kmp-feature-slice

KMP Feature Slice Generator

Procedural workflow for generating complete KMP feature slices. Enforces strict creation order, typed error handling, and compose-arch compliance.

Prerequisites: Read compose-arch skill first — it is the SINGLE SOURCE OF TRUTH for architecture rules.

Companion skills (versions live there — do not duplicate)

• Architecture rules + Component/View/Screen split — compose-arch.
• DI annotations (@BindingContainer, @Provides, @AssistedInject) — metro-di-mobile (Metro 1.0.0).
• Navigation primitives (Value<T>, ChildStack) — decompose.
• HTTP — ktor-client (3.4.3).

When generating a slice, always read these companion skills first for current versions and idioms; never inline a version here.

Project conventions (project-level glue, NOT framework)

This skill references three helpers that are project conventions, not Decompose/Metro/Kotlin APIs. Define them once in your :core:* modules (or substitute equivalents) before following the steps. If your project already has equivalents under different names, mentally remap.

Helper	Lives in	Minimal definition / stand-in
componentScope()	:core:component (typical)	Extension on ComponentContext returning a CoroutineScope tied to component lifecycle. Stand-in: Decompose Essenty's coroutineScope() from essenty-coroutines-extensions — same semantics.
runCatchingApp { ... }	:core:result (typical)	Project-level runCatching variant that maps Throwable to AppResult.Failure (typed-error mapping inside). Stand-in: plain kotlin.runCatching { ... } returning kotlin.Result<T>.
AppResult<T>	:core:result (typical)	Project sealed result type — usually sealed interface AppResult<out T> { data class Success<T>(val value: T); data class Failure(val error: AppError) }. Stand-in: kotlin.Result<T>.

If these don't exist yet in the project, create them once in :core:component / :core:result (or whatever module convention the host project uses) before generating feature slices. Do NOT inline them per-feature.

Below, treat any reference to componentScope() / runCatchingApp / AppResult as a call into these conventions — substitute the stand-ins listed above if the project hasn't adopted them.

Phase 1: Input Collection (LOW FREEDOM)

Collect these inputs before generating any code:

Input	Required	Example
Feature name	Yes	OrderHistory
Data sources	Yes	remote-only, local+remote, local-only
Target platforms	Yes	android,ios,desktop, android,ios,desktop,wasm, all
Error types	Yes	network,validation, network,auth,conflict
Has list/detail?	Yes	list-only, detail-only, list+detail
Navigation	Yes	stack (full screen), slot (dialog/modal), none
Needs pagination?	No	true/false (default: false)
Parent module path	Yes	feature/order-history

Error Type Catalog

Select from these typed error categories (see references/error-patterns.md for sealed class templates):

Error Type	When to Use
network	API calls, timeouts, connectivity
validation	User input, form data
auth	Token expired, unauthorized
conflict	Concurrent modification, duplicate
not-found	Missing resource
storage	Database, file system errors
permission	OS-level permissions (camera, location)

Phase 2: File Manifest (ZERO FREEDOM)

Based on inputs, generate the exact file list. Every feature slice follows this structure:

Mandatory Files (always created)

feature/<name>/
├── api/
│   └── src/commonMain/kotlin/
│       ├── <Name>Component.kt          # Step 1: Interface
│       ├── <Name>Models.kt             # Step 2: Domain models + error types
│       └── <Name>Repository.kt         # Step 3: Repository interface (if data sources != none)
│
└── impl/
    └── src/commonMain/kotlin/
        ├── component/
        │   └── Default<Name>Component.kt   # Step 7: Component implementation
        ├── domain/
        │   ├── usecase/
        │   │   └── Get<Name>UseCase.kt     # Step 5: Primary use case
        │   └── repository/
        │       └── <Name>RepositoryImpl.kt # Step 6: Repository implementation
        ├── data/
        │   └── datasource/
        │       └── <Name>RemoteDataSource.kt  # Step 4: Remote data source
        ├── view/
        │   ├── <Name>ViewState.kt          # Step 8: View state
        │   ├── <Name>ViewEvent.kt          # Step 9: View events
        │   └── <Name>View.kt              # Step 10: View (pure UI)
        ├── screen/
        │   └── <Name>Screen.kt            # Step 11: Screen (thin adapter)
        └── di/
            └── <Name>Module.kt            # Step 12: DI module

Conditional Files

Condition	Additional Files	Insert at step
data-sources: local+remote	<Name>LocalDataSource.kt	Step 4b — same step as RemoteDataSource
local storage uses DataStore (e.g. Settings-style features)	commonMain declares expect factory + interface; platform sourceSets supply actuals — typically androidMain/.../<Name>Settings.android.kt and iosMain/.../<Name>Settings.ios.kt (and desktopMain / wasmJsMain if targeted). Do NOT put DataStore construction in commonMain.	Step 4b — same step as LocalDataSource
has: list+detail	Container Component (<Name>Component.kt) owning ChildStack<Config, Child> + two child Components: <Name>ListComponent.kt and <Name>DetailComponent.kt (each with its own viewState/Event/View/Screen). Impl side mirrors with Default<Name>Component, Default<Name>ListComponent, Default<Name>DetailComponent.	Container interface at Step 1; child interfaces also at Step 1; container impl at Step 7 (drives ChildStack, exposes child factories); child impls at Step 7 (each owns its own state/use cases). View/State/Event/Screen for each child at Steps 8-11.
pagination: true	<Name>Pager.kt in impl/domain/	Step 5b — between Repository (Step 5) and Component (Step 7)
navigation: stack	Navigation config (Config sealed class + ChildStack) in <Name>Component.kt and Default<Name>Component.kt	Steps 1 + 7
navigation: slot	Slot config in <Name>Component.kt and Default<Name>Component.kt	Steps 1 + 7

Component owning navigation alongside state. A Component can expose BOTH val viewState: Value<T> AND val childStack: Value<ChildStack<Config, Child>>. They are independent fields.

List+detail uses the container pattern, not one Component with two states. The container Component holds only the ChildStack and child factories — no viewState of its own. Each child Component (List, Detail) owns its own viewState, events, and use cases. This keeps each child testable in isolation and matches Decompose's Children { } rendering. A real example:

// Container — navigation only, no viewState
interface OrderHistoryComponent {
    val childStack: Value<ChildStack<*, Child>>
    sealed class Child {
        data class List(val component: OrderHistoryListComponent) : Child()
        data class Detail(val component: OrderHistoryDetailComponent) : Child()
    }
}

// Each child is a full Component with its own viewState
interface OrderHistoryListComponent {
    val viewState: Value<OrderHistoryListViewState>
    fun obtainEvent(event: OrderHistoryListEvent)
}

Avoid the anti-pattern of a single Component carrying viewState: Value<ListState> + detailViewState: Value<DetailState> + manual show/hide flags — that scales poorly and breaks the one-Component-one-screen mental model.

Gradle Files

feature/<name>/
├── api/
│   └── build.gradle.kts               # Step 13: API module build
└── impl/
    └── build.gradle.kts               # Step 14: Impl module build

Update settings.gradle.kts — Step 15.

Phase 3: Generation Order (STRICT SEQUENCE)

Generate files in this exact order. Each step depends on previous steps.

Step 1: Component Interface (api/)

interface <Name>Component {
    val viewState: Value<<Name>ViewState>
    fun obtainEvent(event: <Name>Event)

    fun interface Factory {
        fun create(
            componentContext: ComponentContext,
            onNavigate: (<NavigationType>) -> Unit  // from navigation input
        ): <Name>Component
    }
}

Step 2: Domain Models + Typed Errors (api/)

• Domain data classes
• Sealed error hierarchy (see references/error-patterns.md)
• Keep models serializable if used in navigation configs

Step 3: Repository Interface (api/)

• Return AppResult<T> for all operations
• Define typed error mapping

Step 4: Data Sources (impl/data/)

• Remote data source with Ktor client
• (Optional) Local data source with Room/DataStore
  • DataStore needs platform-specific construction → declare expect fun create<Name>DataStore(): DataStore<Preferences> in commonMain and provide actual impls in androidMain (<Name>Settings.android.kt), iosMain (<Name>Settings.ios.kt), and any other targeted platform. The interface stays in commonMain; only the factory is expect/actual.
• Map API DTOs to domain models here

Step 5: Use Cases (impl/domain/usecase/)

• Single execute() function returning Result<T>
• All error handling and mapping here
• Reference error-patterns.md for typed error creation

Step 6: Repository Implementation (impl/domain/repository/)

• Coordinate data sources
• Cache strategy (if local+remote)

Step 7: Component Implementation (impl/component/)

• @Inject + @Assisted pattern
• Value<T> for state (NOT StateFlow)
• componentScope() for coroutines
• Event handling via obtainEvent()
• Navigation via Decompose callbacks

Step 8-9: ViewState + ViewEvent (impl/view/)

• Sealed class for ViewState: Loading, Success, Error (+ Empty if list)
• Sealed class for ViewEvent: all user interactions

Step 10: View (impl/view/)

• Pure UI, only viewState + eventHandler params
• NO remember, NO side effects
• See references/compose-ui-templates.md for templates

Step 11: Screen (impl/screen/)

• Thin adapter: subscribe to component state, pass to View
• Target ~20 lines for a single-view screen. With list+detail and an internal Children { when } block, ~25–30 lines is acceptable — the rule is "no logic", not a hard line count.

Step 12: DI Module (impl/di/)

• @BindingContainer with @Provides bindings — one module per feature.
• For each interface declared in api/ whose impl lives in impl/, prefer @DefaultBinding(<ApiInterface>::class) on the impl class over a hand-written @Provides — Metro auto-binds and you avoid one boilerplate function per interface.
• Use @Provides in the module only when:
  • the binding requires plumbing several deps together (e.g. wrapping a HttpClient into a feature-scoped service), OR
  • the binding wires an api-side Factory to its impl-side counterpart (e.g. mapping OrderHistoryComponent.Factory → DefaultOrderHistoryComponent.Factory).
• object vs class @BindingContainer: both work identically in Metro 1.0. Default to object (no instance state, matches common KMP convention) unless the host project uses class. Canonical example:

@BindingContainer
object OrderHistoryModule {
    // Use cases + repository auto-bind via @Inject / @DefaultBinding — nothing needed here.

    // @Provides only when wiring api Factory ← impl Factory or composing several deps.
    @Provides
    fun provideComponentFactory(
        impl: DefaultOrderHistoryComponent.Factory,
    ): OrderHistoryComponent.Factory = impl
}

Step 13-14: Gradle Build Files

• API module: kotlin multiplatform only, no compose. Apply the Metro plugin if you want use cases to carry @Inject.
• Impl module: kotlin multiplatform + compose + decompose + Metro plugin.

Cross-target api note

Metro 1.0 supports the full KMP target matrix — including js(IR), wasmJs, wasmWasi, and all native targets (see metro-di-mobile skill's "Supported KMP targets" table). So an api/ module that targets android+ios+desktop+js+wasmJs can still apply the Metro plugin and use @Inject on its use cases. Web frontends consuming the api module will compile fine.

The only reason to keep an api/ module Metro-free is when the consumer doesn't run Metro at all — for example, a non-KMP Kotlin/JS app that pulls in only the api artifact and wires use cases by hand. In that case:

• Use cases in api/ stay as plain Kotlin classes (no annotations).
• The impl/ module provides them via @Provides in <Name>Module.kt for the mobile path.
• The web target instantiates use cases manually (GetOrderHistoryUseCase(JsOrderHistoryRepository())) at the call site.

Default to "Metro everywhere" unless you have a concrete reason to opt out.

Consumer-side graph: two-graph wiring

Once the api module carries @Inject annotations, each consumer (mobile vs web) defines its own @DependencyGraph and reuses the same api use cases. The two graphs differ only in which impl modules they pull in.

Mobile graph — pulls the impl module (with Decompose, real DataSources, full Compose UI):

// composeApp/.../di/AndroidAppGraph.kt
@DependencyGraph(
    bindingContainers = [
        NetworkModule::class,
        AndroidPlatformModule::class,
        OrderHistoryModule::class,           // impl-side @BindingContainer
    ]
)
interface AndroidAppGraph {
    val orderHistoryComponentFactory: OrderHistoryComponent.Factory
}

Web graph — only the api module + a web-specific repo impl. No Decompose, no impl module:

// webApp/.../di/WebAppGraph.kt
@DependencyGraph(
    bindingContainers = [WebPlatformModule::class],
)
interface WebAppGraph {
    val getOrderHistoryPageUseCase: GetOrderHistoryPageUseCase
    val getOrderDetailUseCase: GetOrderDetailUseCase
    val orderHistoryRepository: OrderHistoryRepository
}

// webApp/.../orderhistory/JsOrderHistoryRepository.kt
@Inject
@DefaultBinding(OrderHistoryRepository::class)
class JsOrderHistoryRepository(
    private val httpClient: HttpClient,
) : OrderHistoryRepository {
    override suspend fun getPage(cursor: String?): AppResult<OrderPage> = /* ... */
    override suspend fun getDetail(id: String): AppResult<OrderDetail> = /* ... */
}

The use cases (GetOrderHistoryPageUseCase, GetOrderDetailUseCase) live in api with @Inject and resolve from constructor scanning — no @Provides needed in the web module. The @DefaultBinding on JsOrderHistoryRepository plugs the api interface. The web graph deliberately excludes OrderHistoryModule (impl side) since it pulls Decompose lifecycle types React doesn't need.

For createGraph lifetime on JS (top-level lazy vs React Context provider), see metro-di-mobile's "JS / WASM specifics" section.

Step 15: Settings Update

• Add both modules to settings.gradle.kts
• Add impl dependency to composeApp/build.gradle.kts

Phase 4: Validation Checklist

After generation, verify ALL of these:

• compose-arch compliance: Screen has no logic, View has no remember
• Typed errors: All error types from input are covered in sealed hierarchy
• One class per file: No file contains multiple classes
• Value state: Component uses Value<T>, not StateFlow
• Result returns: All use cases return Result<T>
• AppResult in repository: Repository interface returns AppResult<T>
• No Compose imports in Component: Component has zero compose dependencies
• Platform targets match input: Build file targets match requested platforms
• DI bindings complete: All interfaces bound in Module
• Navigation wired: Component factory registered in parent navigation
• Build passes: ./gradlew :<module>:assemble succeeds

Phase 5: Build Verification

# Verify API module compiles
./gradlew :feature:<name>:api:assemble

# Verify impl module compiles
./gradlew :feature:<name>:impl:assemble

# Verify full app compiles with new feature
./gradlew :composeApp:assemble

Reference Files

Load these on demand — do NOT read all upfront:

Reference	When to Load
references/error-patterns.md	Step 2 (domain models + errors)
references/compose-ui-templates.md	Steps 8-11 (UI layer)
references/kotlin-web-templates.md	When target includes wasm or js

Integration with Other Skills

Skill	Relationship
compose-arch	SOURCE OF TRUTH for architecture rules. This skill adds generation order on top.
decompose	Navigation patterns. Load for Step 7 (component) if complex navigation.
metro-di-mobile	DI patterns. Load for Step 12 (module).
kmp	Project structure. Load for Steps 13-15 (gradle).
kotlin-web	Web-specific patterns. Load when targets include wasm/js.