android-kmp-shared-module

name	android-kmp-shared-module
description	KMP shared module setup — expect/actual, commonMain/androidMain/iosMain source sets, what belongs in shared vs platform. Use when adding KMP support, creating a shared module, splitting Android code for multiplatform, or working with expect/actual declarations. Trigger on: "KMP", "multiplatform", "shared module", "commonMain", "expect/actual", "iOS shared", "kotlin multiplatform".

KMP Shared Module

Gradle setup (build.gradle.kts)

plugins {
    alias(libs.plugins.kotlinMultiplatform)
    alias(libs.plugins.androidLibrary)
}

kotlin {
    androidTarget {
        compilations.all { kotlinOptions { jvmTarget = "11" } }
    }
    iosX64(); iosArm64(); iosSimulatorArm64()

    sourceSets {
        commonMain.dependencies {
            implementation(libs.kotlinx.coroutines.core)
            implementation(libs.kotlinx.serialization.json)
            implementation(libs.ktor.client.core)
        }
        androidMain.dependencies {
            implementation(libs.ktor.client.okhttp)
        }
        iosMain.dependencies {
            implementation(libs.ktor.client.darwin)
        }
    }
}

android {
    namespace = "com.example.shared"
    compileSdk = libs.versions.compileSdk.get().toInt()
}

What belongs in commonMain

Put in shared	Keep platform-specific
Domain models	Android Context usage
Repository interfaces	Room / SQLDelight Android config
Use cases	Compose UI
Ktor HTTP client calls	Android permissions
kotlinx.serialization DTOs	Platform file paths
kotlinx.coroutines flows	Firebase (Android only)
Feature flag interfaces	Play Billing

expect / actual pattern

// commonMain
expect class PlatformInfo() {
    val name: String
}

// androidMain
actual class PlatformInfo actual constructor() {
    actual val name: String = "Android ${android.os.Build.VERSION.SDK_INT}"
}

// iosMain
actual class PlatformInfo actual constructor() {
    actual val name: String = UIDevice.currentDevice.systemName()
}

Repository interface in shared, impl per platform

// commonMain — core:domain
interface TaskRepository {
    fun getTasks(): Flow<List<Task>>
    suspend fun syncTasks(): EmptyResult<DataError>
}

// androidMain — core:data (Android module)
class TaskRepositoryImpl(
    private val dao: TaskDao,         // Room — Android only
    private val api: TaskApi,
) : TaskRepository { ... }

Checklist

• No android.* imports in commonMain
• expect/actual only for truly platform-divergent behaviour
• Coroutines and serialization come from KMP-compatible artifacts
• Ktor engine injected per platform (OkHttp / Darwin)
• Domain models are plain Kotlin data classes — no Android annotations