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What is @State in SwiftUI and how does it affect the view?
Tests declarative data flow ownership. A strong answer defines @State as private value-type storage owned by the view that triggers a body re-evaluation on mutation. Red flag: confusing it with external data sources like @ObservedObject or @Binding.
When should you use frame-based layout versus Auto Layout?
Frames suit static or high-frequency UI like particle systems; Auto Layout handles rotation, Dynamic Type, localization, and split screen.
How would you use Energy Log to investigate battery drain?
Tests correlating Instruments energy metrics to app behavior. Strong answers profile with Energy Log, map CPU/Location/Network spikes to user flows or background work, and validate fixes with before/after logs. Red flag: blaming the OS without evidence.
What build settings improve Time Profiler symbols in Release builds?
This tests debug symbol control in Xcode Release builds. Set DEBUG_INFORMATION_FORMAT to DWARF with dSYM, avoid stripping, citing larger size and slower links. Red flag: suggesting you disable optimizations, invalidating the performance profile.
How does Debug Memory Graph find leaks and retain cycles?
This tests knowledge of Xcode's heap visualization versus allocation sampling. A strong answer notes the graph shows objects, draws retain cycles as loops, and contrasts with Leaks time-based sampling. Red flag: expecting leak flags without inspection.
Which Instrument diagnoses scroll stuttering and dropped frames?
Tests whether you connect UI jank to Instruments data. A strong answer names the Core Animation or Time Profiler instrument, cites frame duration and hitch ratio, and correlates spikes with main-thread work.
Explain symbolic breakpoints and debug Auto Layout with one
Tests debugging closed-source frameworks without source lines. A strong answer says a symbolic breakpoint stops on UIViewAlertForUnsatisfiableConstraints, then checks the backtrace or log to find the bad view.
What is an Xcode Scheme? How do you configure a custom one?
This tests Xcode build orchestration. A scheme is a blueprint for build, test, run, profile, analyze actions; a strong answer covers duplicating it, setting launch arguments, and staging configs. A red flag is confusing schemes with build configurations.
What LLDB command prints a UIView description versus an Int?
Tests LLDB formatting at breakpoints. Great answers: po invokes description or debugDescription on objects, while p evaluates and prints raw primitive values with type info. Red flag: mixing them up or recommending console print instead of LLDB.
Difference between .xcodeproj and .xcworkspace, and when to use a workspace
Tests your mental model of Xcode build containers. A project defines one buildable unit; a workspace groups multiple projects so they build together and share derived data. Use it for package managers or modular codebases.
How does Swift async/await improve on completion handlers?
Tests structured concurrency mastery over callback control flow. A strong answer covers inversion of control, suspension points as yield locations, and Tasks as parent-child units.
Explain Copy-on-Write in Swift and implement it for custom structs
Tests value semantics with reference storage and uniqueness checks. A strong answer explains CoW delays copy until mutation via isKnownUniquelyReferenced, lists custom steps: wrap, read, check, clone. Red flag: assuming structs are automatically CoW.
What is a closure capture list? Explain [weak self] versus [unowned self].
Define capture lists, contrast [weak self] safe optionality against [unowned self] crash risk, and match each to lifetime guarantees.
What are Swift generics, why useful, and write a swap function?
This tests parametric polymorphism and type-safe reuse. A strong answer defines generics as placeholder types for reusable code, then writes a swap<T> function using inout parameters. Red flag: confusing generics with Any or omitting inout.
What is a retain cycle in ARC with closures?
It tests reference counting and closure capture semantics. A retain cycle forms when a closure captures self strongly while self holds the closure; break it with weak if self can deallocate, or unowned if it will outlive the closure.
Explain protocols and how extensions provide default implementations
Tests behavior sharing without inheritance. A strong answer defines protocols as requirements, shows extensions injecting default implementations, and contrasts this with base-class inheritance.
Swift struct vs class: differences and when to choose each
Tests value vs reference semantics and let mutability. Strong answers contrast copy behavior with shared references, note allocation tendencies, and choose struct for value semantics or class for identity.
Explain the primary differences between a struct and a class in Swift
Tests value versus reference semantics and their impact on memory and mutation. Strong answers note structs copy on assignment and lack inheritance, while classes share heap instances via ARC. Red flag: claiming structs are always stack-allocated.
What is an optional in Swift? Demonstrate two safe unwrapping methods.
This checks Swift type safety and nil-handling. A strong answer defines Optional as an enum, demonstrates if let binding, and shows the nil-coalescing operator ?? for defaults. Avoid suggesting force-unwrapping with ! as a safe pattern.
What is the difference between let and var in Swift?
This tests value semantics beyond syntax. A strong answer defines let as immutable and var as mutable, notes that let on a reference type only fixes the pointer, and cites clearer intent and compiler optimization.