What is Kotlin Multiplatform?¶

Kotlin Multiplatform (KMP) lets you write shared Kotlin code that runs on multiple platforms — Android, iOS, desktop, web — while keeping platform-specific code where it belongs.

It is not a cross-platform UI framework. You still write native UI on each platform (Jetpack Compose on Android, SwiftUI on iOS). KMP is about sharing the logic underneath the UI.

The core idea¶

In any app, a large portion of the code has nothing to do with the platform:

• Data models (a Song with a name and a BPM is the same on Android and iOS)
• Business logic (calculating the interval between metronome ticks)
• Data persistence and serialization (reading/writing JSON setlists)
• Rules (freemium gate: max 3 setlists in free tier)

KMP lets you write this code once in Kotlin, and use it from both Android and iOS.

How it fits into a project¶

A KMP project has (at minimum) three modules:

shared/
├── commonMain/     ← pure Kotlin, no platform APIs
├── androidMain/    ← Android-specific implementations
└── iosMain/        ← iOS-specific implementations

composeApp/         ← Android app (Jetpack Compose UI)
iosApp/             ← iOS app (Xcode project, SwiftUI UI)

Code in commonMain can be used by both composeApp and iosApp. Code in androidMain or iosMain is only compiled for that platform.

The expect/actual mechanism¶

When you need platform-specific behaviour but want to call it from shared code, KMP uses expect and actual:

// commonMain — declares the interface
expect fun currentTimeMillis(): Long

// androidMain — Android implementation
actual fun currentTimeMillis(): Long = System.currentTimeMillis()

// iosMain — iOS implementation
actual fun currentTimeMillis(): Long = 
    NSDate().timeIntervalSince1970.toLong() * 1000

Shared code calls currentTimeMillis() without knowing which platform it's on. The compiler wires up the right implementation.

What KMP is NOT¶

• Not Kotlin Multiplatform Mobile (KMM) — KMM was the old name for the mobile-focused subset. It's now just called KMP.
• Not Compose Multiplatform — that's a separate JetBrains project that shares UI across platforms using Compose. KMP shares logic; Compose Multiplatform shares UI. They can be combined, but they're distinct.
• Not a replacement for native development — you still need Xcode for iOS and Android Studio for Android. KMP reduces duplication, it doesn't eliminate platform work.

What is shared in SessionClick¶

Further reading¶

• Kotlin Multiplatform overview — kotlinlang.org
• KMP project structure — kotlinlang.org