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](references/solid-principles.md) | Quick reference all principles | | SRP | [single-responsibility.md](references/single-responsibility.md) | Fat views/VMs, splitting files | | OCP | [open-closed.md](references/open-closed.md) | Adding providers, extensibility | | LSP | [liskov-substitution.md](references/liskov-substitution.md) | Protocol contracts, testing | | ISP | [interface-segregation.md](references/interface-segregation.md) | Fat protocols, splitting | | DIP | [dependency-inversion.md](references/dependency-inversion.md) | Injection, testing, mocking | | Concurrency | [concurrency-patterns.md](references/concurrency-patterns.md) | Actors, @MainActor, Sendable | | Anti-Patterns | [anti-patterns.md](references/anti-patterns.md) | Code smells detection |
Templates
| Template | When to use | |----------|-------------| | [view.md](references/templates/view.md) | SwiftUI View with subviews and #Preview | | [viewmodel.md](references/templates/viewmodel.md) | @Observable ViewModel with @MainActor | | [service.md](references/templates/service.md) | API Service, Mock, Cache actor | | [protocol.md](references/templates/protocol.md) | Service protocol, CQRS, Auth | | [model.md](references/templates/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]/ |