iOS System Architecture Cheatsheet

Concept Summary

• iOS uses a four-layer architecture: Core OS, Core Services, Media, and Cocoa Touch • Each layer builds on the one below — higher layers provide more abstraction • The XNU kernel (Mach + BSD) sits at the bottom, managing hardware and security • Apps should always prefer the highest-level API available • Apps never communicate with hardware directly — all access goes through the layers

Diagram

flowchart TD
    A["Cocoa Touch<br>UIKit · SwiftUI · MapKit"] --> B["Media<br>AVFoundation · Metal · Core Animation"]
    B --> C["Core Services<br>Foundation · Core Data · URLSession"]
    C --> D["Core OS<br>XNU Kernel · Security · Accelerate"]
    D --> E["Hardware"]

Key APIs

Layer	Key Frameworks	Purpose
Cocoa Touch	UIKit, SwiftUI, WidgetKit	UI, gestures, app lifecycle
Media	AVFoundation, Metal, Core Graphics	Audio, video, GPU, drawing
Core Services	Foundation, Core Data, URLSession	Data, networking, location
Core OS	Security, Accelerate, LocalAuthentication	Kernel, crypto, biometrics

Common Interview Questions

What are the four layers of iOS architecture and what does each one do?

What kernel does iOS use, and how does it relate to macOS?

Why should developers prefer higher-level frameworks over lower-level APIs?

Common Pitfalls

⚠️ Storing passwords in UserDefaults instead of the Keychain (Core OS Security framework)

⚠️ Doing heavy Core Graphics rendering on the main thread — causes UI jank

⚠️ Using low-level C APIs when a Swift framework already wraps the functionality

Quick Reference Table

Scenario	Recommended Framework	Layer
Build a screen	UIKit / SwiftUI	Cocoa Touch
Make HTTP requests	URLSession	Core Services
Store structured data	Core Data / SwiftData	Core Services
Play audio/video	AVFoundation	Media
Encrypt sensitive data	Security / Keychain	Core OS
Biometric login	LocalAuthentication	Core OS
Custom 2D drawing	Core Graphics	Media
GPU compute / 3D	Metal	Media