This tripped me up more than it should have.
Start by systematically identifying the bugs through code review and runtime analysis, focusing on common lifecycle and scope pitfalls like retain cycles, premature deallocation, and improper memory management. Then, prioritize fixes based on impact and demonstrate a deep understanding of ARC, ownership, and UIKit's lifecycle. Finally, explain how you would prevent similar issues in the future through testing and best practices.
Pro tip: Use Instruments (like Leaks and Allocations) to empirically demonstrate the bugs and validate fixes, showing a data-driven approach rather than just theoretical knowledge.
Reproduce the bugs by running the app and using debugging tools to pinpoint symptoms like crashes, memory leaks, or UI glitches. Identify the specific code paths related to object lifecycle and scope.
Examine the code for common issues: strong reference cycles (e.g., closures, delegates), premature deallocation (e.g., weak references to objects that should be strong), and scope mismanagement (e.g., variables declared in wrong scope).
Prioritize fixes based on severity and impact. Apply appropriate solutions: use weak/unowned references, adjust ownership qualifiers, ensure proper initialization and teardown, and manage object lifetimes correctly.
After fixes, run tests and use Instruments to confirm the issues are resolved and no new issues are introduced. Consider edge cases and scenarios that might trigger similar problems.
Suggest improvements like code reviews, static analysis, unit tests for memory management, and adopting patterns like MVVM with clear ownership to avoid future lifecycle bugs.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
Threading bugs are nasty because they don't always reproduce reliably.
Start by systematically identifying threading issues using tools like Thread Sanitizer and Instruments, then prioritize fixes based on severity and impact. For each issue, explain the root cause, propose a solution, and discuss trade-offs between different synchronization approaches.
Pro tip: Demonstrate maturity by acknowledging that not all threading issues require heavy synchronization; sometimes architectural changes like moving to a serial queue or reducing shared mutable state are better long-term fixes.
Use Xcode's Thread Sanitizer, Instruments (Time Profiler, Threads), and code review to detect data races, deadlocks, and priority inversions. Look for shared mutable state accessed from multiple threads without synchronization.
For each issue, determine why it occurs: unsynchronized access, incorrect queue usage (e.g., concurrent queue with shared resource), or deadlock from nested locks. Consider the specific threading model (GCD, OperationQueue, etc.).
Suggest appropriate fixes: serial queues, locks (NSLock, os_unfair_lock), atomic properties, or barrier flags. Explain how each solution addresses the root cause and any potential drawbacks.
Discuss performance, complexity, and maintainability trade-offs. For example, locks may be faster but error-prone, while serial queues are safer but can introduce latency.
Describe how to verify fixes: rerun Thread Sanitizer, write unit tests for concurrent access, and stress-test under load to ensure issues are resolved without introducing new ones.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
Auto layout with dynamic cell heights is one of those things that looks fine until it doesn't.
Start by reproducing the cell sizing issues and identifying the root causes, such as incorrect Auto Layout constraints or missing self-sizing configuration. Then, systematically fix each bug and verify the solutions with different content sizes and device orientations.
Pro tip: Demonstrate a deep understanding of UITableView's self-sizing mechanism by explaining how estimatedRowHeight and automaticDimension work together, and mention the importance of setting proper content hugging and compression resistance priorities.
Run the app and navigate to the problematic table view. Take screenshots or record the incorrect cell sizes to clearly understand the bugs.
Examine the cell's Auto Layout constraints, ensuring there are no ambiguous or conflicting constraints. Check that all subviews are properly constrained to the cell's contentView.
Confirm that the table view's rowHeight is set to UITableView.automaticDimension and estimatedRowHeight is a non-zero value. Also, ensure the cell's contentView constraints define its height.
After making fixes, test with various content lengths, dynamic type sizes, and device orientations to ensure the cells resize correctly. Use the View Debugger to identify any remaining issues.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
Start by systematically reproducing the issue and inspecting the button's target-action connections in Interface Builder and code. Then trace the event flow through the responder chain and verify the action method signature and thread execution. Finally, implement a robust fix and add tests to prevent regressions.
Pro tip: Demonstrate maturity by discussing how you would prevent similar issues in the future, such as using SwiftUI or programmatic target-action with compile-time checks, and by writing unit tests for UI interactions.
Reproduce the broken button behavior in a controlled environment and isolate whether the issue is consistent or intermittent. Check if the button is enabled, visible, and not obscured by other views.
Examine the button's target-action connections in Interface Builder and code. Verify that the action method exists, has the correct signature (e.g., @IBAction func buttonTapped(_ sender: UIButton)), and is connected to the correct event (e.g., .touchUpInside).
Trace the event through the responder chain to ensure the action reaches the intended target. Check for issues like a nil target, incorrect selector, or a superview intercepting touches.
Apply the fix, such as reconnecting the action, correcting the method signature, or adjusting the view hierarchy. Verify the fix by testing the button in various states and on different devices.
Write unit or UI tests to cover the button action and ensure it works as expected. Consider refactoring to a more robust pattern, like using closures or reactive bindings, to avoid similar issues.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
This is where the conversation got more interesting.
Start by acknowledging the importance of code quality and modularity, then outline a systematic approach: assess current state, identify pain points, propose improvements, and discuss trade-offs. Emphasize alignment with team goals and iterative refactoring.
Pro tip: Focus on measurable outcomes and business impact, not just technical ideals. Show you can balance perfection with pragmatism, especially in a fast-paced environment like Nextdoor.
Review the codebase for common issues like massive view controllers, tight coupling, lack of tests, and inconsistent patterns. Use metrics like code coverage, cyclomatic complexity, and build times.
Identify high-impact areas based on frequency of changes, bug density, and team pain points. Prioritize improvements that reduce risk and increase development velocity.
Suggest modularization (e.g., feature modules, Swift packages), design patterns (MVVM, VIPER), dependency injection, and protocol-oriented programming to improve testability and separation of concerns.
Advocate for incremental refactoring with automated tests to ensure safety. Use feature flags and strangler pattern to gradually replace legacy code without disrupting releases.
Define success metrics (e.g., reduced crash rates, faster build times, increased test coverage) and continuously monitor. Foster a culture of code reviews and knowledge sharing.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.