Kotlin Multiplatform

Shared Kotlin code across iOS, Android, and other platforms.

When to Use

• Sharing business logic

• iOS and Android codesharing

• Multiplatform libraries

• Native UI per platform

Quick Start

// shared/src/commonMain/kotlin/Greeting.kt class Greeting { private val platform = getPlatform()

fun greet(): String {
    return "Hello, ${platform.name}!"
}

}

expect fun getPlatform(): Platform

interface Platform { val name: String }

Core Concepts

Expected/Actual

// commonMain expect class HttpClient() { suspend fun get(url: String): String }

// androidMain actual class HttpClient { actual suspend fun get(url: String): String { return OkHttpClient.newCall(Request.Builder().url(url).build()) .await().body?.string() ?: "" } }

// iosMain actual class HttpClient { actual suspend fun get(url: String): String { return NSURLSession.sharedSession.dataTaskWithURL(NSURL(string = url)!!) } }

Architecture

// Shared ViewModel class UserViewModel(private val repository: UserRepository) { private val _user = MutableStateFlow<User?>(null) val user: StateFlow<User?> = _user.asStateFlow()

fun loadUser(id: String) {
    coroutineScope.launch {
        _user.value = repository.getUser(id)
    }
}

}

// Repository class UserRepository(private val api: ApiClient, private val db: Database) { suspend fun getUser(id: String): User { return db.getUser(id) ?: api.fetchUser(id).also { db.saveUser(it) } } }

Common Patterns

Platform-Specific UI

// Android (Compose) @Composable fun UserScreen(viewModel: UserViewModel) { val user by viewModel.user.collectAsState() user?.let { UserCard(it) } }

// iOS (SwiftUI) struct UserScreen: View { @ObservedObject var viewModel: UserViewModelWrapper

var body: some View {
    if let user = viewModel.user {
        UserCard(user: user)
    }
}

}

Build Configuration

// build.gradle.kts kotlin { androidTarget() iosX64() iosArm64() iosSimulatorArm64()

sourceSets {
    commonMain.dependencies {
        implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.8.0")
        implementation("io.ktor:ktor-client-core:2.3.0")
    }
    androidMain.dependencies {
        implementation("io.ktor:ktor-client-okhttp:2.3.0")
    }
    iosMain.dependencies {
        implementation("io.ktor:ktor-client-darwin:2.3.0")
    }
}

}

Best Practices

Do:

• Share business logic, not UI

• Use Ktor for networking

• Use SQLDelight for database

• Test in commonTest

Don't:

• Force UI sharing

• Ignore iOS memory model

• Skip platform testing

• Mix paradigms

Troubleshooting

Issue Cause Solution

iOS memory issue Freezing rules Check Kotlin/Native docs

Build error Version mismatch Align KMP versions

Type mismatch Expect/actual issue Check signature match

References

• KMP Documentation

• KMP Samples