How Kotlin and iOS Work Together¶

Kotlin is a JVM-based language — so how does it end up running on an iPhone? The answer is Kotlin/Native, a compiler backend that turns Kotlin code into native binaries without a JVM.

The compilation pipeline¶

Kotlin source code (commonMain / iosMain)
        ↓
  Kotlin/Native compiler
        ↓
  LLVM IR (same intermediate format as C/C++/Swift)
        ↓
  Native ARM64 binary code
        ↓
  Packaged as shared.framework
        ↓
  Linked into the iOS app by Xcode

The resulting code runs directly on the device — no JVM, no interpreter. Performance is comparable to Swift or C++.

What Xcode sees: the framework¶

From Xcode's perspective, the shared Kotlin code is just a regular Apple framework (shared.framework). It doesn't know or care that it was written in Kotlin.

Swift code in the iOS app imports it like any other framework:

import shared

let song = Song(name: "Superstition", bpm: 100)

Kotlin classes, functions, and properties are automatically exposed to Swift. Kotlin/Native generates Objective-C headers (.h files) from your Kotlin code, which Swift can consume via its Objective-C interop layer.

Kotlin → Swift type mapping¶

Kotlin/Native translates Kotlin types to their nearest equivalent:

The translation is mostly seamless for simple types. More complex Kotlin patterns (sealed classes, coroutines, generics) require some care.

Coroutines and Swift concurrency¶

Kotlin coroutines (suspend functions) don't map directly to Swift's async/await. This is one of the more painful edges of KMP.

The most common solution is SKIE (Swift Kotlin Interface Enhancer), a Gradle plugin that automatically wraps Kotlin coroutines so they appear as native Swift async functions:

// Kotlin (commonMain)
suspend fun loadSetlists(): List<Setlist>

// Swift — with SKIE, this just works
let setlists = try await repository.loadSetlists()

Without SKIE, you'd need to wrap coroutines manually in callbacks, which is tedious.

Calling Apple APIs from Kotlin (iosMain)¶

In iosMain, you can call Apple's native frameworks directly from Kotlin. Kotlin/Native ships with bindings for the entire Apple SDK:

// iosMain
import platform.AVFAudio.AVAudioEngine
import platform.Foundation.NSDate

actual fun currentTimeMillis(): Long =
    (NSDate().timeIntervalSince1970 * 1000).toLong()

Apple framework names and types keep their original names. It feels slightly foreign in Kotlin, but it works.

Memory management¶

Older versions of Kotlin/Native used a strict ownership model that made sharing objects between threads painful. Since Kotlin 1.7.20, the new memory manager (now the default) uses a model much closer to the JVM — standard garbage collection, no strict thread ownership. This eliminates most of the old pain points.

What this means for SessionClick¶

Further reading¶

• Kotlin/Native overview — kotlinlang.org
• SKIE — Swift-friendly KMP — skie.touchlab.co
• KMP iOS integration guide — kotlinlang.org