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Mobile Ios Swift Concurrency Structured Interview Questions

75 verified Mobile Ios Swift Concurrency Structured interview questions — solve with answers, learn from explanations, test yourself in a real simulation.

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Sample questions

Ios Swift Concurrency StructuredDifficulty 1
In GCD (Grand Central Dispatch), what is a DispatchQueue?
  • aA single thread reserved exclusively for that queue's lifetime
  • bAn abstraction that schedules submitted work onto a pool of threads✓
  • cA compiler directive that disables concurrency checks
  • dA storage container for Swift value types only
Explanation:A DispatchQueue schedules blocks of work (closures) to run on GCD's managed thread pool; it does not own one fixed thread — the system decides which thread actually executes the work.
Ios Swift Concurrency StructuredDifficulty 1
What is the practical difference between queue.sync { ... } and queue.async { ... }?
  • async always runs on the main thread, async never does
  • bsync blocks the calling thread until the block finishes✓
  • casync blocks the calling thread, sync returns immediately
  • dThere is no observable difference in modern Swift
Explanation:sync waits for the submitted block to complete before returning control to the caller, blocking the calling thread; async submits the block and returns control immediately without waiting.
Ios Swift Concurrency StructuredDifficulty 2
let queue = DispatchQueue(label: "serial")
queue.sync {
    queue.sync {
        print("inner")
    }
}

What happens when this runs?
  • aIt deadlocks: the inner sync waits for the serial queue the outer call already occupies✓
  • bIt compiles but does nothing at runtime
  • cIt prints "inner" twice
  • dThe compiler rejects nested sync calls
Explanation:A serial queue runs one block at a time. The outer sync occupies the queue and blocks the calling thread waiting for it to finish; the inner sync tries to enqueue and wait on that same occupied serial queue, so neither can proceed — a classic self-deadlock.
Ios Swift Concurrency StructuredDifficulty 1
In Swift's async/await, what does marking a function async mean?
  • aThe function always runs on a background thread
  • bThe function can never throw an error
  • cThe function is automatically retried on failure
  • dThe function may suspend and later resume✓
Explanation:An async function can suspend at an await point (e.g. waiting on another async operation) and resume later, potentially on a different thread; it does not dictate which specific thread or queue it runs on by itself.
Ios Swift Concurrency StructuredDifficulty 2
What problem does an actor in Swift primarily solve?
  • aIt serializes access to its mutable state✓
  • bIt makes a type conform to Codable automatically
  • cIt removes the need to ever call await
  • dIt guarantees the type is a value type instead of a reference type
Explanation:An actor protects its mutable state by only allowing one task at a time to execute code that touches that state (actor isolation), which is the language's built-in mechanism against data races on shared mutable state.
Ios Swift Concurrency StructuredDifficulty 2
actor Counter {
    private var value = 0
    func increment() { value += 1 }
    func current() -> Int { value }
}

From outside the actor, how must increment() be called?
  • aawait counter.increment()✓
  • bcounter.increment() with no await, since actors are synchronous by default
  • cIt cannot be called from outside the actor at all
  • dOnly from a DispatchQueue.main.sync block
Explanation:Calling an actor's method from outside that actor's isolation is a potential suspension point (the call may have to wait its turn), so Swift requires await at the call site even though increment() itself isn't declared async.

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