[G1-Sync] Manual knowledge update
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Antigravity Agent
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---
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id: mobile-kmp-deep
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title: Kotlin Multiplatform — shared business logic
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category: Coding
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status: draft
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source_trust_level: B
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verification_status: conceptual
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created_at: 2026-05-09
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updated_at: 2026-05-09
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tags: [mobile, kmp, kotlin, vibe-coding]
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tech_stack: { language: "Kotlin", applicable_to: ["iOS", "Android"] }
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applied_in: []
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aliases: [KMP, Kotlin Multiplatform, KMM, Compose Multiplatform, expect/actual, shared module]
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---
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# Kotlin Multiplatform Deep
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> Kotlin 가 iOS + Android + Web + Desktop 동시. **Shared business logic + native UI**. KMM 가 mobile, Compose Multiplatform 가 UI 도. JetBrains official.
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## 📖 핵심 개념
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- Common code: Kotlin (shared).
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- Platform-specific: expect / actual.
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- iOS = Kotlin → Native (LLVM).
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- Android = Kotlin → JVM (default).
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## 💻 코드 패턴
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### Project structure
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```
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my-app/
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├── shared/ # Shared module
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│ ├── commonMain/ # Common Kotlin
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│ ├── androidMain/ # Android-only
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│ ├── iosMain/ # iOS-only
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│ └── jsMain/ # Web (optional)
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├── androidApp/ # Android app
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├── iosApp/ # iOS Xcode project
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└── build.gradle.kts
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```
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### shared/build.gradle.kts
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```kotlin
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kotlin {
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androidTarget()
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iosX64()
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iosArm64()
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iosSimulatorArm64()
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cocoapods {
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summary = "Shared module"
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homepage = "..."
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ios.deploymentTarget = "16.0"
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framework {
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baseName = "shared"
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isStatic = true
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}
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}
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sourceSets {
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commonMain.dependencies {
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implementation("io.ktor:ktor-client-core:2.3.7")
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implementation("io.ktor:ktor-client-content-negotiation:2.3.7")
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implementation("org.jetbrains.kotlinx:kotlinx-serialization-json:1.6.0")
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}
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androidMain.dependencies {
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implementation("io.ktor:ktor-client-okhttp:2.3.7")
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}
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iosMain.dependencies {
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implementation("io.ktor:ktor-client-darwin:2.3.7")
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}
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}
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}
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```
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### Shared model
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```kotlin
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// commonMain/User.kt
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@Serializable
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data class User(
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val id: String,
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val name: String,
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val email: String,
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)
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```
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### Shared API client
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```kotlin
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// commonMain/UserRepository.kt
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class UserRepository(private val client: HttpClient) {
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suspend fun fetchUser(id: String): User {
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return client.get("https://api.example.com/users/$id").body()
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}
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}
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```
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### expect / actual
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```kotlin
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// commonMain/Platform.kt
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expect fun platform(): String
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// androidMain/Platform.kt
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actual fun platform(): String = "Android ${android.os.Build.VERSION.SDK_INT}"
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// iosMain/Platform.kt
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import platform.UIKit.UIDevice
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actual fun platform(): String = "iOS ${UIDevice.currentDevice.systemVersion}"
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```
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→ Common 가 abstract, platform 가 concrete.
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### Network (Ktor)
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```kotlin
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// commonMain
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val client = HttpClient {
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install(ContentNegotiation) {
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json(Json { ignoreUnknownKeys = true })
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}
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install(Logging) { level = LogLevel.HEADERS }
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}
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suspend fun fetchUsers(): List<User> {
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return client.get("https://...").body()
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}
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```
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→ HTTP / JSON / serialization 가 common.
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### DB (SQLDelight)
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```kotlin
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// shared/build.gradle.kts
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plugins {
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id("app.cash.sqldelight") version "2.0.0"
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}
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sqldelight {
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databases {
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create("AppDb") {
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packageName.set("com.example.shared.db")
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}
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}
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}
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```
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```sql
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-- shared/src/commonMain/sqldelight/com/example/User.sq
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CREATE TABLE User (
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id TEXT PRIMARY KEY,
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name TEXT NOT NULL
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);
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selectAll:
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SELECT * FROM User;
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insert:
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INSERT OR REPLACE INTO User VALUES (?, ?);
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```
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```kotlin
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// commonMain
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val driver: SqlDriver = ... // platform 별
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val db = AppDb(driver)
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db.userQueries.insert("1", "Alice")
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val users = db.userQueries.selectAll().executeAsList()
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```
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→ Type-safe SQL. iOS / Android 둘 다.
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### Coroutines + Flow
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```kotlin
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// commonMain
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class UserViewModel(private val repo: UserRepository) {
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val users: Flow<List<User>> = flow {
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emit(repo.fetchUsers())
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}
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}
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```
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→ Flow 가 iOS 에서 도 사용 (KMP-NativeCoroutines).
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### iOS 사용 (Swift)
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```swift
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// iosApp
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import shared
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class UserListVM: ObservableObject {
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@Published var users: [User] = []
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private let repo = UserRepository(client: ...)
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func load() async {
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do {
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let result = try await repo.fetchUsers()
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await MainActor.run { self.users = result }
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} catch {
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print(error)
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}
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}
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}
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```
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→ Kotlin class 가 Swift 에서 native 처럼.
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### Async on iOS (KMP-NativeCoroutines)
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```kotlin
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// commonMain
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@NativeCoroutines
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suspend fun fetchUsers(): List<User> = ...
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```
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```swift
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// iOS
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import KMPNativeCoroutinesAsync
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let users = try await asyncFunction(for: repo.fetchUsersNative())
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```
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→ Suspend → Swift async/await.
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### Compose Multiplatform (UI도)
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```kotlin
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// commonMain (Compose UI)
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@Composable
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fun UserCard(user: User) {
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Card {
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Column(Modifier.padding(16.dp)) {
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Text(user.name, style = MaterialTheme.typography.h6)
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Text(user.email)
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}
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}
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}
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```
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→ iOS + Android 가 같은 UI.
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→ iOS Compose 가 stable (2024+).
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### vs SwiftUI / Jetpack Compose only
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```
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KMP shared logic + native UI:
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- Native 친화 (각 platform UI)
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- 작은 cost.
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Compose Multiplatform:
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- Single UI codebase
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- iOS 도 Compose
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- "iOS 답지" 의 약간
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→ Logic 만 = KMP.
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UI 도 = Compose Multiplatform.
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```
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### vs Flutter / React Native
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```
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Flutter / RN:
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- Single 언어 (Dart / JS)
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- Single UI codebase
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- Native bridge
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KMP:
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- Native 언어 (Kotlin + Swift)
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- Native UI (보통)
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- Logic 만 share
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→ KMP 가 native-friendly.
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RN/Flutter 가 빠른 dev.
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```
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### Pros
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```
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- Native iOS UX.
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- Logic share = bug 1번 fix.
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- Type-safe (Kotlin).
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- Coroutines (async).
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- Native interop.
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```
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### Cons
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```
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- iOS dev 가 Kotlin 이해.
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- Library 가 작음 (vs RN).
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- Build time (LLVM iOS = 느린).
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- Tooling 가 RN/Flutter 보다 약함.
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```
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### Real-world
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- **Cash App** (Square): 큰 KMP user.
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- **Netflix**: 일부 mobile.
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- **Philips**: hue lighting.
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- **VMware**: Workspace ONE.
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- **Touchlab**: KMM consultancy.
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### LiveData / StateFlow → Swift
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```kotlin
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// commonMain
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class ViewModel {
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val state: StateFlow<UserState> = ...
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}
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```
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```swift
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// iOS
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ViewModel().state.collect { state in
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// ...
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}
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```
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→ StateFlow 가 Combine publisher 처럼.
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### Image loading (Coil 3 multiplatform)
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```kotlin
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// Compose Multiplatform
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AsyncImage(
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model = "https://...",
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contentDescription = null,
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)
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```
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### Settings (Multiplatform)
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```kotlin
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// com.russhwolf:multiplatform-settings
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val settings: Settings = ...
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settings.putString("theme", "dark")
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val theme = settings.getString("theme", "light")
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```
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### Test
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```kotlin
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// commonTest
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class UserRepoTest {
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@Test
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fun fetchesUser() = runTest {
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val user = repo.fetchUser("1")
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assertEquals("Alice", user.name)
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}
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}
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```
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→ Common test 가 모든 platform.
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### Migration to KMP
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```
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1. Existing Android app.
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2. shared module 추가.
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3. 1 logic (e.g. API client) 이동.
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4. iOS 가 framework 사용.
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5. 점진 — 1 module 씩.
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```
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→ Strangler fig 식.
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### Build
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```bash
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# Gradle (Android)
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./gradlew :androidApp:installDebug
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# iOS
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cd iosApp && pod install
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xed iosApp.xcworkspace
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# Xcode build & run.
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# Release framework
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./gradlew :shared:linkReleaseFrameworkIosArm64
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```
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### Future
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```
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- Compose Multiplatform iOS 가 stable + 인기 ↑.
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- KMP 의 toolchain 빠름.
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- More libraries multiplatform.
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- "Kotlin everywhere".
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```
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## 🤔 의사결정 기준
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| 상황 | 추천 |
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| Native UX 중요 + share logic | KMP (Kotlin shared, Swift/Kotlin UI) |
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| Single UI codebase | Compose Multiplatform |
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| 빠른 prototype | RN / Flutter |
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| 큰 회사 + Android 강 | KMP |
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| iOS dev 가 Swift 만 | KMP shared logic only |
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| Web 도 share | KMP / RN Web |
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| Performance critical | Native or KMP |
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## ❌ 안티패턴
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- **모든 거 share (UI 까지)**: native UX 잃음.
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- **Platform-specific 가 common**: leak.
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- **Coroutines + Swift 직접**: KMP-NativeCoroutines 사용.
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- **iOS framework 가 큰**: bundle 폭발 — minimize.
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- **Only 1 platform test**: 다른 platform 깨짐.
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- **Big bang migration**: gradual.
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## 🤖 LLM 활용 힌트
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- expect / actual 가 KMP 의 핵심.
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- Ktor / SQLDelight / kotlinx 가 multiplatform-ready.
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- Compose Multiplatform = UI 까지 share.
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- KMP-NativeCoroutines 가 Swift async/await.
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## 🔗 관련 문서
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- [[Mobile_KMP_Compose]]
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- [[Mobile_Flutter_Patterns]]
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- [[Android_Kotlin_Coroutines_Scopes]]
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