Kotlin Multiplatform — 하나의 코드베이스로 JVM, JS, Native를 타겟팅하는 방법

┌─────────────────────────────────┐
│        commonMain               │ ← 플랫폼 무관한 순수 코틀린
│  (비즈니스 로직, 데이터 모델)    │
├─────────┬───────────┬───────────┤
│ jvmMain │  iosMain  │  jsMain   │ ← 플랫폼별 구현
│ (JVM)   │  (iOS)    │  (JS)     │
└─────────┴───────────┴───────────┘

my-kmp-project/
├── build.gradle.kts
├── shared/
│   ├── build.gradle.kts
│   └── src/
│       ├── commonMain/kotlin/    ← 공유 코드
│       ├── commonTest/kotlin/    ← 공유 테스트
│       ├── jvmMain/kotlin/       ← JVM 전용
│       ├── iosMain/kotlin/       ← iOS 전용
│       ├── jsMain/kotlin/        ← JS 전용
│       └── ...
├── androidApp/                   ← Android 앱
├── iosApp/                       ← iOS 앱 (Xcode 프로젝트)
└── webApp/                       ← 웹 앱

plugins {
    kotlin("multiplatform") version "1.9.0"
}

kotlin {
    // 타겟 플랫폼 선언
    jvm()
    iosArm64()
    iosSimulatorArm64()
    js(IR) {
        browser()
        nodejs()
    }

    sourceSets {
        val commonMain by getting {
            dependencies {
                implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.7.3")
                implementation("org.jetbrains.kotlinx:kotlinx-serialization-json:1.6.0")
                implementation("io.ktor:ktor-client-core:2.3.0")
            }
        }
        val commonTest by getting {
            dependencies {
                implementation(kotlin("test"))
            }
        }
        val jvmMain by getting {
            dependencies {
                implementation("io.ktor:ktor-client-okhttp:2.3.0")
            }
        }
        val iosMain by getting {
            dependencies {
                implementation("io.ktor:ktor-client-darwin:2.3.0")
            }
        }
        val jsMain by getting {
            dependencies {
                implementation("io.ktor:ktor-client-js:2.3.0")
            }
        }
    }
}

// commonMain — 선언만
expect fun platformName(): String

// jvmMain
actual fun platformName(): String = "JVM ${System.getProperty("java.version")}"

// iosMain
actual fun platformName(): String = "iOS ${UIDevice.currentDevice.systemVersion}"

// jsMain
actual fun platformName(): String = "JavaScript"

// commonMain
expect class UUID {
    fun toHexString(): String
}

// jvmMain
actual class UUID(private val uuid: java.util.UUID) {
    actual fun toHexString(): String = uuid.toString()

    companion object {
        fun randomUUID() = UUID(java.util.UUID.randomUUID())
    }
}

// iosMain
actual class UUID(private val uuid: platform.Foundation.NSUUID) {
    actual fun toHexString(): String = uuid.UUIDString()

    companion object {
        fun randomUUID() = UUID(platform.Foundation.NSUUID())
    }
}

// commonMain
expect class AtomicInt(value: Int) {
    fun get(): Int
    fun set(value: Int)
    fun incrementAndGet(): Int
}

// jvmMain — 기존 클래스를 그대로 사용
actual typealias AtomicInt = java.util.concurrent.atomic.AtomicInteger

// commonMain
class ApiClient(private val httpClient: HttpClient) {

    suspend fun getUser(id: Long): UserDto {
        return httpClient.get("https://api.example.com/users/$id").body()
    }

    suspend fun createUser(request: CreateUserRequest): UserDto {
        return httpClient.post("https://api.example.com/users") {
            contentType(ContentType.Application.Json)
            setBody(request)
        }.body()
    }
}

@Serializable
data class UserDto(
    val id: Long,
    val name: String,
    val email: String
)

@Serializable
data class CreateUserRequest(
    val name: String,
    val email: String
)

// jvmMain — OkHttp 엔진
fun createApiClient(): ApiClient {
    val httpClient = HttpClient(OkHttp) {
        install(ContentNegotiation) { json() }
    }
    return ApiClient(httpClient)
}

// iosMain — Darwin 엔진
fun createApiClient(): ApiClient {
    val httpClient = HttpClient(Darwin) {
        install(ContentNegotiation) { json() }
    }
    return ApiClient(httpClient)
}

// commonMain에서 UI 작성
@Composable
fun App() {
    MaterialTheme {
        var count by remember { mutableStateOf(0) }

        Column(
            modifier = Modifier.fillMaxSize(),
            horizontalAlignment = Alignment.CenterHorizontally,
            verticalArrangement = Arrangement.Center
        ) {
            Text("클릭 횟수: $count", style = MaterialTheme.typography.headlineMedium)
            Spacer(modifier = Modifier.height(16.dp))
            Button(onClick = { count++ }) {
                Text("클릭")
            }
        }
    }
}