SOLID Swift - Apple Best Practices 2026

Agent Workflow (MANDATORY)

Before ANY implementation, use TeamCreate to spawn 3 agents:

1. fuse-ai-pilot:explore-codebase - Analyze existing architecture
2. fuse-ai-pilot:research-expert - Verify Swift/Apple docs via Apple Docs MCP + Context7
3. XcodeBuildMCP - Build validation after code changes. Then run fuse-ai-pilot:sniper.

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DRY - Reuse Before Creating (MANDATORY)

Before writing ANY new code:

1. Grep the codebase for similar protocols, services, or logic
2. Check shared locations: Core/Extensions/, Core/Utilities/, Core/Protocols/
3. If similar code exists -> extend/reuse instead of duplicate
4. If code will be used by 2+ features -> create it in Core/ directly

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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]/

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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

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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

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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]/