Intermediate everything in iOS & Swift, page 4
How do UITableView and UICollectionView reuse cells?
This tests your understanding of UIKit's flyweight pattern and scroll performance. A strong answer covers the reuse pool, dequeueReusableCell mapping identifiers to new index paths, and registration.
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 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 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.
KVO: Swift's Built-In Observer Pattern
KVO lets an object subscribe to property changes on another object automatically. It shines when you need UI updates driven by model state, but forgetting to remove observers before deallocation crashes your app with an NSException.
Thread Sanitizer catches data races in Swift
Thread Sanitizer turns flaky race crashes into reproducible reports by monitoring memory accesses at runtime. Use it in Xcode to catch unsynchronized cross-thread reads and writes. It only catches races that execute during your test run, so coverage matters.
Profiling iOS Memory with Instruments
Instruments X-rays your heap to catch objects that outstay their welcome. Profile image-heavy features or when jetsam kills your app. Never trust Simulator memory numbers; always validate on physical hardware.
@objc: Swift's Bridge to Objective-C
@objc is the bridge badge that exposes Swift code to the Objective-C runtime so UIKit selectors and KVO work. You need it for target-action and Core Data. Forget it and you get a runtime crash, not a compiler error.
XCFrameworks: One Bundle for Every Apple Platform
An XCFramework is a shipping container for compiled Apple libraries: one bundle holds device, simulator, and Mac binaries so Xcode picks the right slice automatically. Use it to distribute closed-source SDKs.
Dependency Resolution in iOS Package Managers
Dependency resolution finds one version of every library that satisfies all transitive requirements. It runs when SPM, CocoaPods, or Carthage builds your graph. The footgun is tight upper bounds; they multiply unresolvable diamond conflicts.
XCTestCase: One Fresh Instance Per Test
A XCTestCase subclass is a lab bench for isolated experiments. Each test method is a fresh invocation checked by the runner. The silent killer is shared mutable state on the case: without setUp, tests flake because execution order is never guaranteed.
MatchedGeometryEffect: Shared Element Hero Transitions
MatchedGeometryEffect lets two views share one spatial identity, so a thumbnail can appear to fly into a detail sheet across view hierarchies. It only works if both views exist simultaneously during a withAnimation block; otherwise the transition snaps.
Custom Rendering with draw(_:)
draw(_:) is the callback where UIKit asks your view for pixels. Override it to render custom shapes or text with Core Graphics when stock views fall short. The footgun is calling it directly or doing non-drawing work inside; use setNeedsDisplay() instead.
withAnimation: The Implicit Animation Transaction
withAnimation wraps state changes in a transaction, telling SwiftUI to tween affected layout automatically. Call it from button actions or view model methods to animate many views from one state flip.
withCheckedThrowingContinuation: Bridge Callbacks to Async/Await
withCheckedThrowingContinuation bridges legacy completion-handler APIs into async/await. Wrap one-shot callback-based work so callers use try await instead of nested blocks. The footgun is resuming twice or never, which checked runtimes catch in debug builds.
Unwind Segues: Jump Back in the Stack
An unwind segue jumps from a deep screen to an earlier view controller without manual popping. Use it after multi-step flows to return to a root screen. If multiple screens share the same action, UIKit picks the nearest one and silently routes users wrong.
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