Kotlin Multiplatform

Shared code with platform-specific edges

Kotlin Multiplatform (KMP) lets you write common code once, typically business logic, networking, data and validation, and compile it for the JVM, Android, iOS (via Kotlin/Native), desktop and web. Code lives in source sets such as commonMain, androidMain and iosMain. When common code needs a platform API it declares an expect function or class and each platform provides an actual implementation. Multiplatform libraries such as kotlinx.coroutines, kotlinx.serialization, Ktor client and SQLDelight work in common code. Compose Multiplatform from JetBrains extends Compose UI to other platforms. Support levels differ per target, so check the docs for your version.

expect and actual

Common declaration, platform implementations.

// commonMain
expect fun platformName(): String

class Greeter {
    fun greet(): String = "Hello from ${platformName()}"
}

// androidMain
actual fun platformName(): String = "Android ${android.os.Build.VERSION.SDK_INT}"

// iosMain
actual fun platformName(): String =
    platform.UIKit.UIDevice.currentDevice.systemName()

Share logic first, UI later

A common adoption path is sharing domain and data code while keeping native UIs, then deciding later whether shared UI with Compose Multiplatform is worth it.

Quick check: How does common KMP code access a platform-specific API?

• It declares an `expect` declaration with `actual` implementations per platform
• It uses reflection at runtime
• It cannot; common code is pure math only
• It calls Java directly on every platform