How Kotlin Does Multiplatform

## What Kotlin Multiplatform is **Kotlin Multiplatform (KMP)** is a language and tooling capability that lets you write code once in Kotlin and compile it for several **targets** (platforms) at once. It is built into the Kotlin compiler — you enable it with the `kotlin("multiplatform")` Gradle plugin. ## The core idea: one front-end, many back-ends The Kotlin compiler has two halves: - a **front-end** that parses and type-checks your Kotlin source (the same for every platform), and - a **back-end** that emits platform-specific output. KMP reuses the one front-end and swaps back-ends to produce: - **JVM bytecode** — for Android and server/desktop apps. - **Native binaries** — via **Kotlin/Native**, which uses **LLVM** to produce machine code for iOS, macOS, watchOS, Linux, Windows. - **JavaScript** — for browsers/Node. - **WebAssembly (Wasm)** — a newer target for the web. ## What you actually share Shared code lives in the **`commonMain`** source set. It can only use the **Kotlin common standard library** plus multiplatform-aware libraries (e.g. `kotlinx.coroutines`, `kotlinx.serialization`, Ktor client). Typical shared pieces: - business/domain logic - networking and serialization - data models and validation - view-model / presentation logic UI is often kept **native per platform** (Jetpack Compose on Android, SwiftUI on iOS), though **Compose Multiplatform** lets you share UI too. ## Bridging platform gaps When common code needs something only a platform provides (e.g. the current time source, a UUID, secure storage), you use **`expect`/`actual`**: `commonMain` declares an `expect` API, and each platform source set supplies the `actual` implementation. ```kotlin // commonMain expect fun platformName(): String // androidMain actual fun platformName(): String = "Android ${android.os.Build.VERSION.SDK_INT}" // iosMain actual fun platformName(): String = "iOS" ``` ## Why it matters You avoid re-implementing the same logic in Kotlin, Swift, and JavaScript. Bugs are fixed once. Unlike some cross-platform frameworks, KMP does **not** force a shared UI or a separate runtime — it produces idiomatic native artifacts, so a KMP iOS framework is a normal `.framework` callable from Swift.