Swift MVVM Skill

Help the agent produce better MVVM code for Swift projects.

Core stance

• Keep the ViewModel UI-framework agnostic. ViewModels should not import SwiftUI, UIKit, or AppKit.
• The ViewModel should be mostly (1) state, (2) intent methods, and (3) dependency coordination.
• Push work into smaller, testable units (pure structs/functions, use cases, controllers, repositories, mappers, formatters).
• Use dependency injection with protocols so everything is mockable.
• Extensions are encouraged for organization (especially protocol conformances).

When to use

Use this skill when the user asks to:

• Add or refactor features in SwiftUI/UIKit/AppKit and keep architecture clean.
• Create or improve a ViewModel (state, intents, effects) and move logic out of Views/ViewControllers.
• Fix state-management issues (wrong wrappers, threading warnings, unstable bindings).
• Improve testability (protocol-based dependencies, mocks/fakes, deterministic state updates).
• Reduce a “massive ViewModel” by splitting concerns.

First check (before writing code)

1. Identify UI tech: SwiftUI, UIKit, or AppKit.
2. Identify deployment target:
   • If iOS 17+/macOS 14+ is available, prefer Observation (@Observable).
   • Otherwise prefer Combine-based ObservableObject + @Published.
3. Match existing project style (naming, folders, DI approach, networking layer).

MVVM responsibilities

View layer

SwiftUI View / UIKit UIViewController / AppKit NSViewController:

• Declares layout and binds to state.
• Sends user intents (tap, selection, text changes) to the ViewModel.
• Owns UI-only concerns (navigation, presenting alerts, AppKit panels, first responder, etc.).

ViewModel

• Owns screen state (loading/data/error, derived UI values).
• Coordinates effects through injected dependencies.
• Exposes intent methods (onAppear(), refresh(), didTap…) rather than views calling random internals.

Domain / Services

• Encapsulate fetching, caching, persistence, decoding, validation.
• Must be independent of UI frameworks.

Hard rule: No UI framework imports in ViewModels

ViewModels should not import:

• SwiftUI
• UIKit
• AppKit

If a ViewModel needs a platform behavior, define a tiny protocol in a non-UI module (Foundation-only), and provide a platform implementation in the View layer (or platform adapter module).

Example:

// In a Foundation-only target / file.
protocol FileRevealing {
  func reveal(_ url: URL)
}

// In AppKit layer.
import AppKit

struct WorkspaceFileRevealer: FileRevealing {
  func reveal(_ url: URL) {
    NSWorkspace.shared.activateFileViewerSelecting([url])
  }
}

Any “AppKit-y” or “UIKit-y” behavior inside the ViewModel is a **smell**.

## Preferred ViewModel shapes

Pick one based on scope.

### Pattern A: Simple screen

- `State` struct (nested)
- intent methods
- async `load()` with cancellation

### Pattern B: Complex screen

- `State` struct (nested)
- `Action` enum + `send(_:)`
- reducer-like switch for state transitions
- side effects delegated to injected units

## Keep state structured (avoid a ViewModel with 30 vars)

Prefer:

```swift
struct State: Equatable {
  var view = ViewState()
  var content = ContentState()
  var alerts = AlertsState()

  struct ViewState: Equatable {
    var isLoading = false
    var title = ""
  }

  struct ContentState: Equatable {
    var rows: [Row] = []
    var emptyMessage: String? = nil
  }

  struct AlertsState: Equatable {
    var error: ErrorState? = nil
  }
}

Refactoring a massive ViewModel (priority order)

When reducing a large VM, refactor in this order:

1. Extract pure logic first

• Move non-IO computations into pure structs or pure functions.
• Examples: filtering, sorting, mapping domain models to row models, formatting, validation, state reducers.

2. Extract side effects into controllers (still testable)

• Create small “effect” units that do IO and orchestration.
• Keep them behind protocols and inject into the VM.
• Examples: LoadExamplesUseCase, ExamplesController, AnalyticsTracking, FileRevealing.

3. Leave the VM as state + intents

• VMs forward intents to pure logic/effect units and assign state.

Concurrency & cancellation rules

• UI state changes should happen on the main actor.
• Store ongoing tasks and cancel them when a new request starts or when the view disappears.

SwiftUI integration rules

Observation (@Observable)

• Hold a ViewModel instance using @State (owner) and pass it down.

Combine (ObservableObject)

• Use @StateObject when the view creates/owns the ViewModel.
• Use @ObservedObject when the view is given the ViewModel.

Dependency injection rules

• ViewModel takes dependencies in its initializer.
• Dependencies are protocols; provide a production implementation and a mock/fake for tests.
• Prefer injecting small units:
  • Pure logic: RowBuilder, Validator, Reducer
  • Effects: UseCase, Controller, Repository
  • Platform adapters: FileRevealing, URLOpening, etc.

Avoid massive ViewModels

Smells:

• Imports UI frameworks.
• Does URLSession/JSON decoding directly.
• Builds NSAlert/UIAlertController.
• Knows about NSWorkspace/UIApplication.
• Formats everything inline with complex logic.
• Handles navigation, analytics, networking, caching all together.

Refactor moves:

• Extract a pure RowBuilder or Reducer.
• Extract a UseCase for business rules.
• Extract a Repository for IO.
• Extract a side-effects Controller to orchestrate multiple services.
• Extract platform adapters (tiny protocols) for UI actions.

Output expectations

When writing/refactoring:

• Provide code in diff-friendly chunks, grouped by file.
• Prefer small, composable functions.
• Use extensions for organization (e.g., protocol conformances, grouping helpers).
• Name clearly: FooViewModel, FooState, FooUseCase, FooRepository, FooController, FooRowBuilder.
• Add tests for ViewModel state transitions and extracted pure logic.

Testing guidance

Prefer Swift Testing (import Testing, @Test, #expect, #require) for new tests.

• Test pure logic units directly (fast, deterministic).
• Test ViewModel state transitions (success/failure/cancellation) by injecting mocks.

Templates

Copy/paste from:

• templates/ObservationViewModel.swift
• templates/CombineViewModel.swift
• templates/AppKitViewController.swift
• templates/ProtocolAdapters.swift
• templates/ServiceUseCaseControllerAndPureLogic.swift
• templates/ViewModelTests.swift (Swift Testing)

Additional resources (load only if needed)

• MVVM Overview: references/mvvm-overview.md
• Module Structure: references/module-structure.md
• File Naming Conventions: references/file-naming-conventions.md
• Where Things Go: references/where-things-go.md
• Common Patterns: references/common-patterns.md
• Adding New Features: references/adding-new-features.md
• Integration Patterns: references/integration-patterns.md
• Anti-Patterns: references/anti-patterns.md
• Testing Considerations: references/testing-considerations.md
• Services vs Feature Services: references/services-vs-feature-services.md
• Controller vs Coordinator: references/controller-vs-coordinator.md
• State Management: references/state-management.md