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Explain MVVM, its improvements over MVC, and Swift data binding techniques.
Tests architectural separation of concerns and pattern comparison skills. Define ViewModel as state transformer, cite MVC's massive view controller, and list Combine or closures for binding. Red flag: calling MVVM as MVC with no testability.
How do you complete a file download after the app backgrounds?
This tests iOS background execution and URLSession background transfers. Configure a background URLSession, handle AppDelegate events, and respect the 30-second completion handler limit. Never claim NSTimer or beginBackgroundTask sustains background downloads.
How does the Coordinator pattern decouple navigation, and what are its components?
This tests iOS separation of concerns. Explain that Coordinators extract navigation logic from view controllers for reuse, and describe AppCoordinator tree with child coordinators. A red flag is treating it as a router or ignoring parent-child retention.
Diagnose and fix slow iOS app launch time
Pre-main covers dylib loading, rebasing, and static initializers; main covers didFinishLaunching and first frame; profile with Instruments, then defer work, reduce dynamic libraries, and trim…
Outline the key steps and APIs for State Preservation and Restoration
Tests UIKit's automatic state restoration pipeline. Strong answers cover: opting in via AppDelegate, setting restorationIdentifiers, implementing encode/decodeRestorableStateWithCoder, and the system snapshot cycle.
Compare MVVM with unidirectional flow (TCA, Redux)
MVVM is lightweight with bidirectional binding and per-screen view models; unidirectional flow centralizes a single state mutated only by reducers via actions, gaining predictability and testability at…
Implement SwiftUI navigation between list and detail with product ID
Tests modern SwiftUI navigation and clean data flow. Great answer: NavigationStack with NavigationLink(value:) and navigationDestination(for:), plus a bound path for programmatic control.
Explain the Coordinator pattern and the navigation problem it solves in MVC
This tests if you see view controllers bloat from absorbing navigation flow. A good answer names 3 problems (delegate bloat, massive view controllers, embedded routing) and says coordinators own creation and routing.
Pass data back from a modally presented view controller delegate versus closure
This tests decoupled UIKit communication and memory safety. A strong answer outlines a weak delegate protocol and a closure with weak self, contrasting coupling and retain cycles. Red flag: singletons or direct parent references.
Handle a deep link URL in SceneDelegate and navigate to order detail.
Handles willConnectTo and openURLContexts; parses path with URLComponents; validates ID; pushes detail onto nav stack.
How do you pop to root in SwiftUI NavigationStack?
This tests declarative state-driven navigation. A strong answer binds NavigationStack to a NavigationPath, passes the binding down, and clears the path to pop to root. A red flag is suggesting UIKit popToRootViewController or legacy isActive hacks.
What NavigationView limitations did NavigationStack solve for programmatic navigation?
This tests why NavigationView programmatic navigation was brittle. A strong answer contrasts scattered NavigationLink isActive booleans and broken multi-destination stacks against NavigationStack's unified path array. A red flag is calling it a simple rename.
How would you architect the app root for onboarding vs authenticated flows?
Tests window root swapping and state-driven architecture. Use a coordinator to own the window, swap rootViewController between nav and tab controllers on auth changes, and crossfade. Red flag: onboarding modal over tabs or keeping both hierarchies alive.
Design state-driven navigation for a conditional wizard in UIKit and SwiftUI
This tests separating navigation state from UI. Model the flow as a state machine enum, derive the stack from state, and unit test transitions in plain Swift. A red flag is imperative pushes inside views or view controllers.
When is UserDefaults right, what are its limits, and native types?
Tests if you treat UserDefaults as a small preference store, not a database. Good answers name native plist types, cite no encryption and small size limits, and name Core Data or Keychain for heavy or sensitive data. Red flag: storing images or passwords.
How do you persist a custom Swift struct to JSON in Documents?
Tests fluency with Codable and FileManager sandbox APIs. Good answer: conform to Codable, encode with JSONEncoder, resolve Documents directory via FileManager URLs, write Data atomically, and reverse with JSONDecoder.
Why store auth tokens in Keychain, not UserDefaults?
UserDefaults is an unencrypted plist readable from backups and on jailbroken devices; use Keychain Services, which stores encrypted items with access control; save with SecItemAdd and read with…
Describe a pattern for background Core Data fetches and UI updates
This tests NSManagedObjectContext concurrency and queue confinement. Strong answer: create a background context, fetch, pass objectIDs or structs to main, then main context fetches by ID to update UI. Red flag: passing NSManagedObjects across threads.
Describe lightweight vs heavyweight Core Data migration and a heavyweight example
This tests inferred versus manual Core Data migrations. Contrast lightweight additive changes with heavyweight transforms like entity splits, citing promoting a string to a new entity. A red flag is claiming renames always need custom mapping.
Optimize a slow NSFetchedResultsController screen
Set fetchBatchSize so rows load in pages, use relationshipKeyPathsForPrefetching to avoid per-row faulting round trips, and add indexes matching sort and predicate to make queries fast.