iOS Developer Interview Questions & Answers (with Model Answers)

Model Answer

I lean toward SwiftUI for new development because of its declarative model, faster iteration, and reduced boilerplate, especially when I can target recent iOS versions. I fall back to UIKit when I need fine-grained control, complex custom layouts, or features SwiftUI does not yet cover well, and the two interoperate so I can mix them. For existing UIKit codebases I introduce SwiftUI incrementally rather than rewriting. The decision balances the deployment target, complexity, and team familiarity.

Model Answer

Swift uses automatic reference counting, so I watch for strong reference cycles, especially between objects and their closures or delegates. I break cycles with weak or unowned references, using weak when the reference can become nil and unowned when it should always outlive the closure. I profile with Instruments' leak and allocation tools to confirm objects deallocate as expected. Delegates are typically weak to avoid the classic view-controller retain cycle.

Model Answer

I keep view controllers or views thin and move logic into testable units using a pattern like MVVM, so the UI binds to a view model free of UIKit dependencies. I inject dependencies rather than instantiating them directly, which makes mocking in tests straightforward. I separate networking and persistence behind protocols so I can swap implementations and test in isolation. This keeps the codebase modular and resilient as it grows.

Model Answer

I use Swift's structured concurrency with async/await for asynchronous work, which makes code far more readable than nested completion handlers. I use actors to protect mutable shared state from data races, and I keep UI updates on the main actor. For older code I may still encounter Grand Central Dispatch, so I understand queues and avoid blocking the main thread. The priority is correctness without data races and a responsive UI.

Model Answer

I implemented a complex synchronisation feature that kept local Core Data in sync with a backend, including conflict resolution. I designed a clear sync strategy, handled background fetch and app lifecycle transitions, and made the merge logic deterministic. I tested edge cases like interrupted syncs and offline edits extensively. The feature shipped reliably with low crash rates because I treated the edge cases as first-class.

Structs are value types, copied on assignment, while classes are reference types sharing a single instance. Structs do not support inheritance but are safer for concurrency and avoid unintended shared mutation. Swift favours structs for models and value semantics, reserving classes for when you need identity, inheritance, or reference semantics.

@State declares a source of truth owned locally by a view, and SwiftUI re-renders the view when it changes. @Binding creates a two-way reference to state owned elsewhere, letting a child view read and mutate a parent's state without owning it. Using them correctly keeps a single source of truth and avoids inconsistent UI.

Optionals make the possible absence of a value explicit in the type system, so the compiler forces you to handle the nil case. Optional chaining lets you safely call into a chain that may be nil, returning nil instead of crashing. This eliminates a whole class of null-pointer errors common in other languages, provided you avoid force-unwrapping carelessly.

The main thread, backed by the main run loop, handles all UI rendering and user interaction, so all UIKit and SwiftUI updates must occur on it. Blocking it with heavy work causes dropped frames and unresponsiveness, and the watchdog may terminate the app. You offload heavy work to background queues or async tasks and dispatch UI updates back to the main actor.

Options range from UserDefaults for small key-value settings, to the keychain for secrets, to Core Data or SwiftData for structured relational data, to plain files for documents. I choose based on the data's size, structure, query needs, and sensitivity. For complex object graphs with relationships and queries I reach for Core Data or SwiftData, while simple flags belong in UserDefaults.

We saw crashes in our analytics that never occurred in development. I symbolicated the crash logs and found a force-unwrap failing on a specific server response. I replaced the force-unwrap with safe handling and added a fallback, then verified with the same payload. The crash disappeared and I audited the codebase for similar unsafe unwraps.

Our app needed a modern settings screen and the team wanted to adopt SwiftUI gradually. I built the new screen in SwiftUI and embedded it in the existing UIKit navigation using a hosting controller. I kept the data layer shared so behaviour stayed consistent. The migration shipped smoothly and gave the team a low-risk template for adopting SwiftUI elsewhere.

A scrolling list was janky on older devices. I profiled with Instruments and found expensive cell layout and large images decoded on the main thread. I moved image decoding off the main thread, cached sized images, and simplified the cell layout. Scrolling became smooth at sixty frames per second on the target devices.

We needed a fix live before a marketing event but review timing was uncertain. I prepared a focused, low-risk build, requested an expedited review with a clear justification, and had a staged rollout ready. The expedited review was granted and the fix went out in time. I documented the expedited process for future urgent releases.