Swift engineer behavioural interview questions

As asked

Tell me about a time you shipped code that caused a crash or serious regression in a production iOS app. Walk me through how you discovered it, how you diagnosed the root cause, how you fixed it, and what you changed afterward to prevent similar issues.

Sample answer outline

A strong answer gives a concrete scenario (not hypothetical), names specific signals (Crashlytics report, crash rate spike, specific symbolicated stack trace), explains root cause clearly (force unwrap on nil, unbalanced transaction finish, race condition), describes a hotfix or kill switch, and articulates a durable change: adding tests for that path, adding a crash-free-session monitoring threshold, or establishing a review checklist.

Expect these follow-ups

• How do you prioritize a crash that affects 0.1% of users but is 100% reproducible for them?
• What monitoring do you set up before a release to catch regressions early?

As asked

Describe a specific performance problem in an iOS app you worked on, how you measured it, what you changed, and the quantified result. What was harder than you expected?

Sample answer outline

A strong answer names the symptom (slow scroll, 3-second launch, high memory), the measurement tool (Instruments, MetricKit, or frame rate counter), the root cause found (image decoding on main thread, N+1 Core Data fetch, missing shadow path), the fix, and a before/after metric. The harder-than-expected part should show genuine problem-solving depth rather than a rehearsed story.

Expect these follow-ups

• How did you verify the improvement held up across different device tiers?
• Did the fix introduce any new tradeoffs you had to manage?

As asked

Tell me about a time when an iOS release deadline was so aggressive that it put pressure on code quality or testing coverage. What specifically did you consider cutting, what did you actually cut, and what was the result?

Sample answer outline

A strong answer gives a specific project, names the specific corners that were under pressure (skipping unit tests, no accessibility review, a hack in the data layer), explains the candidate's response (negotiated scope, shipped a clearly-scoped v1 with logged tech debt, or pushed back with data), and reflects honestly on the tradeoff and its consequences.

Expect these follow-ups

• How do you track technical debt you knowingly accumulate under deadline pressure?
• Have you ever advocated for slipping a deadline and succeeded? What made the case convincing?

As asked

Have you ever migrated a UIKit-based screen or module to SwiftUI in a production app? Walk me through the decisions you made, the interop strategy you used, and what you would do differently.

Sample answer outline

A strong answer discusses using UIHostingController or UIViewRepresentable to wrap SwiftUI into UIKit or vice versa for incremental migration, picking screens with self-contained state rather than deeply integrated ones, handling data flow differences between delegation and @Binding, and managing navigation state across the boundary. What they would do differently might be starting with navigation container migration or waiting for iOS 16 NavigationStack.

Expect these follow-ups

• How do you test SwiftUI views that contain UIKit components wrapped with UIViewRepresentable?
• What view controller lifecycle events have no equivalent in SwiftUI and how did you handle them?

As asked

Tell me about a time you disagreed with a colleague about an architectural decision for an iOS feature, how you handled the disagreement, and what happened.

Sample answer outline

A strong answer names the specific disagreement (MVC vs MVVM, singleton vs dependency injection, SwiftUI vs UIKit for a new screen), explains the candidate's reasoning with technical evidence rather than opinion, describes how they listened to the other side, and how the decision was reached (prototype, measurement, consensus, or escalation). The outcome should show maturity: either their approach was adopted, the team's was, or a hybrid was found.

Expect these follow-ups

• What criteria do you use when you cannot reach consensus on an architecture question?
• How do you avoid architecture debates slowing down feature delivery?

As asked

Describe a situation where a junior iOS developer submitted code that had significant issues during review. How did you handle the review, how did you communicate the feedback, and what was the outcome?

Sample answer outline

A strong answer gives context (what the code did, what the issues were, the junior's experience level), describes constructive feedback (naming specific issues, explaining the why, linking documentation or examples rather than rewriting everything), and shows the follow-up (pairing session, follow-up review, improvement over time). Candidate should show balance between shipping velocity and code quality.

Expect these follow-ups

• How do you calibrate the depth of a code review for a junior versus a senior engineer?
• What do you do when a junior's approach is not wrong but just different from what you would do?

As asked

Describe a large iOS feature you led technically from design to production. What was the feature, how did you break down the work, how did you handle unknowns, and what would you do differently?

Sample answer outline

A strong answer names a concrete feature (not vague), describes decomposition (spike for unknowns, incremental milestones, API contract early), explains how they handled unexpected technical blockers (third-party SDK bug, App Store guideline change, performance surprise on older devices), and gives an honest retrospective. Leadership signals: early communication of risks, involving QA and design in technical decisions.

Expect these follow-ups

• How do you track technical risk on a feature that depends on a new iOS API that might change in beta?
• How do you balance building for the future versus shipping what works now?

As asked

Tell me about a UI test in your iOS project that was flaky: sometimes passing, sometimes failing with no code changes. What did you do to diagnose and fix it?

Sample answer outline

A strong answer describes the symptoms (random CI failures, non-reproducible locally), the debugging process (enabling failure artifacts, recording video of the test run, adding explicit waits for async UI settling, checking for shared mutable state between tests), the root cause (race condition in animation completion, shared singleton state not reset between tests, simulator performance variance), and the fix. Candidate should mention XCUIElement.waitForExistence(timeout:) and proper test isolation.

Expect these follow-ups

• What is your strategy for deciding when a flaky test should be deleted versus fixed?
• How would you set up parallel test execution to catch timing-dependent failures faster?

As asked

You receive a Crashlytics report showing crashes exclusively when the app is in the background, in a background URLSession callback. The stack trace points to your Core Data stack. What do you do?

Sample answer outline

A strong answer starts with reproducing in the debugger by simulating background launch via the scheme's background fetch checkbox, then examines whether the Core Data context is being accessed from the wrong thread in the background callback (background URLSession callback runs on a delegate queue, not the main thread), checks for missing performBlock calls, and considers that the persistent store may not be loaded yet at crash time. Fix: always access Core Data from its context's queue, lazily initialize the stack, and add assertion handlers in debug builds.

Expect these follow-ups

• How do you reproduce an app crash that only happens when the OS wakes the app in the background?
• What xctrace command would you use to capture a crash in a background process?

As asked

Users on iPhone 8 report the app getting killed while browsing a gallery of high-resolution photos. You see didReceiveMemoryWarning firing frequently. Walk me through your response: immediate mitigation, diagnosis, and long-term fix.

Sample answer outline

A strong answer covers: respond to didReceiveMemoryWarning by purging in-memory image caches (NSCache auto-evicts but check custom caches), use Instruments VM Tracker to find what is holding memory (images, decoded bitmaps, cached objects), check if images are decoded at full resolution rather than downsampled to display size, and consider loading images at smaller sizes using ImageIO/CGImageSource for thumbnails. Long term: use WWDC-recommended downsample function, implement paging with lazy loading, and add memory footprint to CI monitoring.

Expect these follow-ups

• How do you downsample a UIImage to a target display size without loading the full bitmap into memory?
• What is the difference between virtual memory, resident memory, and dirty memory in iOS?

As asked

You deploy an iOS update and Crashlytics shows a spike in DecodingError crashes. The backend team tells you they changed a JSON field from a string to an integer two days ago without coordinating. How do you handle this immediately and how do you prevent it?

Sample answer outline

A strong answer describes the immediate fix: add a flexible Codable decoder that handles both types (e.g. a custom AnyCodable or decoding with do/catch trying both), roll out a hotfix build, and ask the backend team to version the API or add a compatibility shim. Prevention: versioned API endpoints (/v2/), contract testing (Pact or similar), staging environment CI that validates API responses against the iOS model types before deployment, and never using required fields for data the server might change types on.

Expect these follow-ups

• How do you write a Codable type that accepts both String and Int for the same field?
• What is contract testing and how would you implement it for an iOS app without a large testing infrastructure?

As asked

You have a critical business release scheduled for tomorrow and Apple App Review rejects the build for a guideline violation you were not expecting. Walk me through your decision process and actions.

Sample answer outline

A strong answer describes: reading the rejection reason carefully, checking the Human Interface Guidelines and App Store Review Guidelines for the specific violation, deciding whether to fix, appeal, or both in parallel, using the Reply to App Review button to clarify intent if the rejection seems mistaken, preparing a hotfix if it is a legitimate violation, and communicating the delay to stakeholders honestly. Candidate should know that Expedited Review exists for critical situations.

Expect these follow-ups

• What types of rejections are almost always reversible via appeal versus always requiring a fix?
• How would you reduce the probability of unexpected rejections on future releases?

As asked

Product wants to use NavigationStack (iOS 16 only) for a major navigation refactor, but your app still supports iOS 14. How do you approach this?

Sample answer outline

A strong answer presents the options: raise the minimum deployment target (most pragmatic if usage data supports it), use @available checks with a UIKit fallback for iOS 14 (NavigationView plus UINavigationController wrapped), or hide the refactor behind an abstraction (a Router protocol with two implementations). Candidate should check analytics for the percentage of users on iOS 14 to make the business case and know that Apple drops two-year-old OS versions rapidly.

Expect these follow-ups

• What percentage of iOS users run the current major version within six months of release?
• How do you test an @available conditional branch for an older OS version without owning an old device?

As asked

A crash report shows a third-party analytics SDK crashing on startup on iOS 17.2. The SDK vendor has not released a fix. Your app ships to production in three days. What do you do?

Sample answer outline

A strong answer describes: verifying the crash is actually from the SDK (symbolicate fully, check the stack), checking if an earlier SDK version avoids the issue (try pinning to last known stable), using a feature flag to disable SDK initialization on affected OS versions as a kill switch, filing a bug with the vendor with a minimal reproducible sample, and exploring a temporary replacement (a competing SDK or disabling the feature). Communicating to stakeholders before the deadline.

Expect these follow-ups

• How do you minimize risk from third-party SDKs in general when choosing which to adopt?
• What is a thin wrapper pattern for third-party SDKs and how does it help in this scenario?

As asked

It is June, Apple has just announced iOS 18 at WWDC, and the first beta breaks a key feature in your app due to a UIKit behavioral change. The stable release is in September. How do you plan your response?

Sample answer outline

A strong answer describes: file a feedback report with Apple with a minimal sample (Feedback Assistant), check the release notes and migration guide for the API change, write a fix that is backward compatible with iOS 17 using @available, test against each beta as it drops (automate if possible), coordinate with QA on a beta testing track, and plan a release that ships before iOS 18 GA so existing users are not broken on day one of the new OS.

Expect these follow-ups

• How do you prioritize iOS beta compatibility work against the regular feature roadmap?
• What is your process for monitoring each Xcode and iOS beta that ships over the summer?

As asked

Tell me about a time you discovered that a feature you shipped, or were about to ship, had a meaningful accessibility problem. How did you identify it, what did you fix, and how do you build accessibility into your process now?

Sample answer outline

A strong answer gives a specific scenario (VoiceOver not reading a custom control, insufficient contrast ratio, focus order wrong after a dynamic content update), the discovery method (turning on VoiceOver during QA, an accessibility audit tool, a user complaint), the fix in code (accessibilityLabel, accessibilityTraits, accessibilityElements), and the process change (adding accessibility to the definition of done, running the Accessibility Inspector regularly).

Expect these follow-ups

• How do you test VoiceOver behavior in XCUITest?
• What is the difference between accessibilityLabel and accessibilityHint, and when should you use each?

As asked

Tell me about a product feature that required collecting or using sensitive user data on iOS (location, contacts, health, camera). How did you handle the privacy implications, the permission flow, and any pushback you encountered?

Sample answer outline

A strong answer names the sensitive data type, describes how the candidate proactively engaged design and product on the permission prompt copy and the value exchange (why should the user say yes), how they handled the deny case gracefully in code, and any App Review guidance they followed. A strong signal: minimal data collection, on-device processing where possible, and a clear data retention plan discussed with the team.

Expect these follow-ups

• What happens in your app when a user grants location access then revokes it in Settings mid-session?
• How does App Tracking Transparency affect your analytics if the user denies it?

As asked

Walk me through the most technically challenging custom animation or interactive transition you built in an iOS app. What approach did you take, what went wrong, and how did you make it performant?

Sample answer outline

A strong answer names a specific animation (custom view controller transition, gesture-driven card stack, parallax scroll, fluid resizing), describes the approach (UIViewPropertyAnimator for interactive, CAKeyframeAnimation for physics-like curves, UIPercentDrivenInteractiveTransition), names the performance issues encountered (offscreen rendering, main thread work), and the fix. Candidate should differentiate between layer animations and view animations and know which avoids the main thread.

Expect these follow-ups

• How do UIViewPropertyAnimator's pause and resume capabilities differ from a standard UIView.animate?
• What Core Animation timing functions exist beyond the built-in ease curves?

As asked

You discover that a bug deployed two weeks ago has been writing incorrect data to Core Data, affecting roughly 30% of active users. The fix is straightforward but the corrupted records need repair. How do you handle this?

Sample answer outline

A strong answer covers: ship the bug fix immediately, do not wait for a perfect solution, identify the scope of corruption (what records, which users, what values are wrong), write a migration that detects and repairs corrupted records on next launch (NSMigrationManager or a custom migration script), test the migration on a copy of production data before shipping, and communicate to users if data loss is visible. Consider a server-side repair job if data is synced.

Expect these follow-ups

• How do you test a Core Data migration that changes existing records without risking further data loss?
• What process change prevents a bug of this kind from being in production for two weeks before detection?