uikit-mvvm

Approach: Production-First Iterative Refactoring — This skill is built for production enterprise codebases where stability and reviewability matter more than speed. Architecture changes are delivered through iterative refactoring — small, focused PRs (≤200 lines, single concern) tracked in a refactoring/ directory. Critical safety issues ship first; cosmetic improvements come last.

UIKit MVVM Architecture (iOS 13+)

Enterprise-grade UIKit MVVM architecture skill. Opinionated: prescribes Combine-bound ViewModels, Coordinator navigation, constructor injection via factories, ViewState enum, DiffableDataSource, and programmatic Auto Layout. Adopts a production-first iterative refactoring approach — every pattern is chosen for testability, reviewability, and safe incremental adoption in large teams. UIKit MVVM remains the dominant production architecture for large-scale iOS apps.

Architecture Layers

View/ViewController  → UIViewController + UIView. Binds to ViewModel via Combine/closures/GCD. Renders state.
ViewModel Layer      → Zero UIKit imports. Exposes ViewState<T>. Uses @Published (Combine) or closures (GCD).
Coordinator Layer    → Manages navigation flow. Creates ViewModels + VCs. Owns UINavigationController.
Repository Layer     → Protocol-based data access. Hides data source details.
Service Layer        → URLSession, persistence. Injected via protocol. May use GCD or async/await.

Quick Decision Trees

"Should this ViewController have a ViewModel?"

Is there business logic, networking, or complex state?
├── YES → Create a ViewModel (see "Which binding mechanism" below for Combine vs closures)
└── NO → Is it a container/flow controller (tab bar, navigation)?
    ├── YES → Coordinator manages it, no ViewModel needed
    └── NO → No ViewModel needed unless it simplifies testing

"Which binding mechanism should I use?"

Is this a legacy codebase with heavy GCD / completion handlers?
├── YES → Is the goal to extract ViewModels first (keep GCD)?
│   ├── YES → Closures / Bindable<T> + GCD in service layer
│   │   └── Upgrade to Combine later (see migration-patterns.md)
│   └── NO → Adopt Combine during extraction
│       └── @Published + sink (production standard)
└── NO → What is the minimum iOS target?
    ├── iOS 13+ → Combine: @Published + sink
    │   └── One-shot events? → PassthroughSubject
    └── < iOS 13 → Closures / Bindable<T> wrapper