observable and AsyncStream builds on this branch.

Key Changes

Navigation

• UINavigationController → NavigationStack + NavigationPath
• UITabBarController → SwiftUI TabView
• Multiple coordinator classes → single AppCoordinator: ObservableObject, ServiceLocatorProvider
• pushViewController(_:animated:) → path.append(item)
• present(_:animated:) → .sheet(isPresented:)

App Lifecycle

• AppDelegate + SceneDelegate → SwiftUI @main App
• Deep links: scene(_:openURLContexts:) → .onOpenURL { }

Dependency Injection

• ServiceLocator.shared singleton → instance-based ServiceLocator threaded through coordinators, sessions, and ViewModels
• ServiceLocatorProvider protocol + @Service property wrapper with static subscript(_enclosingInstance:) for declarative service resolution
• Session teardown no longer calls serviceLocator.reset() — live views may still resolve services via @Service during SwiftUI teardown; the next session's activate() overwrites with fresh instances

ViewModel → Coordinator Communication

• Navigation closures (var onShowDetail: ((FeaturedItem) -> Void)?) — same as main branch

What Stays the Same

• Combine (@Published + .sink) for reactive state
• MVVM architecture, session-scoped DI, all service protocols

Why NavigationStack + NavigationPath over NavigationLink

This branch uses programmatic navigation exclusively — NavigationStack with NavigationPath managed by the coordinator. NavigationLink is deliberately avoided:

• Navigation belongs in the coordinator, not the view. NavigationLink couples navigation decisions to SwiftUI views, making it hard to trigger navigation from ViewModels, deep links, or programmatic flows.
• NavigationLink(isActive:) and NavigationLink(tag:selection:) are deprecated since iOS 16. Apple replaced them with navigationDestination(for:) + NavigationPath, which is exactly what this branch uses.
• NavigationPath is type-erased and composable. The coordinator can path.append(any Hashable) without Views knowing the destination type. NavigationLink requires the destination View at the call site.
• Testing is simpler. Navigation is testable via closures on ViewModels (onShowDetail, onShowProfile) — no need to tap UI elements.

// How navigation works in this branch:
// 1. View calls ViewModel closure
viewModel.didTapFeaturedItem(item)

// 2. ViewModel fires navigation closure (set by coordinator)
onShowDetail?(item)

// 3. Coordinator appends to NavigationPath
coordinator.homePath.append(item)

// 4. NavigationStack picks up the change via .navigationDestination(for:)

If you support iOS 17+: how this code evolves

If your deployment target is iOS 17+, you can remove Combine entirely. Here's how each pattern changes:

ViewModels — ObservableObject + @Published → @Observable:

// iOS 16 (this branch)                    // iOS 17+ (observation)
class HomeViewModel: ObservableObject {     @Observable class HomeViewModel {
    @Published var items = []                   var items = []
    @Published var isLoading = false             var isLoading = false
}                                           }

Views — @ObservedObject / @StateObject → @Bindable / @State:

// iOS 16 (this branch)                    // iOS 17+
@ObservedObject var viewModel: HomeVM       @Bindable var viewModel: HomeVM
@StateObject var viewModel = HomeVM()       @State var viewModel = HomeVM()

Service events — AnyPublisher → AsyncStream:

// iOS 16 (this branch)                    // iOS 17+
favoritesService.favoritesDidChange         let stream = favoritesService.favoritesChanges
    .sink { self.favoriteIds = $0 }         Task { for await ids in stream {
    .store(in: &cancellables)                   self.favoriteIds = ids
                                            }}

See the observation branch for the complete migration (PR #6).

Test plan

• All 73 unit tests pass
• Build succeeds on iOS 16+ simulator
• Home → Detail → back navigation
• Profile modal present/dismiss
• Items → Detail → back navigation
• Deep links work via .onOpenURL
• Login → main flow transition
• Logout works without crash (services remain registered during teardown)

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