SOLID Swift - Apple Best Practices 2026
Agent Workflow (MANDATORY)
Before ANY implementation, use TeamCreate to spawn 3 agents:
-
fuse-ai-pilot:explore-codebase - Analyze existing architecture
-
fuse-ai-pilot:research-expert - Verify Swift/Apple docs via Apple Docs MCP + Context7
-
XcodeBuildMCP - Build validation after code changes. Then run fuse-ai-pilot:sniper.
DRY - Reuse Before Creating (MANDATORY)
Before writing ANY new code:
-
Grep the codebase for similar protocols, services, or logic
-
Check shared locations: Core/Extensions/ , Core/Utilities/ , Core/Protocols/
-
If similar code exists -> extend/reuse instead of duplicate
-
If code will be used by 2+ features -> create it in Core/ directly
Architecture (Features Modular MANDATORY)
Layer Location Max Lines
Views Features/[Feature]/Views/
80
ViewModels Features/[Feature]/ViewModels/
100
Services Features/[Feature]/Services/
100
Protocols Features/[Feature]/Protocols/
30
Shared Core/{Models,Protocols,Services,Extensions,Utilities}/
NEVER use flat Sources/ structure - always Features/[Feature]/
Critical Rules (MANDATORY)
Rule Value
File limit 100 lines (split at 90)
ViewModels @MainActor @Observable
Protocols Features/[Feature]/Protocols/ or Core/Protocols/ ONLY
Models Sendable structs
Documentation /// on all public APIs
Previews Every View MUST have #Preview
Reference Guide
Concepts
Topic Reference When to consult
SOLID Overview solid-principles.md Quick reference all principles
SRP single-responsibility.md Fat views/VMs, splitting files
OCP open-closed.md Adding providers, extensibility
LSP liskov-substitution.md Protocol contracts, testing
ISP interface-segregation.md Fat protocols, splitting
DIP dependency-inversion.md Injection, testing, mocking
Concurrency concurrency-patterns.md Actors, @MainActor, Sendable
Anti-Patterns anti-patterns.md Code smells detection
Templates
Template When to use
view.md SwiftUI View with subviews and #Preview
viewmodel.md @Observable ViewModel with @MainActor
service.md API Service, Mock, Cache actor
protocol.md Service protocol, CQRS, Auth
model.md Model, DTO, Error, Enum
Forbidden
Anti-Pattern Fix
Files > 100 lines Split at 90
Protocols in impl files Move to Protocols/ directory
ObservableObject
Use @Observable
Completion handlers Use async/await
Missing #Preview
Add preview for every View
Non-Sendable in async Use struct with let
Flat Sources/ structure Use Features/[Feature]/