Interview guides / ios

Janky home feed: iOS Interview Question

Janky home feed interview question with a clear example answer for iOS engineers. Practice with AceMobileInterview.

This is a common iOS interview topic: Janky home feed.

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

1. 1.Download the buggy Xcode zip. The home feed freezes/stutters on load with a large JSON payload. Find and fix the planted performance issues (≈45–60 min).

   ## Scenario Candidates get a UIKit feed that “works” but freezes on large payloads. ## How to investigate - Reproduce with the bundled large `feed.json` - Time Profiler / Main Thread Checker - Search for `Data(contentsOf` / `JSONDecoder().decode` on the main path ## Planted bug 1: Synchronous network + decode on main **Symptom:** UI freezes ~1s on appear **Root cause:** `FeedViewController.load()` calls `Data(contentsOf:)` and `JSONDecoder` on the main thread **Fix:** `DispatchQueue.global(qos: .userInitiated).async` (or async/await) for load+decode; hop back to main only to assign `items` and `reloadData()` ## Planted bug 2: Image downsample on main **Symptom:** Scroll hitch when cells appear **Root cause:** `UIImage(data:)` + redraw in `cellForRowAt` on main **Fix:** Decode/downsample off-main; cache `UIImage`; cancel work on reuse ## Planted bug 3: Architecture smell **Symptom:** Hard to test / reason about **Root cause:** Networking lives inside the view controller **Fix:** Extract `FeedLoading` protocol + `LocalFeedLoader`; VC only binds results ## Optimization tips - Prefer `URLSession` even for local files when teaching async patterns - Use `autoreleasepool` when decoding many images - Avoid doing I/O inside `cellForRowAt` ## What to say in the interview - How you proved the main-thread hitch before changing code - What you measured after (scroll FPS / time to first content)