Interview questions in iOS & Swift, page 6
Purpose and setup of an Objective-C bridging header
The bridging header imports ObjC headers into Swift; Xcode auto-creates it or you add it manually and set the build setting.
Diagnosing choppy scrolling performance
Reach for Instruments, especially the Time Profiler and Core Animation/Hangs tools, and look for main-thread work and dropped frames.
Struct versus class performance and memory tradeoffs
Structs are value types, often stack-allocated with no ARC; classes are heap-allocated reference types with retain counts.
Exposing Swift classes to Objective-C
Subclass NSObject or mark members @objc, import the generated ModuleName-Swift.h, and accept that Swift-only types like enums with associated values cannot cross.
What an autoreleasepool is and when to add one manually
An autorelease pool holds objects until it drains; wrap tight loops creating many temporary ObjC objects in autoreleasepool to cap peak memory.
dispatch_async versus Task.detached for offloading work
Dispatch_async submits a closure to a queue; Task.detached starts an unstructured async task off the current actor without inheriting context.
The dynamic keyword and message dispatch
Dynamic forces Objective-C message dispatch via objc_msgSend, enabling KVO and swizzling, at the cost of skipping inlining and devirtualization.
Verifying and avoiding copy-on-write overhead
Confirm extra copies via isKnownUniquelyReferenced or instruments and the retain trace; ensure unique references, mutate inout/in place, reserveCapacity, avoid aliasing.
Debugging a closure capture leak in a third-party library
Use the Memory Graph Debugger to inspect retain paths, the Leaks and Allocations instruments, and malloc stack logging; trace the cycle before patching.
map versus compactMap versus flatMap
Map transforms each element one-to-one; compactMap transforms then drops nils; flatMap transforms to sequences then concatenates.
Copy-on-write and implementing it for a custom struct
CoW shares a buffer until mutation, deep-copying only when the buffer is non-unique; Array, Dictionary, Set, String use it; implement with a class storage box and isKnownUniquelyReferenced.
View identity in SwiftUI and the .id() modifier
Identity links a view to its persistent state and transitions; structural identity comes from position, explicit identity from .id(); changing the id resets state and forces a new view.
Faulting in Core Data
A fault is a placeholder object whose property data is not yet loaded; accessing a property fires the fault and fetches from the store, saving memory.
Minimizing Storyboard merge conflicts
Split into many small storyboards or xibs, or move to programmatic or SwiftUI layout; process-wise coordinate ownership and merge often.
Decoding a heterogeneous JSON array by a type field
Peek the type field with a partial decode, switch on it, and decode the concrete type, wrapping each in an enum or boxed protocol value.
Retain cycles and how to break them
A cycle is two objects (or a closure and its owner) holding mutual strong references so neither deallocates; break it with a [weak] or [unowned] capture list.
Purpose of the guard statement
Guard requires its condition to hold or else exits the scope, and unwrapped optionals stay in scope afterward; this flattens nesting versus if.
Closures and trailing closure syntax
Closures are self-contained function blocks that capture surrounding state; a final closure parameter can be passed as a trailing closure outside the parentheses.
do-try-catch versus Optional return types
Throwing functions with do/try/catch convey why something failed via typed Error values; Optionals only say success or absence. Use throws when the caller needs the reason.
Protocols with associated types and opaque types
A PAT leaves a placeholder type unresolved, so it has no single concrete shape and historically could not be used as a bare existential; opaque types with some let you return one fixed underlying conforming…
We are hiring for this. Every open role lists the topics its interview covers, so you can prepare for the real thing rather than guessing.
See open roles