Create your Kotlin Multiplatform app

Write common declarations

The common source set contains shared code that can be used across multiple target platforms. It's designed to contain code that is platform-independent. If you try to use platform-specific APIs in the common source set, the IDE will show a warning:

1. Open the shared/src/commonMain/kotlin/com/jetbrains/greeting/greetingkmp/Greeting.kt file where you can find an automatically generated Greeting class with a greet() function:

   class Greeting { private val platform = getPlatform() fun greet(): String { return "Hello, ${platform.name}!" } }

2. Add a bit of variety to the greeting. Import kotlin.random.Random from the Kotlin standard library. This is a multiplatform library that works on all platforms and is included automatically as a dependency.

3. Update the shared code with the reversed() call from the Kotlin standard library to reverse the text:

   import kotlin.random.Random class Greeting { private val platform: Platform = getPlatform() fun greet(): String { val firstWord = if (Random.nextBoolean()) "Hi!" else "Hello!" return "$firstWord Guess what this is! > ${platform.name.reversed()}!" } }

Writing the code only in common Kotlin has obvious limitations because it can't use any platform-specific functionality. Using interfaces and the expect/actual mechanism solves this.

Check out platform-specific implementations

The common source set can define expected declarations (interfaces, classes, and so on). Then each platform source set, in this case androidMain and iosMain, has to provide actual platform-specific implementations for the expected declarations.

While generating the code for a specific platform, the Kotlin compiler merges expected and actual declarations and generates a single declaration with actual implementations.

1. When creating a Kotlin Multiplatform project with IntelliJ IDEA, you get a template with the Platform.kt file in the commonMain module:

   interface Platform { val name: String }

   It's a common Platform interface with information about the platform.

2. Switch between the androidMain and the iosMain modules. You'll see that they have different implementations of the same functionality for the Android and the iOS source sets:

   // Platform.android.kt in the androidMain module: import android.os.Build class AndroidPlatform : Platform { override val name: String = "Android ${Build.VERSION.SDK_INT}" }
   // Platform.ios.kt in the iosMain module: import platform.UIKit.UIDevice class IOSPlatform: Platform { override val name: String = UIDevice.currentDevice.systemName() + " " + UIDevice.currentDevice.systemVersion }

   • The name property implementation from AndroidPlatform uses the Android-specific code, namely the android.os.Build dependency. This code is written in Kotlin/JVM. If you try to access a JVM-specific class, such as java.util.Random here, this code will compile.

   • The name property implementation from IOSPlatform uses iOS-specific code, namely the platform.UIKit.UIDevice dependency. It's written in Kotlin/Native, meaning you can write iOS code in Kotlin. This code becomes a part of the iOS framework, which you will later call from Swift in your iOS application.

3. Check the getPlatform() function in different source sets. Its expected declaration doesn't have a body, and actual implementations are provided in the platform code:

   // Platform.kt in the commonMain source set expect fun getPlatform(): Platform
   // Platform.android.kt in the androidMain source set actual fun getPlatform(): Platform = AndroidPlatform()
   // Platform.ios.kt in the iosMain source set actual fun getPlatform(): Platform = IOSPlatform()

Here, the common source set defines an expected getPlatform() function and has actual implementations, AndroidPlatform() for the Android app and IOSPlatform() for the iOS app, in the platform source sets.

While generating the code for a specific platform, the Kotlin compiler merges expected and actual declarations into a single getPlatform() function with its actual implementations.

That's why expected and actual declarations should be defined in the same package – they are merged into one declaration in the resulting platform code. Any invocation of the expected getPlatform() function in the generated platform code calls a correct actual implementation.

Now you can run the apps and see all of this in action.

Run your application on Android

1. In the list of run configurations, select composeApp.

2. Choose an Android virtual device next to the list of configurations and click Run.

   If you don't have a device in the list, create a new Android virtual device.

   

   

Run your application on iOS

If you haven't launched Xcode as part of the initial setup, do that before running the iOS app.

In IntelliJ IDEA, select iosApp in the list of run configurations, select a simulated device next to the run configuration, and click Run.

If you don't have an available iOS configuration in the list, add a new run configuration.

See also

• See how to create and run multiplatform tests to check that the code works correctly.

• Learn more about the project structure.

• If you want to convert your existing Android project into a cross-platform app, complete this tutorial to make your Android app cross-platform.