Common vs Platform Code: expect/actual

The Idea

KMP splits code into two places:

• Common code (commonMain) — runs everywhere. Pure Kotlin, no platform APIs.
• Platform code (androidMain, iosMain) — uses Android or iOS APIs.

When common code needs a platform feature, you use the expect/actual mechanism. You declare a contract in commonMain, then provide an implementation in each platform source set.

Step 1: Declare an Interface in commonMain

The cleanest pattern is to define an interface in commonMain and implement it per platform.

// commonMain
interface Platform {
    val name: String
    fun osVersion(): String
}

class Greeting(private val platform: Platform) {
    fun greet(): String = "Running on ${platform.name} (${platform.osVersion()})"
}

Step 2: Implement on Android

// androidMain
import android.os.Build

class AndroidPlatform : Platform {
    override val name: String = "Android"
    override fun osVersion(): String = Build.VERSION.RELEASE
}

Step 3: Implement on iOS

// iosMain
import platform.UIKit.UIDevice

class IOSPlatform : Platform {
    override val name: String = UIDevice.currentDevice.systemName()
    override fun osVersion(): String = UIDevice.currentDevice.systemVersion
}

expect/actual: A Function Example

Sometimes you do not want a class. Use expect fun to declare, actual fun to implement.

// commonMain
expect fun currentTimeMillis(): Long

// androidMain
import android.os.SystemClock
actual fun currentTimeMillis(): Long = SystemClock.elapsedRealtime()

// iosMain
import platform.Foundation.NSDate
import platform.Foundation.timeIntervalSince1970
actual fun currentTimeMillis(): Long =
    (NSDate().timeIntervalSince1970 * 1000).toLong()

The compiler checks that every target has an actual. If you add a new target, you must supply it.

Diagram: commonMain declares the contract; each platform source set supplies an actual implementation.

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How to Choose

Prefer interfaces for testability. You can pass a fake Platform in tests.

Expected Output

Calling Greeting(AndroidPlatform()).greet():

Running on Android (15)

Calling Greeting(IOSPlatform()).greet():

Running on iOS (18.0)

Practice Exercise

1. In commonMain, define an interface Logger with fun log(message: String).
2. In androidMain, implement it with android.util.Log.d.
3. In iosMain, implement it with println (or NSLog).
4. Add an expect fun platformName(): String and an actual in both source sets.
5. Write a common function that prints platformName() using your Logger.