kmp-boundaries

Kotlin Multiplatform Boundary Design

Core rules for any KMP boundary:

• Keep commonMain semantic — describe what the product needs, not Android/iOS mechanics: currentRegion(), never currentRegionFromAndroidLocale(context).
• Split by capability — Clipboard, ShareSheet, Haptics, Biometrics as separate interfaces, not one Platform god object.
• Keep actuals thin — they translate, they don't decide; a business if/when inside an actual belongs in common, tested with a fake.
• Prefer a common interface + per-platform binding over expect class whenever you need fakes / DI / lifecycle / runtime selection.
• Introduce an intermediate source set (skikoMain, appleMain) only when two platforms genuinely share an actual.

Two boundaries get the most detail below: the Activity-owned platform-UI boundary, and the AGP-9 KMP-library constraints.

Related: android-skills:kmp-ktor (network boundary), compose/references/multiplatform.md (Compose-MP mechanics), android-skills:kotlin-coroutines (scope ownership). For the iOS↔Swift bridge — Kotlin→Swift naming, type widths (Int is 32-bit), SKIE suspend→async / Flow→AsyncSequence, sealed-class exhaustiveness, SwiftUI embedding — load references/ios-interop.md when authoring the iOS-side actual.

Platform-UI bindings are Activity-owned, not Context-owned

The single most common Android boundary mistake: passing applicationContext / LocalContext.current into a binding that actually needs an Activity, then papering over the lifecycle gap with Intent.FLAG_ACTIVITY_NEW_TASK. That flag is a smell — it hides that this is a foreground-UI operation. Hold an Activity instead.

// commonMain — semantic interface; DOCUMENT what `suspend` means
interface ShareSheet {
    /** Launches the system share sheet. Returns when the sheet is PRESENTED — not when the user
     *  completes or cancels. (Otherwise callers write incorrect retry/confirmation logic.) */
    suspend fun shareText(text: String)
}

// androidMain — thin: build the intent and launch it. Activity-owned.
class AndroidShareSheet(private val activity: Activity) : ShareSheet {
    override suspend fun shareText(text: String) {
        activity.startActivity(Intent.createChooser(
            Intent(Intent.ACTION_SEND).setType("text/plain").putExtra(Intent.EXTRA_TEXT, text), null,
        ))
    }
}

You don't app-wide-inject an Activity (it's framework-created and lifecycle-bound) — construct the binding in an activity scope in the Android app module (Hilt @InstallIn(ActivityComponent::class), where Activity is a default binding; Koin scoped). commonMain only ever sees the interface; the Activity never leaves the app module. If a longer-lived (app-scoped) object needs it, hold it behind a lifecycle-aware provider (set in onResume, cleared in onPause) so a destroyed Activity can't leak.

AGP-9 KMP-library constraints (structural — they shape what can live in shared code)

AGP 9 replaces com.android.library with com.android.kotlin.multiplatform.library for the Android side of a KMP module, and rejects com.android.application + kotlin.multiplatform outright. The new plugin enforces a single-variant architecture:

• BuildConfig is unavailable — compile-time constants come from BuildKonfig or an injected AppConfiguration interface. Don't design commonMain APIs that assume BuildConfig.X exists.
• No build variants — variant-specific deps/resources/signing live in the app module; a debug/release decision surfaces as a runtime config value injected into common code, not a build-variant split inside the KMP module.
• No NDK / JNI — extract native (C/C++) into a separate com.android.library module, wrapped behind a common interface the KMP module consumes.
• Compose-MP resources need explicit enable — add androidResources { enable = true } inside kotlin { android { … } }, or Res.string.* / Res.drawable.* crash at runtime on Android (the build still succeeds — easy to miss).
• Consumer ProGuard rules need migration — consumerProguardFiles("rules.pro") from the old android {} block is silently dropped; use consumerProguardFiles.add(file("rules.pro")) in the new DSL.
• The KMP module can't also be com.android.application — the Android entry point (MainActivity, Application class, launcher manifest, applicationId / targetSdk / versionCode / versionName) moves to a separate androidApp module that depends on the shared library. MainActivity, app-level Hilt setup, and nav-host wiring all move out of the shared androidMain.
• kapt is incompatible with AGP 9's built-in Kotlin — migrate annotation processors to KSP (2.3.1+), or fall back to com.android.legacy-kapt for processors with no KSP equivalent.

Concern	Pre-AGP-9 (monolithic)	AGP 9 KMP library
MainActivity, Application class, launcher manifest	androidMain of shared module	Separate androidApp module
applicationId, versionCode, targetSdk	Shared module's android {}	androidApp only
Compile-time constants (env, flags)	BuildConfig field	BuildKonfig in common, or runtime DI
NDK / JNI native code	androidMain (any module)	Separate com.android.library, behind a common interface

For migrating an existing project, see JetBrains' kotlin-tooling-agp9-migration skill for the full mechanics.