Kotlin Multiplatform: Sharing Code Between iOS and Android Without the Headaches

Beyond the Cross-Platform Compromise

For a long time, the mobile development landscape presented a stark choice: build two separate, native applications for maximum performance and platform fidelity, or adopt a cross-platform framework that often sacrificed one or both. Technologies like React Native or Flutter bundle a runtime and render UI through their own abstractions, which can lead to performance bottlenecks and challenges in achieving a truly native look and feel.

Kotlin Multiplatform takes a fundamentally different approach. Instead of trying to share everything, including the UI, it focuses on what makes the most sense: the business logic. You write your data models, validation rules, API networking, database interactions, and other platform-agnostic code once in Kotlin. This shared module then compiles natively—to JVM bytecode for Android and to native frameworks (via the Kotlin/Native compiler) for iOS. The user interface remains fully native, built with SwiftUI/Jetpack Compose or UIKit/Views, giving you the best of both worlds.

How Kotlin Multiplatform Works: The Architecture

The core of a KMP project is the shared module, written in pure Kotlin. This module contains the common code intended to run on all target platforms. However, Kotlin is a pragmatic language, and KMP embraces this by allowing for platform-specific implementations where necessary.

Expect/Actual Declarations

This is the genius mechanism that handles platform differences elegantly. In your shared common code, you declare an expected function or property.

// In commonMain source set expect fun getCurrentTime(): Long

Then, in the platform-specific source sets (e.g., androidMain and iosMain), you provide the actual implementation.

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

This pattern allows you to maintain a clean, shared API while leveraging the unique strengths of each operating system under the hood.

The Tangible Benefits: Why Developers Are Adopting KMP

• Uncompromised Native UI: Your apps use Swift and Kotlin for their respective interfaces, ensuring perfect platform integration, access to the latest OS features, and optimal performance for user interactions.
• Massive Code Reuse: Studies and real-world cases show teams can share 70-80% of their codebase (logic, networking, caching), dramatically reducing development time, bugs, and effort for bug fixes and feature updates.
• Team Flexibility & Skill Utilization: iOS and Android developers can continue working with their familiar native tools and languages. They collaborate on the shared Kotlin logic, which has a gentle learning curve, especially for Android developers.
• Gradual Adoption: You don't need to bet your entire company on it. Start by sharing a simple utility or your API models. You can migrate pieces of your logic incrementally, reducing risk.
• Performance: Since the shared code compiles to native binaries, there's no interpreter or bridge overhead for the logic layer. It runs at full speed.

Addressing the Real Concerns: Is It Production Ready?

KMP has evolved rapidly. With the release of Kotlin 1.9.20 and the stabilization of the new K2 compiler, the technology has reached a significant milestone in stability and performance.

The iOS Integration Story

A common concern is iOS integration. The shared Kotlin module compiles into a native .framework (for iOS) or a .xcframework (for macOS and iOS). This framework can be added to your Xcode project like any other dependency. Your Swift code calls into it directly. Tooling like the Kotlin Multiplatform Mobile (KMM) plugin for Android Studio and IntelliJ IDEA streamlines this process, and CocoaPods integration is also well-supported.

Maturity and Community

Major companies like Netflix, Philips, McDonald's, and Cash App are using KMP in production. The ecosystem is growing, with excellent libraries from JetBrains (like kotlinx.coroutines, kotlinx.serialization) and a vibrant community creating multiplatform libraries for HTTP (Ktor), dependency injection (Koin), and SQLite (SQLDelight).

Getting Started Without the Headache

1. Start Small: Identify a low-risk, logic-heavy component in your app, like API client networking, date formatting utilities, or complex validation rules.
2. Use the Right Tools: Install the KMM plugin in Android Studio. It provides a wizard to create a new multiplatform project with both Android and iOS targets correctly configured.
3. Leverage the Documentation: The official Kotlin Multiplatform documentation is comprehensive and includes hands-on tutorials.
4. Embrace the Model: Architect your shared code to be purely about business logic. Keep all UI, platform-specific services (like deep linking, notifications), and hardware interactions in the native layers.

Conclusion: A Pragmatic Path Forward

Kotlin Multiplatform is not a silver bullet that makes platform-specific knowledge obsolete. Instead, it is a pragmatic and powerful tool that respects the strengths of native development while eliminating the massive redundancy of writing the same business logic twice. It reduces bugs, accelerates feature development, and aligns team efforts—all while delivering the high-quality, performant native applications that users expect. For teams looking to optimize their mobile development process without the traditional cross-platform compromises, KMP offers a compelling and headache-reducing solution.