Interview guides / ios

Double-charge checkout race: iOS Interview Question

Double-charge checkout race interview question with a clear example answer for iOS engineers. Practice with AceMobileInterview.

This is a common iOS interview topic: Double-charge checkout race.

Below is a concise answer you can adapt in a real interview, then go deeper in the full AceMobileInterview track.

1. 1.Pay can fire twice on double-tap; cart totals flicker under concurrency. Fix races and shared mutable state (≈45–60 min).

   ## Scenario Checkout is not idempotent under rapid taps; cart is mutated off-main unsafely. ## How to investigate - Double-tap Pay with network delay enabled (mock sleep) - Thread Sanitizer if available - Trace `cart.items` writes ## Planted bug 1: No re-entry guard on Pay **Symptom:** Two `checkout()` network calls **Root cause:** Button stays enabled; async method has no `isCheckingOut` flag **Fix:** Disable button / set flag at start; clear in `defer` on main; ignore taps while in flight ## Planted bug 2: Unsynchronized cart mutation **Symptom:** Flickering totals / rare crash **Root cause:** `Cart.shared.items.append` from background + main **Fix:** Isolate mutations on one queue (actor / serial queue) or only mutate on main ## Planted bug 3: UI update from background **Symptom:** Occasional UIKit threading warnings **Root cause:** Completion handler sets `totalLabel.text` off-main **Fix:** `DispatchQueue.main.async` for all UIKit ## Optimization tips - Client-side single-flight is necessary but say you’d also want server idempotency keys - Prefer structured concurrency (`Task` + cancellation) over fire-and-forget ## What to say in the interview - Difference between UI guard vs true idempotency - How you’d test with stress taps