iOS/Mac Architecture Guidelines: A Practical Approach
Building robust, maintainable iOS and Mac applications requires a solid architectural foundation. Over the years, I’ve found that combining principles from Clean Architecture, Domain-Driven Design (DDD), and modern reactive patterns creates a powerful framework for mobile and desktop development. Let me walk you through the key principles that guide my approach.
Foundation: Best of Both Worlds
The architecture I advocate starts with two well-established patterns: Clean Architecture and Layered Architecture (from DDD). Rather than picking one over the other, I recommend taking the best aspects of both.
Domain-Driven Design and Layered Architecture represent some of the most thoroughly researched approaches to software design. There’s an enormous body of knowledge here, making it an excellent foundation for developers who want to deeply understand software design principles. While many DDD concepts are tailored for backend systems and may not directly apply to mobile or desktop apps, that’s perfectly fine. You can skip what doesn’t fit your context, or better yet, develop a basic understanding of these concepts to have more meaningful conversations with your backend teams.
Clean Architecture takes Layered Architecture and refines it for modern application development. Its key improvements focus on two critical areas:
- enforcing inward-pointing dependencies through interfaces and ports toward the domain layer,
- and with that, achieving better testability and framework independence.
However, Clean Architecture deliberately leaves many questions unanswered and lacks detailed guidance for numerous practical scenarios. This is precisely where the depth of Layered Architecture and DDD becomes valuable, filling in the gaps with comprehensive software design principles.
Unidirectional Data Flow: Predictable State Management
A crucial extension to this foundation is implementing Unidirectional Data Flow, which brings predictability and clarity to how data moves through your application. This is accomplished through several key components:
Data Stores serve as immutable single sources of truth for each Entity or group of related Entities. These stores are managed exclusively by a Data Store Manager, which is the only component authorized to modify data within stores. This pattern draws inspiration from Redux but adapts it for a more native iOS/Mac experience. Instead of a purely functional store, we use regular classes for store management and leverage use-cases for orchestrating changes. This approach allows for a smoother transition from standard Clean Architecture implementations and gives you flexibility in controlling the level of protection and complexity based on your application’s needs.
Use-cases orchestrate all actions and coordinate changes to data, acting as the bridge between presentation and domain layers. They apply changes to Entities and ensure that all state changes flow through well-defined pathways.
In the Domain Layer, entities remain immutable. These are still Rich Domain Entities that encapsulate business rules, but their immutability means they don’t modify internal state directly. Instead, they provide new instances with applied changes, ensuring thread safety and making state changes explicit and traceable.
Composition Root: Clear Dependency Management
The Composition Root is where your entire dependency graph comes together. This is where all components are instantiated and injected into their dependent consumers. A critical principle here is separating the instantiation (construction) phase from the execution phase. This separation dramatically simplifies your codebase by allowing components to focus solely on control flow and data flow, without being cluttered with construction logic.
All instantiation and injection should flow through a Dependency-Injection Container. I strongly recommend striving for Local Registries rather than Global ones. Local registries prevent uncontrollable coupling across your application, making it easier to reason about dependencies and maintain modularity as your codebase grows.
Presentation Layer: MVVM-C for Clear Separation
For the presentation layer, I recommend MVVM-C (Model-View-ViewModel-Coordinator), which provides clear separation of concerns:
Views should handle only presentation concerns—displaying data and capturing user interactions. They should be as “dumb” as possible, delegating all logic to their ViewModel.
ViewModels have two primary responsibilities: reading data from Stores and preparing it for View consumption, and handling actions from Views by delegating to Use-cases (which are injected during the instantiation phase). This keeps ViewModels focused on presentation logic without directly manipulating domain state.
Coordinators handle all screen transitions and navigation flows, completely hiding these concerns from ViewModels. The ViewModel informs the Coordinator what happened (e.g., “user completed checkout”), and the Coordinator knows how to accomplish it (e.g., “dismiss modal, show success screen, then return to home”). This separation makes ViewModels more testable and prevents navigation logic from leaking throughout your presentation layer.
Conclusion
This architectural approach combines battle-tested principles from Clean Architecture and DDD with modern reactive patterns to create a robust foundation for iOS and Mac applications. By enforcing unidirectional data flow, maintaining clear boundaries between layers, and carefully managing dependencies through composition roots, you’ll build applications that are maintainable, testable, and scalable. The key is understanding these principles deeply enough to adapt them to your specific context while maintaining the core benefits they provide.