Senior System Design: Modular Architecture

At the senior level, your job is less about writing code and more about designing systems. You must consider the entire lifecycle of the app, from the first byte fetched to the final frame rendered. You are an architect of mobile experiences.

1. Modularization & Library Evolution

Monolithic apps are a liability. They lead to slow build times and “spaghetti code.”

• SPM Local Packages: Use Swift Package Manager to enforce strict boundaries.
• Static vs. Dynamic Linking: Understand the trade-offs. Static linking is faster at runtime (less dynamic jumping) but can increase binary size if used incorrectly. Dynamic linking allows for shared code but can slow down app launch time (dyld).
• Interface Segregation: Create “API-only” modules that contain only protocols. This allows features to interact without depending on each other’s full implementation.

2. Resilience: Offline-First & Data Sync

High-signal apps work everywhere, even in airplane mode.

• Multi-tiered Caching: Implement a system that checks Memory (Fastest) -> Disk (Fast) -> Network (Slowest). Use NSCache for memory and SwiftData for disk.
• Conflict Resolution: What happens if the user edits data offline and it conflicts with the server? Master strategies like “Last Write Wins” or “Manual Merge.”
• Background Tasks: Use BGTaskScheduler to perform heavy sync or data processing while the device is charging and on Wi-Fi.

3. Performance Engineering & Optimization

• Startup Time: Audit your main() function and UIApplicationDelegate. Defer non-critical SDK initialization (like Analytics or Ads) until the user actually needs them.
• Memory Footprint: Use the “Allocations” and “VM Tracker” instruments to minimize your app’s memory usage. A low memory footprint prevents the OS from killing your app in the background.
• LLVM & Compiler Flags: Understand how -O (optimization) and -Osize (optimize for size) affect your binary.

4. Scalability & Team Growth

• Code Generation: Use tools like Sourcery or Swift Macros to reduce boilerplate.
• Documentation: Use DocC to generate beautiful, interactive documentation for your modules.
• Observability: Don’t just log errors; track “Time to Interactive” (TTI) and “App Hangs” using MetricKit.

Checkpoint Task

Map out the module dependency graph for a “Social Media” app. Identify the “Core” modules and explain how you would prevent circular dependencies between the “Feed” and “Profile” features.