--- name: kotlin-multiplatform-8193b6372745 description: >- Kotlin Multiplatform (KMP) is JetBrains' technology for sharing Kotlin code between Android, iOS, web, and desktop applications while keeping the UI native on each platform. Use when a user asks to set up a KMP shared module, share business logic, networking or a data layer between Android and iOS, write expect/actual declarations, use Ktor or SQLDelight in commonMain, write a shared ViewModel, test shared code, or migrate a KMP project to Android Gradle Plugin 9. license: Apache-2.0 compatibility: "Kotlin 2.4 on JDK 17 or newer; AGP 9.4 needs Gradle 9.6 or newer (AGP 9.0 needs 9.1). The Android target needs the Android SDK and AGP 9; building or testing iOS targets needs macOS with Xcode." metadata: author: terminal-skills version: 1.1.0 category: development tags: "kotlin, kmp, cross-platform, mobile, ios" repository: https://github.com/JetBrains/kotlin --- # Kotlin Multiplatform — Shared Business Logic for Mobile ## Overview Kotlin Multiplatform (KMP) compiles one Kotlin codebase for Android, iOS, desktop (JVM) and web. Business logic, networking and the data layer live in a shared module; each platform keeps its own UI (or shares it too with Compose Multiplatform). Platform differences are isolated behind `expect`/`actual` declarations. ## Instructions ### Project Structure Create the project with the Kotlin Multiplatform wizard in IntelliJ IDEA or Android Studio (New Project > Kotlin Multiplatform). Shared code and app entry points are separate modules: ``` tasklane/ ├── shared/ # Kotlin Multiplatform module (build.gradle.kts below) │ └── src/ │ ├── commonMain/kotlin/ # Platform-independent code │ ├── commonMain/sqldelight/ # .sq files (SQLDelight) │ ├── androidMain/, iosMain/, jvmMain/ # actuals per platform (iosMain comes from the default hierarchy) │ └── jvmTest/kotlin/ # Tests that run on any machine ├── androidApp/ # Android app (Jetpack Compose), depends on :shared └── iosApp/ # Xcode project (SwiftUI), links the Shared framework ``` ### Gradle Setup ```kotlin // shared/build.gradle.kts plugins { kotlin("multiplatform") version "2.4.20" kotlin("plugin.serialization") version "2.4.20" id("app.cash.sqldelight") version "2.4.0" id("com.android.kotlin.multiplatform.library") version "9.4.1" } kotlin { jvm() android { // replaces androidTarget() and the top-level android {} block namespace = "com.tasklane.shared" compileSdk = 36 minSdk = 24 } listOf(iosArm64(), iosSimulatorArm64()).forEach { target -> target.binaries.framework { baseName = "Shared" // Swift: import Shared isStatic = true } } sourceSets { commonMain.dependencies { implementation("io.ktor:ktor-client-core:3.6.0") implementation("io.ktor:ktor-client-content-negotiation:3.6.0") implementation("io.ktor:ktor-serialization-kotlinx-json:3.6.0") implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.11.0") implementation("app.cash.sqldelight:coroutines-extensions:2.4.0") implementation("org.jetbrains.androidx.lifecycle:lifecycle-viewmodel:2.11.0") } commonTest.dependencies { implementation(kotlin("test")) implementation("org.jetbrains.kotlinx:kotlinx-coroutines-test:1.11.0") implementation("io.ktor:ktor-client-mock:3.6.0") } // One HTTP engine and one SQLite driver per platform androidMain.dependencies { implementation("io.ktor:ktor-client-okhttp:3.6.0"); implementation("app.cash.sqldelight:android-driver:2.4.0") } jvmMain.dependencies { implementation("io.ktor:ktor-client-okhttp:3.6.0"); implementation("app.cash.sqldelight:sqlite-driver:2.4.0") } iosMain.dependencies { implementation("io.ktor:ktor-client-darwin:3.6.0"); implementation("app.cash.sqldelight:native-driver:2.4.0") } } } sqldelight { databases { create("TaskDatabase") { packageName.set("com.tasklane.db") } } } ``` Useful tasks: `./gradlew :shared:jvmTest` (shared tests on the JVM), `:shared:iosSimulatorArm64Test` (macOS only), `:shared:allTests`. Xcode builds the framework through a run-script phase that calls `./gradlew :shared:embedAndSignAppleFrameworkForXcode`. ### Local Database with SQLDelight ```sql -- shared/src/commonMain/sqldelight/com/tasklane/db/Task.sq -- SQLDelight generates TaskDatabase, TaskEntity and typed query functions from this file. CREATE TABLE TaskEntity ( id TEXT NOT NULL PRIMARY KEY, title TEXT NOT NULL, status TEXT NOT NULL DEFAULT 'TODO', pending_sync INTEGER NOT NULL DEFAULT 0, created_at INTEGER NOT NULL ); selectAll: SELECT * FROM TaskEntity ORDER BY created_at DESC; upsert: INSERT OR REPLACE INTO TaskEntity(id, title, status, created_at) VALUES (?, ?, ?, ?); markPendingSync: UPDATE TaskEntity SET pending_sync = 1 WHERE id = ?; ``` ### Platform-Specific Code with expect/actual ```kotlin // shared/src/commonMain/kotlin/com/tasklane/shared/Platform.kt // 'expect' declares what each platform must implement. expect fun platformName(): String expect class DriverFactory { fun createDriver(): SqlDriver } // shared/src/androidMain/kotlin/com/tasklane/shared/Platform.android.kt actual fun platformName(): String = "Android ${android.os.Build.VERSION.SDK_INT}" actual class DriverFactory(private val context: Context) { actual fun createDriver(): SqlDriver = AndroidSqliteDriver(TaskDatabase.Schema, context, "tasks.db") } // shared/src/iosMain/kotlin/com/tasklane/shared/Platform.ios.kt actual fun platformName(): String = UIDevice.currentDevice.systemName() + " " + UIDevice.currentDevice.systemVersion actual class DriverFactory { actual fun createDriver(): SqlDriver = NativeSqliteDriver(TaskDatabase.Schema, "tasks.db") } // shared/src/jvmMain/kotlin/com/tasklane/shared/Platform.jvm.kt actual fun platformName(): String = "JVM ${System.getProperty("java.version")}" actual class DriverFactory { actual fun createDriver(): SqlDriver = JdbcSqliteDriver(JdbcSqliteDriver.IN_MEMORY).also { TaskDatabase.Schema.create(it) } } ``` Imports: `app.cash.sqldelight.db.SqlDriver`, `app.cash.sqldelight.driver.android.AndroidSqliteDriver`, `app.cash.sqldelight.driver.native.NativeSqliteDriver`, `app.cash.sqldelight.driver.jdbc.sqlite.JdbcSqliteDriver`, `platform.UIKit.UIDevice`. ### Networking with Ktor ```kotlin // shared/src/commonMain/kotlin/com/tasklane/shared/TaskApi.kt // (imports from io.ktor.client.*, io.ktor.http.*, kotlinx.serialization.* omitted) @Serializable data class TaskDto(val id: String, val title: String, val status: String, val createdAt: Long) class TaskApi(baseUrl: String, authToken: String, engine: HttpClientEngine? = null) { private val config: HttpClientConfig<*>.() -> Unit = { install(ContentNegotiation) { json(Json { ignoreUnknownKeys = true }) } // don't crash on extra API fields defaultRequest { url(baseUrl) bearerAuth(authToken) contentType(ContentType.Application.Json) // required for setBody(task) } } // Without an engine argument Ktor picks the one on the classpath: OkHttp on Android, Darwin on iOS private val client = if (engine != null) HttpClient(engine, config) else HttpClient(config) suspend fun fetchTasks(): List = client.get("api/tasks").body() suspend fun createTask(task: TaskDto): TaskDto = client.post("api/tasks") { setBody(task) }.body() } ``` ### Shared Business Logic ```kotlin // shared/src/commonMain/kotlin/com/tasklane/shared/TaskRepository.kt // Runs on every target — write once, test once. Uses asFlow/mapToList from // app.cash.sqldelight.coroutines and kotlin.time.Clock, kotlin.uuid.Uuid from the standard library. class TaskRepository(private val api: TaskApi, db: TaskDatabase) { private val queries = db.taskQueries // Emits the cached list again whenever the table changes fun getTasks(): Flow> = queries.selectAll().asFlow().mapToList(Dispatchers.Default) // Pull the server state into the local cache suspend fun syncTasks() { val remote = api.fetchTasks() queries.transaction { remote.forEach { queries.upsert(it.id, it.title, it.status, it.createdAt) } } } // Save locally first; if the request fails, flag the row for a later sync suspend fun createTask(title: String): String { val task = TaskDto(Uuid.random().toString(), title, "TODO", Clock.System.now().toEpochMilliseconds()) queries.upsert(task.id, task.title, task.status, task.createdAt) try { api.createTask(task) } catch (e: Exception) { queries.markPendingSync(task.id) } return task.id } } ``` ### ViewModel (Shared UI Logic) ```kotlin // shared/src/commonMain/kotlin/com/tasklane/shared/TaskListViewModel.kt // androidx.lifecycle.ViewModel is multiplatform; viewModelScope is cancelled when the ViewModel is cleared. data class TaskListUiState(val tasks: List = emptyList(), val isLoading: Boolean = true, val error: String? = null) class TaskListViewModel(private val repository: TaskRepository) : ViewModel() { private val _uiState = MutableStateFlow(TaskListUiState()) val uiState: StateFlow = _uiState.asStateFlow() init { repository.getTasks() .onEach { tasks -> _uiState.update { it.copy(tasks = tasks, isLoading = false) } } .launchIn(viewModelScope) refresh() } fun refresh() { viewModelScope.launch { _uiState.update { it.copy(isLoading = true, error = null) } try { repository.syncTasks() } catch (e: Exception) { _uiState.update { it.copy(error = e.message) } } _uiState.update { it.copy(isLoading = false) } } } fun createTask(title: String) { viewModelScope.launch { repository.createTask(title) } } } ``` ## Examples ### Example 1: Test the shared repository without an emulator **User request:** "Add a test for the sync logic in our shared module that I can run on CI without Android or iOS devices." ```kotlin // shared/src/jvmTest/kotlin/com/tasklane/shared/TaskRepositoryTest.kt class TaskRepositoryTest { private val engine = MockEngine { request -> assertEquals("Bearer test-token", request.headers[HttpHeaders.Authorization]) respond( content = """[{"id":"t1","title":"Ship v2","status":"TODO","createdAt":1790000000000,"assignee":"mira"}]""", headers = headersOf(HttpHeaders.ContentType, "application/json"), ) } @Test fun syncStoresRemoteTasks() = runTest { val db = TaskDatabase(DriverFactory().createDriver()) // in-memory SQLite on the JVM val repository = TaskRepository(TaskApi("https://api.tasklane.dev/", "test-token", engine), db) repository.syncTasks() assertEquals(listOf("Ship v2"), repository.getTasks().first().map { it.title }) } } ``` `./gradlew :shared:jvmTest` compiles commonMain and jvmMain, generates the SQLDelight code and ends with `BUILD SUCCESSFUL`; the report is in `shared/build/reports/tests/jvmTest/index.html`. The unknown `assignee` field is ignored because of `ignoreUnknownKeys`. ### Example 2: Fix the build after upgrading to Android Gradle Plugin 9 **User request:** "After bumping AGP to 9 the build fails: The 'com.android.library' (or 'com.android.application') plugin is not compatible with the 'org.jetbrains.kotlin.multiplatform' plugin since AGP 9.0." ```diff // shared/build.gradle.kts plugins { kotlin("multiplatform") version "2.4.20" - id("com.android.library") version "9.4.1" + id("com.android.kotlin.multiplatform.library") version "9.4.1" } kotlin { - androidTarget() + android { + namespace = "com.tasklane.shared" + compileSdk = 36 + minSdk = 24 + } } -android { - namespace = "com.tasklane.shared" - compileSdk = 36 - defaultConfig { minSdk = 24 } -} ``` Then move `src/main` to `src/androidMain`, `src/test` and `src/androidUnitTest` to `src/androidHostTest`, `src/androidTest` and `src/androidInstrumentedTest` to `src/androidDeviceTest`. Android tests are off until `withHostTest {}` or `withDeviceTest {}` is added inside `android {}`. Keep the application itself (`com.android.application`, `MainActivity`) in a separate `androidApp` module that depends on `:shared`. `./gradlew :shared:tasks` now configures and lists `assembleAndroidMain`. ## Guidelines 1. **Share logic, not UI** — Share business logic, networking, data layer in Kotlin; keep UI native (Jetpack Compose on Android, SwiftUI on iOS) 2. **expect/actual for platform APIs** — Use it for file system, biometrics, notifications. Check the standard library first: `kotlin.uuid.Uuid` and `kotlin.time.Clock`/`Instant` are common code (kotlinx-datetime 0.8 no longer ships `Instant` and `Clock`). `expect class` is still Beta and warns unless `-Xexpect-actual-classes` is set; `expect fun` is stable, and a plain interface implemented per platform avoids the warning 3. **Ktor for HTTP** — Ktor is the standard multiplatform HTTP client; it uses OkHttp on Android and URLSession (the Darwin engine) on iOS 4. **SQLDelight for local DB** — SQLDelight generates type-safe Kotlin from SQL; each target needs its own driver dependency 5. **Kotlin Serialization** — Use `@Serializable` data classes; works across all platforms unlike Gson or Moshi 6. **Coroutines + Flow** — Both are multiplatform. From Swift a `suspend` function is called with a completion handler or `async`; a `Flow` has no Swift counterpart, so expose callbacks or add SKIE or KMP-NativeCoroutines 7. **Start with the shared module** — Build it with tests first (`jvmTest` needs no device); then wrap it with platform UIs. `viewModelScope` uses `Dispatchers.Main`, which a desktop JVM app gets from `kotlinx-coroutines-swing` 8. **Compose Multiplatform for UI** — If you want shared UI too, use Compose Multiplatform (covers Android, iOS, desktop, web) 9. **AGP 9** — `com.android.library` and `com.android.application` no longer work in a multiplatform module. 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See the License for the specific language governing permissions and limitations under the License. # Mega AI Hub resource metadata Creator / publisher: TerminalSkills Original project: https://github.com/TerminalSkills/skills/blob/3bfbf7f76459d89422eddc0e5f572ad0e6c393c9/skills/kotlin-multiplatform/SKILL.md Attribution basis: Source repository record Fields of work: Development & Engineering, Product Management Roles: Backend Developer, Frontend Developer, Full-Stack Developer, Mobile Developer, Product Manager Specialties: Cross Platform, Mobile Tools: Jetbrains Country applicability: Country not specified Classification basis: Source catalog labels Instruction SHA-256: c4112b9ea016dc420ed790d3c2b93f0377599b46794a9f39a77f5b90182c979b