Kotlin Multiplatform Development Help

Create your first cross-platform app

Here you will learn how to create and run your first Kotlin Multiplatform application using Android Studio.

Create the project from a template

  1. In Android Studio, select File | New | New Project.

  2. Select Kotlin Multiplatform App in the list of project templates, and click Next.

    Mobile Multiplatform project template
  3. Specify a name for your first application, and click Next.

    Mobile Multiplatform project - general settings
  4. In the iOS framework distribution list, select the Regular framework option.

    Mobile Multiplatform project - additional settings
  5. Keep the default names for the application and shared directories. Click Finish.

The project will be set up automatically. It may take some time to download and set up the required components when you do this for the first time.

Examine the project structure

To view the full structure of your multiplatform project, switch the view from Android to Project.

Select the Project view

Each Kotlin Multiplatform project includes three modules:

  • shared is a Kotlin module that contains the logic common for both Android and iOS applications – the code you share between platforms. It uses Gradle as the build system to help automate your build process.

  • androidApp is a Kotlin module that builds into an Android application. It uses Gradle as the build system. The androidApp module depends on and uses the shared module as a regular Android library.

  • iosApp is an Xcode project that builds into an iOS application. It depends on and uses the shared module as an iOS framework. The shared module can be used as a regular framework or as a CocoaPods dependency, based on what you've chosen in the previous step in iOS framework distribution. In this tutorial, it's a regular framework dependency.

Basic Multiplatform project structure

The shared module consists of three source sets: androidMain, commonMain, and iosMain. Source set is a Gradle concept for a number of files logically grouped together where each group has its own dependencies. In Kotlin Multiplatform, different source sets in a shared module can target different platforms.

The common source set uses common Kotlin code, and platform source sets use Kotlin code specific to each target. Kotlin/JVM is used for androidMain and Kotlin/Native for iosMain:

Source sets and modules structure

When the shared module is built into an Android library, common Kotlin code is treated as Kotlin/JVM. When it is built into an iOS framework, common Kotlin is treated as Kotlin/Native:

Common Kotlin, Kotlin/JVM, and Kotlin/Native

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 Greeting.kt file and try to access one of the Java classes, java.util.Random().nextBoolean(), inside the greet() function:

    import java.util.Random fun greet(): String { val firstWord = if (Random().nextBoolean()) "Hi!" else "Hello!" }

    Android Studio highlights that the Random class is unresolved because you can't call specific Java functions from the common Kotlin code.

  2. Follow the IDE's suggestions and replace it with 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. The code should now compile successfully.

  3. Add a bit of variety to the greeting. Update the shared code with the reversed() function 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\nGuess what it is! > ${platform.name.reversed()}!" } }

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

Add platform-specific implementations

The common source set can define an interface or an expected declaration. Then each platform source set, in this case androidMain and iosMain, has to provide actual platform-specific implementations for the expected declarations from the common source set.

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 project in Android Studio, 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.kt in androidMain module: import android.os.Build class AndroidPlatform: Platform { override val name: String = "Android ${Build.VERSION.SDK_INT}" }
    // Platform.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 platform code, namely the android.os.Build dependency. This code is written in Kotlin/JVM. If you try to access java.util.Random here, this code will compile.

    • The name property implementation from IOSPlatform uses iOS platform 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 commonMain module: expect fun getPlatform(): Platform
    // Platform.kt in androidMain module: actual fun getPlatform(): Platform = AndroidPlatform()
    // Platform.kt in iosMain module: 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 to ensure everything works.

Explore the expect/actual mechanism (optional)

The template project uses the expect/actual mechanism for functions, but it also works for most Kotlin declarations, such as properties and classes. Let's implement an expected property:

  1. Open Platform.kt in the commonMain module and add the following at the end of the file:

    expect val num: Int

    The Kotlin compiler complains that this property has no corresponding actual declarations in the platform modules.

  2. Try to provide the implementation right away with:

    expect val num: Int = 42

    You'll get an error saying that expected declarations must not have a body, in this case an initializer. The implementations must be provided in actual platform modules. Remove the initializer.

  3. Select the num property. Press Option + Enter and choose "Create actual property for module ModuleName.shared.main (JVM)". IDE generates the actual property in androidMain/Platform.kt. You can then complete the implementation:

    actual val num: Int = 1
  4. Now provide the implementation for the iosMain module. Add the following to iosMain/Platform.kt:

    actual val num: Int = 2
  5. Add the num property to the greet() function to see the differences:

    fun greet(): String { val firstWord = if (Random.nextBoolean()) "Hi!" else "Hello!" return "$firstWord [$num]\nGuess what it is! > ${platform.name.reversed()}!" }

Run your application

You can run your multiplatform application for both Android or iOS from Android Studio.

Run your application on Android

  1. Create an Android virtual device.

  2. In the list of run configurations, select androidApp.

  3. Choose your Android virtual device and click Run.

    Run multiplatform app on Android
    First mobile multiplatform app on Android

Run your application on iOS

  1. Launch Xcode in a separate window. The first time you do this, you may also need to accept the license terms and allow Xcode to perform some necessary initial tasks.

  2. In Android Studio, select iosApp in the list of run configurations and click Run.

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

    Run multiplatform app on iOS
    First mobile multiplatform app on iOS

Run on a new iOS simulated device

If you want to run your application on a simulated device, you can add a new run configuration.

  1. In the list of run configurations, click Edit Configurations.

    Edit run configurations
  2. Click the + button above the list of configurations and select iOS Application.

    New run configuration for iOS application
  3. Name your configuration.

  4. Select the Xcode project file. To do so, navigate to your project, for example KotlinMultiplatformSandbox, open theiosApp folder, and select the .xcodeproj file.

  5. In the Execution target list, select a simulated device and click OK.

    New run configuration with iOS simulator
  6. Click Run to run your application on the new simulated device.

Run on a real iOS device

  1. Connect a real iPhone device to Xcode.

  2. Make sure to code sign your app. For more information, see the official Apple documentation.

  3. Create a run configuration by selecting an iPhone in the Execution target list.

  4. Click Run to run your application on the iPhone device.

Next step

In the next part of the tutorial, you'll learn how to update the UI elements using platform-specific libraries.

Proceed to the next part

See also

Get help

Last modified: 01 November 2023