Kotlin Multiplatform

Kotlin Multiplatform (KMP) is a technology developed by JetBrains that enables writing shared code across multiple platforms while maintaining the flexibility to access native APIs on each system. Unlike traditional cross-platform solutions, KMP adopts a hybrid approach where only business logic is shared, leaving the user interface native to each platform. This architecture ensures optimal performance and an authentic user experience on each target environment.

Fundamentals of Kotlin Multiplatform

• Modular architecture enabling business logic sharing (models, logic, networking, persistence) while keeping UI layers native
• Compilation to target platforms via Kotlin/JVM for Android, Kotlin/Native for iOS, and Kotlin/JS for web
• expect/actual mechanism allowing definition of common interfaces and their platform-specific implementations
• Native interoperability with Java on Android and Objective-C/Swift on iOS without performance overhead

Strategic benefits

• Up to 40% reduction in development time through business logic sharing across platforms
• Simplified maintenance with a single codebase for application logic, reducing inconsistency risks
• Guaranteed native performance as each platform compiles to its native code (no JavaScript bridge)
• Progressive adoption possible by gradually integrating KMP into existing applications
• Mature ecosystem with multiplatform libraries (Ktor, SQLDelight, Kotlinx.serialization) and official Google support

Practical architecture example

Here's a typical Kotlin Multiplatform project structure with shared module and platform-specific implementations:

// shared/src/commonMain/kotlin/Repository.kt
class UserRepository(private val api: ApiClient) {
    suspend fun getUser(id: String): Result<User> {
        return try {
            val response = api.fetchUser(id)
            Result.success(response.toUser())
        } catch (e: Exception) {
            Result.failure(e)
        }
    }
}

// shared/src/commonMain/kotlin/Platform.kt
expect class PlatformLogger() {
    fun log(message: String)
}

// shared/src/androidMain/kotlin/PlatformLogger.kt
import android.util.Log

actual class PlatformLogger {
    actual fun log(message: String) {
        Log.d("KMP", message)
    }
}

// shared/src/iosMain/kotlin/PlatformLogger.kt
import platform.Foundation.NSLog

actual class PlatformLogger {
    actual fun log(message: String) {
        NSLog("KMP: $message")
    }
}

Implementing a KMP project

1. Configure the project with Kotlin Multiplatform plugin in build.gradle.kts defining targets (Android, iOS, JVM, JS)
2. Structure architecture in modules: commonMain for shared code, androidMain/iosMain for platform-specific implementations
3. Implement shared data layer (API clients, models, repositories) using multiplatform libraries
4. Define expect interfaces for platform-dependent features (storage, logging, analytics)
5. Develop native UIs (Jetpack Compose for Android, SwiftUI for iOS) consuming the shared module
6. Set up testing with commonTest for shared logic and platform-specific tests
7. Configure CI/CD to compile and distribute across all target platforms

Essential tools and libraries

• Ktor: multiplatform HTTP client for asynchronous network calls
• SQLDelight: type-safe code generation for multiplatform SQL databases
• Kotlinx.serialization: native JSON serialization/deserialization without reflection
• Kotlinx.coroutines: concurrency and asynchronous operations management
• Koin or Kodein: multiplatform dependency injection
• Compose Multiplatform: experimental extension enabling UI sharing (under active development)
• KMP-NativeCoroutines: improved coroutines interoperability with Swift/Objective-C

Kotlin Multiplatform represents a major evolution in mobile and multiplatform development, offering an optimal balance between code reuse and native performance. With official Google support for Android and growing adoption by companies like Netflix, VMware, and Philips, KMP emerges as a pragmatic alternative to JavaScript cross-platform frameworks, particularly suited for teams seeking native performance without compromising user experience. Investment in this technology guarantees significant reduction in development and maintenance costs while preserving delivery quality and velocity.