Interview guides / ios

Corrupt draft cache: iOS Interview Question

Corrupt draft cache interview question with a clear example answer for iOS engineers. Practice with AceMobileInterview.

This is a common iOS interview topic: Corrupt draft cache.

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

1. 1.Drafts sometimes vanish; truncated JSON can crash the next launch. Fix persistence races and atomicity (≈45–60 min).

   ## Scenario Draft store writes JSON unsafely and crashes on bad data. ## How to investigate - Force-kill mid-save (or call save repeatedly while editing) - Inspect the drafts file after a crash - Trace `JSONSerialization` / `Data.write` ## Planted bug 1: Non-atomic write **Symptom:** Truncated file after kill **Root cause:** Writing directly to the final path **Fix:** Write to `*.tmp` then `FileManager.replaceItemAt` / atomic options ## Planted bug 2: Crash on decode failure **Symptom:** Launch crash loop **Root cause:** `try!` decode of cache **Fix:** `try?` / `do-catch` → empty draft; optionally quarantine bad file ## Planted bug 3: Unserialized read/write **Symptom:** Intermittent corruption **Root cause:** Saves from textDidChange without coalescing; overlapping writes **Fix:** Serial queue or actor; debounce saves (e.g. 300ms) ## Optimization tips - Mention File Coordination / `NSFileCoordinator` for shared containers - For modern apps, SwiftData/Core Data reduce some of these footguns—but know the file-level answer ## What to say in the interview - How you reproduced partial writes - Why atomic replace matters on mobile (process death is normal)