Separation of Concerns

Divide a system into distinct sections, each addressing a separate concern. A concern is a set of information that affects the code.

Common Concerns to Separate

• UI/Presentation: How things look
• Business logic/Domain: What things do
• Data access: Where things are stored
• Navigation: How to move between screens
• State management: How data flows
• External services: Third-party integrations

Violation Signs

• Components fetching data AND rendering AND handling business rules
• Business logic importing React or framework-specific code
• Database queries mixed with validation logic
• UI components aware of API endpoint URLs
• Navigation logic inside business rules

→ See [layers-example.md](references/layers-example.md) for complete before/after refactoring.

Layer Dependencies

┌─────────────────────────────────────────┐
│           Presentation Layer            │
│  (Screens, Components, Hooks, ViewModels)│
└─────────────────────┬───────────────────┘
                      │ depends on
                      ▼
┌─────────────────────────────────────────┐
│             Domain Layer                │
│    (Entities, UseCases, Ports)          │
└─────────────────────┬───────────────────┘
                      │ depends on
                      ▼
┌─────────────────────────────────────────┐
│          Infrastructure Layer           │
│   (Adapters, Repositories, Services)    │
└─────────────────────────────────────────┘

Domain has NO dependencies on Presentation or Infrastructure

Quick Checks by Layer

When Concerns Can Stay Together

• Prototypes: Speed over architecture for throwaway code
• Simple utilities: Pure functions without dependencies
• Trivial components: A button that just styles and calls onPress
• Co-location benefit: Sometimes keeping related code together aids understanding

The key question: "If I change this concern, how many unrelated things break?"