Kotlin Multiplatform

Kotlin Multiplatform (KMP) lets you share code between platforms — Android, iOS, desktop, web, and server — while still writing platform-specific code where it matters. Unlike some cross-platform approaches, you decide how much to share: just business logic, or the user interface too.

How a KMP project is organized

shared/
└── src/
    ├── commonMain/kotlin/     # code for every platform
    ├── androidMain/kotlin/    # Android-specific code
    └── iosMain/kotlin/        # iOS-specific code (compiled to native)
androidApp/                    # the Android app, uses :shared
iosApp/                        # the Xcode project, uses the shared framework

Code in commonMain can use only multiplatform libraries — the Kotlin standard library, kotlinx.coroutines, kotlinx.serialization, kotlinx-datetime, Ktor Client, SQLDelight, and many more.

expect and actual

When common code needs a platform feature, declare it with expect and implement it per platform with actual:

// commonMain
expect fun platformName(): String

class Greeting {
    fun greet() = "Hello from ${platformName()}!"
}

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

// iosMain
import platform.UIKit.UIDevice

actual fun platformName(): String =
    UIDevice.currentDevice.systemName() + " " + UIDevice.currentDevice.systemVersion

The iOS version calls Apple's UIKit directly — Kotlin/Native can use iOS frameworks. In many cases, an interface in common code with platform-specific implementations (passed in through dependency injection) is a simpler alternative to expect/actual.

Calling shared code from Swift

The shared module compiles to an iOS framework, which Swift imports like any other:

import Shared

let message = Greeting().greet()

Compose Multiplatform

Compose Multiplatform (from JetBrains) extends Jetpack Compose to iOS, desktop, and web, so the UI can be shared as well. Teams choose between sharing only logic (native UI on each platform, written in SwiftUI and Compose) or sharing both.

When to use it

KMP suits teams that ship on Android and iOS and want one implementation of networking, validation, and business rules, without giving up native performance or platform APIs. Start small — share one module, such as the networking layer — and expand as the team gains confidence.