Kotlin Multiplatform (KMP)

Kotlin Multiplatform (KMP) is a cross-platform approach that shares Kotlin business logic across iOS, Android, desktop, web, and server while letting each platform keep its native UI. Unlike fully cross-platform UI frameworks, KMP focuses on sharing the layers below the UI — networking, data access, validation, and domain logic — compiled to each target. JetBrains' Compose Multiplatform optionally shares the UI layer too, for teams that want fuller reuse.

Components

• Kotlin Multiplatform: compiles shared Kotlin to JVM (Android), native (iOS via Kotlin/Native), JS/Wasm (web), and server targets, using the expect/actual mechanism for platform-specific implementations.
• Shared modules: common networking (Ktor), JSON serialization (kotlinx.serialization), coroutines, and database (SQLDelight) code reused across platforms.
• Native UI: SwiftUI on iOS and Jetpack Compose on Android consume the shared logic, preserving fully native look, feel, and performance.
• Compose Multiplatform (optional): shares the UI layer across Android, iOS, desktop, and web when maximum reuse is desired.
• Tooling: Gradle, Android Studio, and Xcode integration, with shared code exposed to iOS as a framework.

Strengths

KMP lets teams share the most bug-prone, business-critical logic exactly once while keeping fully native UIs, so apps feel platform-native with no compromise on user experience. It is incremental — you can adopt it module by module inside existing native apps rather than committing to a rewrite, which dramatically lowers risk. Sharing networking, validation, models, and persistence reduces duplicated logic and prevents the subtle divergence that creeps in when two teams reimplement the same rules. Kotlin is a strong, modern language, and shared code can extend to backend and web for end-to-end reuse.

Trade-offs

KMP shares logic, not UI, by default, so you still build UIs twice unless you adopt Compose Multiplatform, whose iOS support is comparatively newer. Tooling and the Kotlin/Native iOS toolchain add setup complexity, and the ecosystem is younger than React Native or Flutter, with fewer ready-made libraries. iOS developers must work with Kotlin-generated frameworks, which can feel unfamiliar. Debugging across the Kotlin/Native boundary is harder than in a single-language app.

When to Use It

Choose KMP when you want native UIs and UX on each platform but want to eliminate duplicated business logic — it is ideal for teams with existing native iOS and Android apps seeking incremental, low-risk code sharing. It fits apps where platform fidelity matters but the shared logic (sync, offline, validation, domain rules) is substantial. If you instead want a single shared UI and the fastest path to one cross-platform codebase, Flutter or React Native may suit better. Teams adopting it usually have mature native apps and want to stop maintaining two divergent copies of critical logic, gaining consistency and fewer cross-platform bugs without rewriting their UIs. Starting with a single shared module — networking or data — proves the approach with low risk before broadening reuse across the codebase.