Approach: Production-First Iterative Refactoring — This skill is built for production enterprise codebases using The Composable Architecture. Architecture changes are delivered through iterative refactoring — small, focused PRs tracked in a refactoring/ directory. AI tools consistently generate outdated TCA code (pre-1.4 patterns, WithViewStore, Environment, Combine Effects). Every rule here exists to prevent those mistakes.

TCA SwiftUI Architecture (iOS 16+, TCA 1.7+)

Enterprise-grade skill for The Composable Architecture (point-free/swift-composable-architecture). Opinionated: prescribes @Reducer macro, @ObservableState, struct-of-closures dependencies, delegate actions for child-parent communication, and modern navigation via @Presents/StackState. TCA's API changed substantially across versions — AI tools trained on pre-1.7 code consistently generate WithViewStore, IfLetStore, Environment, and other removed patterns. This skill encodes the modern (1.7+) patterns validated against real enterprise codebases.

If the project uses TCA <1.7, consult references/migration.md for pre-macro patterns and incremental upgrade path.

Architecture Layers

View Layer         → SwiftUI Views. Direct store access, NO WithViewStore.
                     Owns store via let/var. Uses @Bindable for $ bindings.
Reducer Layer      → @Reducer struct. State + Action + body. Pure logic.
                     Returns Effect for async work. ALWAYS runs on main thread.
Effect Layer       → .run { send in } for async. .send for sync delegate actions.
                     Cancellable, mergeable, concatenatable.
Dependency Layer   → @DependencyClient structs. Struct-of-closures, NOT protocols.
                     liveValue (app) / testValue (auto-unimplemented) / previewValue.

Quick Decision Trees

"Should this be its own Feature reducer?"

Does this component have its own screen/view?
├── YES → Own @Reducer struct
└── NO → Does it have business logic you want to unit test?
    ├── YES → Own @Reducer struct, compose via Scope
    └── NO → Is its state optional (not always visible)?
        ├── YES → Own @Reducer, use ifLet (avoids unnecessary work)
        └── NO → Keep logic in parent reducer

"What navigation pattern do I need?"

What type of navigation?
├── Modal (sheet/alert/dialog/fullScreenCover)?
│   → Tree-based: @Presents + PresentationAction + ifLet
├── Single drill-down?
│   → Tree-based: optional state + navigationDestination
├── Multi-level push (NavigationStack)?
│   → Stack-based: StackState + StackAction + forEach
├── Deep linking is critical?
│   → Stack-based (easy to construct state array)
└── Modals FROM within a NavigationStack?
    → BOTH: stack for push nav, tree for sheets/alerts

"How should child communicate with parent?"

Child needs to tell parent something happened?
├── Use delegate action: case delegate(DelegateAction)
│   Parent observes ONLY .delegate cases, never child internals
│
Child needs to share logic between action cases?
├── Use mutating func on State or private helper method
│   NEVER send an action to share logic — it traverses the entire reducer tree unnecessarily,
│   creating hidden coupling and making performance profiling difficult
│
Sibling reducers need to communicate?
└── Delegate up to parent, parent coordinates
    NEVER send actions between siblings directly — siblings should not know about each other

Workflows

Workflow: Create a New TCA Feature

When: Building a new screen or self-contained feature from scratch.

1. Define State as @ObservableState struct inside @Reducer — conform to Equatable
2. Define Action enum inside @Reducer — NO Equatable conformance needed (TCA 1.4+)
3. Use three-category action pattern: view(ViewAction), internal(InternalAction), delegate(DelegateAction) (rules.md)
4. Implement var body: some ReducerOf<Self> — use Reduce<State, Action> { } for explicit generics (helps Xcode autocomplete)
5. Add child features via Scope, ifLet, or forEach BEFORE parent Reduce block
6. Create dependencies with @DependencyClient macro (dependencies.md)
7. Wire view: direct store.property access, @Bindable var store for $ bindings
8. Write exhaustive TestStore tests (testing.md)

Workflow: Decompose a God Reducer

When: A single reducer handles logic for multiple screens, has 800+ lines, or testing is impossible to isolate.

1. Identify feature boundaries — each screen = own reducer
2. Scan for anti-patterns using detection checklist (anti-patterns.md)
3. Create refactoring/ directory with per-feature plan files (refactoring-workflow.md)
4. Extract child reducers bottom-up: leaf features first, then mid-level, then root
5. Use Scope(state:action:) for always-present children
6. Use .ifLet(\.$destination, action: \.destination) for optional children — effects auto-cancel on dismissal
7. Use .forEach(\.path, action: \.path) for stack navigation
8. Convert child-parent communication to delegate actions — parent must NEVER observe child internal actions
9. Write non-exhaustive integration tests for cross-feature flows (testing.md)

Workflow: Migrate Legacy TCA to Modern (1.7+)

When: Modernizing pre-1.7 TCA code that uses WithViewStore, IfLetStore, Environment, etc.

1. Ensure TCA 1.4+ with @Reducer macro on all reducers (prerequisite)
2. Migrate feature-by-feature, bottom-up (leaf features first) (migration.md)
3. Per feature: add @ObservableState to State AND update view simultaneously — never one without the other
4. Replace @PresentationState with @Presents
5. Replace WithViewStore(store, observe: { $0 }) with direct store.property access
6. Replace IfLetStore/ForEachStore/SwitchStore with native SwiftUI + store.scope
7. Replace NavigationStackStore with NavigationStack(path: $store.scope(...))
8. For iOS 16: wrap views in WithPerceptionTracking { }
9. Run existing tests — all must pass before proceeding to next feature
10. See full migration checklist and syntax transformations in migration.md

Code Generation Rules

<critical_rules> AI tools consistently generate outdated TCA code. Every output must use MODERN TCA (1.7+). ALWAYS:

1. Use @Reducer macro — never bare struct Feature: Reducer conformance
2. Use @ObservableState on ALL State types — never generate State without it
3. Define State AND Action INSIDE the @Reducer struct body — never in extensions (macros can't see them)
4. Use @Reducer enum for Destination/Path reducers (TCA 1.8+) — eliminates massive boilerplate
5. Action does NOT need Equatable — remove it. Why: TCA 1.4+ uses case key paths for cancellation and receive() matching, making Equatable conformance unnecessary and a maintenance burden
6. Access store properties directly: store.count, store.send(.tapped) — NEVER use WithViewStore
7. Use @Bindable var store when $store bindings are needed
8. Use .run { send in } for effects — never EffectTask, .task { }, .fireAndForget { }
9. Use enum-with-cases for cancel IDs — never empty enum types (pruned in release builds)
10. Never capture whole @ObservableState in effect closures — extract needed values first
11. Never do expensive work in reducers — they run on main thread. Offload to .run effects. </critical_rules>

<fallback_strategies> When working with TCA code, you may encounter cryptic compiler errors. If you fail to fix the same error twice:

1. "compiler is unable to type-check this expression": State/Action likely defined in extension instead of inside @Reducer struct. Move them inside.
2. "Circular reference resolving attached macro 'Reducer'": Don't nest @Reducer struct Y inside extension of another @Reducer struct X. Extract to top level.
3. Macro + property wrapper conflict: Avoid property wrappers inside @ObservableState State. Use @ObservationStateIgnored as workaround (changes won't trigger re-renders).
4. "A 'reduce' method should not be defined in a reducer with a 'body'": Never define BOTH reduce(into:action:) AND var body in the same reducer.
5. NavigationStack dismiss fights: Ensure .navigationDestination is OUTSIDE ForEach/List, not inside. </fallback_strategies>

Modernization Migration Cheat-Sheet

When migrating legacy TCA code or writing new features, these 7 tokens come up repeatedly. They are NOT optional — AI tools consistently default to the outdated form on the left. Always use the right column.

Legacy / outdated	Modern (TCA 1.7+, 2026)	Why
environment.mainQueue	@Dependency(\.continuousClock)	mainQueue was the pre-1.0 scheduler injection. In modern TCA, timing is a Clock dependency — override it with ImmediateClock or TestClock in tests.
AnySchedulerOf<DispatchQueue> in State	@Dependency(\.continuousClock)	Same as above. Schedulers are no longer passed through State — they are dependencies. Use try await clock.sleep(for: .seconds(0.3)) in .run effects.
@Bindable var store (iOS 17+ only)	iOS 16: @Perception.Bindable var store — iOS 17+: @Bindable var store	@Perception.Bindable is TCA's backport of SwiftUI's @Bindable for iOS 16 because Apple's @Bindable requires iOS 17. Without it, $store.binding fails to compile on iOS 16 targets.
Bool showLogoutAlert in State	@Presents var alert: AlertState<Action>?	Alerts belong in AlertState, not in booleans — this gives you message text, button state, and action dispatching in one value, and it auto-dismisses through standard @Presents.
[Product] in State	IdentifiedArrayOf<Product>	Plain arrays force O(n) updates-by-ID and confuse ForEachStore. IdentifiedArrayOf (import IdentifiedCollections) is the canonical TCA collection — constant-time lookup by id, stable animations, required by forEach(\.items, ...).
var filtered: [T] { state.items.filter { ... } } (computed)	Stored var filtered: IdentifiedArrayOf<T> updated in the reducer	Computed vars on State hide reducer work from TCA's observation system and run on every access. Store the derived value and recompute it in the action that changes the source — it shows up in the action log and is testable.
.run { try? await api.delete(id) }	.run { send in try await api.delete(id); await send(.deleteSucceeded) } catch: { error, send in await send(.deleteFailed(error)) }	try? inside effects is a silent-failure bug — the UI thinks the delete worked but nothing changed on the server. Use the catch: parameter on .run and emit an explicit failure action.

Dismiss-in-delegate-handler pattern

When a child feature finishes (save, cancel, delete) and the parent uses @Presents, the parent MUST dismiss in the same handler that processes the delegate action:

case .destination(.presented(.editor(.delegate(.saved(let item))))):
    state.items.append(item)
    state.destination = nil    // ← REQUIRED. Without this, the sheet stays open.
    return .none

Forgetting state.destination = nil is the #1 TCA navigation bug — the save succeeds but the sheet never dismisses. For an alternative, child features can use @Dependency(\.dismiss) and call await dismiss() inside their own effects.

unimplemented(placeholder:) for non-Void test dependencies

@DependencyClient's testValue auto-generates unimplemented(...) for () -> Void closures, but returning non-Void requires a placeholder. Without it, the test crashes with "dependency has no implementation":

@DependencyClient
struct ProfileClient {
    var fetch: @Sendable (User.ID) async throws -> Profile = {
        _ in unimplemented("ProfileClient.fetch", placeholder: Profile.mock)
    }
}

The placeholder: value is used only when the test does NOT assert against fetch — it prevents the crash while still failing the test if the call is unexpected.

Confidence Checks

Before finalizing generated or refactored TCA code, verify ALL:

[ ] @Reducer macro — present on all feature structs
[ ] @ObservableState — present on all State types
[ ] State/Action — defined INSIDE @Reducer struct, not in extensions
[ ] No WithViewStore — direct store access everywhere
[ ] No Equatable on Action — removed (unnecessary since TCA 1.4)
[ ] Delegate actions — child-parent communication uses .delegate pattern
[ ] Effect closures — capture only needed values, not whole state
[ ] Cancel IDs — enum with cases, NOT empty enum types
[ ] Navigation — @Presents for modals, StackState for push nav, never nested NavigationStack
[ ] Dependencies — @DependencyClient struct-of-closures, not protocols
[ ] Tests — TestStore with exhaustive assertions, @MainActor annotated
[ ] Schedulers — `@Dependency(\.continuousClock)`, NOT `mainQueue` or `AnySchedulerOf`
[ ] iOS 16 bindings — `@Perception.Bindable` used instead of `@Bindable` when deployment target is iOS 16
[ ] Collections in State — `IdentifiedArrayOf` used instead of `[T]`
[ ] Derived state — stored property updated in reducer, NOT a computed var on State
[ ] `.run` effects — use `catch:` parameter, never `try?` inside effect closures
[ ] Alert state — `AlertState<Action>` in `@Presents`, NOT a `Bool showXAlert`
[ ] File size — new files <= 400 lines; oversized files have split task in refactoring/

Companion Skills

Project need	Companion skill	Apply when
Swift Concurrency patterns	skills/swift-concurrency/SKILL.md	Writing async effects, actor isolation, Sendable compliance
GCD/OperationQueue legacy code	skills/gcd-operations/SKILL.md	Legacy async work before migrating to TCA effects
Comprehensive testing guidance	skills/ios-testing/SKILL.md	Advanced testing patterns beyond TCA TestStore
Security audit	skills/ios-security/SKILL.md	Auditing Keychain usage, network security in TCA apps

References

Reference	When to Read
references/rules.md	Do's and Don'ts quick reference: modern TCA patterns and critical anti-patterns
references/reducer-architecture.md	@Reducer macro rules, feature decomposition, parent-child scoping, state design
references/effects.md	Effect API (.run, .send, .merge, .cancel), cancellation, long-running effects, anti-patterns
references/dependencies.md	@DependencyClient, DependencyKey, liveValue/testValue, module boundaries, test guards
references/navigation.md	Tree-based (@Presents/ifLet), stack-based (StackState/forEach), dismissal, deep linking
references/testing.md	TestStore exhaustive/non-exhaustive, TestClock, case key paths, per-feature checklist
references/migration.md	Version progression, per-feature migration checklist, syntax transformations, known issues
references/anti-patterns.md	AI-specific mistakes, god reducer signs, performance pitfalls, enterprise concerns
references/performance.md	Action costs, _printChanges, .signpost, scope performance, high-frequency action mitigation
references/refactoring-workflow.md	refactoring/ directory protocol, per-feature plans, PR sizing, phase ordering