Kotlin Multiplatform: A Comprehensive Guide to Cross-Platform Development

Kotlin Multiplatform (KMP) is revolutionizing cross-platform development by enabling developers to write shared business logic and reuse it across multiple platforms. Created by JetBrains, KMP is a powerful framework that supports Android, iOS, web, desktop, and backend applications. Unlike traditional frameworks like Flutter or React Native, KMP allows you to keep native UI elements while sharing only the business logic. This approach ensures native performance and flexibility, making it a top choice for modern app development.

What is Kotlin Multiplatform (KMP)?

Kotlin Multiplatform (KMP) is a cross-platform development framework that allows developers to share code across multiple platforms, including:

• Mobile: Android & iOS
• Web: JavaScript-based applications
• Desktop: macOS, Windows, Linux
• Backend: JVM-based server-side applications

Unlike frameworks like Flutter or React Native, which enforce a single UI, KMP focuses on sharing business logic while allowing developers to use platform-specific UI components. This ensures a native look and feel for each platform without compromising performance.

How is KMP Different from Other Cross-Platform Frameworks?

Here’s a quick comparison of KMP with other popular frameworks:

Key Benefits of Kotlin Multiplatform

KMP offers several advantages for cross-platform development:

1. Code Reusability

• Write core logic once and use it across multiple platforms.
• Reduces duplicate code and maintenance efforts.
• Ideal for business logic, networking, and data handling.

2. Native Performance

• No interpreters or virtual machines (unlike React Native or Flutter).
• Uses Kotlin/JVM for Android, Kotlin/Native for iOS, and Kotlin/JS for web.
• Compiles to optimized native binaries.

3. Flexibility

• Developers can use platform-specific APIs when needed.
• Supports the expect/actual mechanism for platform-specific implementations.

4. Seamless Interoperability

• Android: Works seamlessly with Java/Kotlin codebases.
• iOS: Can be used with Swift and Objective-C via .framework files.
• Web/Desktop: Easily integrates with JavaScript and other desktop technologies.

5. Works with Existing Projects

• Unlike other frameworks that require a full rewrite, KMP can be gradually integrated into existing Android or iOS apps.

Use Cases for Kotlin Multiplatform

KMP is versatile and can be used in various scenarios:

1. Mobile Apps (Android & iOS)

• Apps that require shared business logic but different UI on each platform.
• Example: Shared authentication, API calls, and data storage.

2. Cross-Platform Libraries

• Build reusable SDKs that work on multiple platforms.
• Example: Networking libraries, analytics SDKs, and encryption tools.

3. Desktop Applications

• Use KMP + Compose Multiplatform to create desktop applications.
• Example: Internal tools, dashboards, or admin panels.

4. Web Development

• Kotlin/JS allows writing shared logic that can run on browsers or Node.js.
• Example: Form validation logic used on both mobile and web.

5. Embedded & IoT Systems

• Kotlin/Native allows writing software for low-level systems like IoT devices.
• Example: Data collection apps in industrial automation.

How Kotlin Multiplatform Works

KMP uses three Kotlin compilers to target multiple platforms:

1. Kotlin/JVM (for Android & Backend)

• Compiles Kotlin code into Java bytecode.
• Used for Android applications and backend services (Spring Boot, Ktor).

2. Kotlin/Native (for iOS, macOS, Windows, Linux)

• Compiles Kotlin to native machine code using LLVM.
• Allows interoperability with Swift/Objective-C (iOS) and C/C++.

3. Kotlin/JS (for Web)

• Compiles Kotlin into JavaScript to run in browsers.
• Works with React, Node.js, or plain JavaScript.

Setting Up a KMP Project

A typical Kotlin Multiplatform project follows this structure:

/MyKMPProject  
  ├── /shared        (Shared logic)  
  │   ├── /src  
  │   │   ├── /commonMain   (Code shared across all platforms)  
  │   │   ├── /androidMain  (Android-specific code)  
  │   │   ├── /iosMain      (iOS-specific code)  
  │   │   ├── /jsMain       (Web-specific code)  
  │   ├── build.gradle.kts  (Gradle configuration)  
  ├── /androidApp    (Android app)  
  ├── /iosApp        (iOS app)  
  ├── /webApp        (Web app)  

Gradle Configuration

Add this to build.gradle.kts:

plugins {  
    kotlin("multiplatform") version "1.9.20"  
}  

kotlin {  
    androidTarget() // Android  
    ios()           // iOS  
    js(IR)          // Web  

    sourceSets {  
        val commonMain by getting {  
            dependencies {  
              implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.7.3")  
            }  
        }  
        val androidMain by getting  
        val iosMain by getting  
    }  
}  

Key Takeaways

• Kotlin Multiplatform (KMP) allows sharing business logic while keeping native UI.
• Uses Kotlin/JVM, Kotlin/Native, and Kotlin/JS to target multiple platforms.
• Follows a modular structure with shared and platform-specific modules.
• Uses expect/actual for platform-specific implementations.