iOS System Design Interview Questions

📱 Question 1: Design an Instagram-like Feed Hard

🎯 Problem Statement:

Design the architecture for an Instagram-like feed that can:

• Display images and videos efficiently
• Support infinite scrolling with pagination
• Handle offline mode and caching
• Optimize for battery life and data usage
• Scale to millions of users

🔑 Key Considerations:

• Memory Management: How to prevent OOM with images?
• Prefetching: When and how much to prefetch?
• Caching Strategy: Disk vs memory, eviction policies
• Video Playback: Autoplay, preloading, battery impact
• Network Optimization: Request batching, retries, CDN

✅ Expected Solution Components:

1. Architecture: MVVM with Coordinator pattern
2. Data Layer: Repository pattern with caching
3. Image Loading: SDWebImage or custom solution with NSCache
4. Prefetching: UITableViewDataSourcePrefetching
5. Video: AVPlayer pool, reuse strategy
6. Pagination: Cursor-based or offset-based
7. Offline: Core Data or Realm for persistence

💡 Pro Tips:

• Discuss trade-offs: memory vs network vs UX
• Mention UICollectionView vs UITableView choice
• Talk about cell reuse and performance
• Consider accessibility and VoiceOver

🔄 Question 2: Design a Real-Time Chat System Hard

🎯 Problem Statement:

Design a WhatsApp-like messaging system for iOS:

• Real-time message delivery (WebSocket)
• Local message storage and sync
• Media sharing (images, videos, files)
• Typing indicators and read receipts
• Offline message queuing

🔑 Key Considerations:

• Networking: WebSocket vs long polling?
• Database: Core Data vs Realm vs SQLite?
• Sync Strategy: How to handle offline/online transitions?
• Message Ordering: Timestamps, local vs server time
• Encryption: E2E encryption approach

✅ Solution Architecture:

1. Transport Layer: URLSession WebSocket with reconnection logic
2. Message Queue: Pending, sent, delivered, read states
3. Local Storage: Core Data with background contexts
4. Media Upload: Background URLSession with progress tracking
5. Push Notifications: APNs for background delivery

🖼️ Question 3: Image Caching System Medium

🎯 Problem Statement:

Design an efficient image caching system that:

• Downloads and caches images from URLs
• Manages memory and disk cache
• Handles cache eviction intelligently
• Supports image transformations (resize, crop)
• Thread-safe operations

✅ Key Components:

class ImageCache {
  private let memoryCache: NSCache<NSString, UIImage>
  private let diskCache: FileManager
  private let downloadQueue: OperationQueue
  
  func loadImage(url: URL, completion: @escaping (UIImage?) -> Void)
  func clearCache(olderThan: TimeInterval)
  func prefetch(urls: [URL])
}

💡 Discussion Points:

• Cache eviction: LRU vs LFU vs TTL
• Disk cache size limits and cleanup
• Image format optimization (WebP, HEIC)
• Concurrent downloads and request deduplication

⚡ Question 4: App Launch Optimization Hard

🎯 Problem Statement:

Your app takes 3 seconds to launch. How do you reduce it to under 1 second?

🔍 Investigation Steps:

1. Measure: Use Instruments (Time Profiler, System Trace)
2. Identify: What's happening during launch?
   • Pre-main time (dyld loading)
   • Main thread initialization
   • First view controller creation

✅ Optimization Strategies:

• Reduce dyld time: Minimize dynamic frameworks, use static linking
• Defer work: Move non-critical init to background or after first frame
• Lazy loading: Initialize only what's needed for first screen
• Async operations: Use GCD for parallel initialization
• Asset optimization: Compress images, use asset catalogs

🧵 Question 5: Main Thread Optimization Medium

🎯 Problem Statement:

Your app's scrolling is janky. Frame rate drops to 30 FPS. How do you fix it?

🔍 Debugging Approach:

• Use Instruments (Core Animation, Time Profiler)
• Enable "Color Offscreen-Rendered" in simulator
• Check for main thread blocking operations
• Measure cell prepare time

✅ Common Solutions:

1. Move to background: Image decoding, text sizing, layout calculations
2. Optimize layers: Reduce layer count, avoid offscreen rendering
3. Cell optimization: Reuse cells properly, pre-calculate heights
4. Rendering: Use CALayer directly for complex views
5. Image handling: Downscale images to display size