Kotlin Multiplatform: Platform Abstraction Decisions

Expert guidance for KMP architecture in Amethyst - deciding what to share vs keep platform-specific.

When to Use This Skill

Making platform abstraction decisions:

• "Should I create expect/actual or keep Android-only?"
• "Can I share this ViewModel logic?"
• "Where does this crypto/JSON/network implementation belong?"
• "This uses Android Context - can it be abstracted?"
• "Is this code in the wrong module?"
• Preparing for iOS/web/wasm targets
• Detecting incorrect placements

Abstraction Decision Tree

Central question: "Should this code be reused across platforms?"

Follow this decision path (< 1 minute):

Q: Is it used by 2+ platforms?
├─ NO  → Keep platform-specific
│         Example: Android-only permission handling
│
└─ YES → Continue ↓

Q: Is it pure Kotlin (no platform APIs)?
├─ YES → commonMain
│         Example: Nostr event parsing, business rules
│
└─ NO  → Continue ↓

Q: Does it vary by platform or by JVM vs non-JVM?
├─ By platform (Android ≠ iOS ≠ Desktop)
│  → expect/actual
│  Example: Secp256k1Instance (uses different security APIs)
│
├─ By JVM (Android = Desktop ≠ iOS/web)
│  → jvmAndroid
│  Example: Jackson JSON parsing (JVM library)
│
└─ Complex/UI-related
   → Share it in commonsUI; expect/actual or a slot only for the platform leaf
   Example: Navigation. One shell in commonsUI, picked by window size
   (ScreenLayoutSpec); only the Activity / Window entry point is native

Final check:
Q: Maintenance cost of abstraction < duplication cost?
├─ YES → Proceed with abstraction
└─ NO  → Duplicate (simpler)

Real Examples from Codebase

Crypto → expect/actual:

// commonMain - expect declaration
expect object Secp256k1Instance {
    fun signSchnorr(data: ByteArray, privKey: ByteArray): ByteArray
}

// androidMain - uses Android Keystore
// jvmMain - uses Desktop JVM crypto
// iosMain - uses iOS Security framework

Why: Each platform has different security APIs.

JSON parsing → jvmAndroid:

// quartz/build.gradle.kts
val jvmAndroid = create("jvmAndroid") {
    api(libs.jackson.module.kotlin)
}

Why: Jackson is JVM-only, works on Android + Desktop, not iOS/web.

Navigation → shared, entry points → platform-specific:

• Shared (commonsUI, moving there): the nav host, bottom bar / rail / permanent drawer, chosen by window size via ScreenLayoutSpec
• Android: MainActivity hosts it; Desktop: a Window (+ MenuBar, tray) hosts it Why: Android runs on laptops too, and the new Desktop app renders the same UI. See commons/plans/2026-09-27-one-ui-android-desktop.md. Today's desktopApp sidebar shell is legacy.

Mental Model: Source Sets as Dependency Graph

Think of source sets as a dependency graph, not folders.

┌─────────────────────────────────────────────┐
│ commonMain = Contract (pure Kotlin)         │
│ - Business logic, protocol, data models     │
│ - No platform APIs                          │
└────────────┬────────────────────────────────┘
             │
             ├──────────────────────┬────────────────────
             │                      │
             ▼                      ▼
   ┌───────────────────┐  ┌──────────────────┐
   │ jvmAndroid        │  │ iosMain          │
   │ JVM libs shared   │  │ iOS common       │
   │ - Jackson         │  │                  │
   │ - OkHttp          │  └────┬─────────────┘
   └───┬───────────┬───┘       │
       │           │           │
       ▼           ▼           ├─→ iosArm64Main
  ┌─────────┐ ┌──────────┐     └─→ iosSimulatorArm64Main
  │android  │ │jvmMain   │
  │Main     │ │(Desktop) │
  └─────────┘ └──────────┘

Future: jsMain, wasmMain

Key insight: jvmAndroid is NOT a platform - it's a shared JVM layer.

The jvmAndroid Pattern

Unique to Amethyst. Shares JVM libraries between Android + Desktop.

When to Use jvmAndroid

Use jvmAndroid when:

• ✅ JVM-specific libraries (Jackson, OkHttp, url-detector)
• ✅ Android implementation = Desktop implementation (same JVM)
• ✅ Library doesn't work on iOS/web

Do NOT use jvmAndroid for:

• ❌ Pure Kotlin code (use commonMain)
• ❌ Platform-specific APIs (use androidMain/jvmMain)
• ❌ Code that should work on all platforms

Example from quartz/build.gradle.kts

// Must be defined BEFORE androidMain and jvmMain
val jvmAndroid = create("jvmAndroid") {
    dependsOn(commonMain.get())

    dependencies {
        api(libs.jackson.module.kotlin)  // JSON parsing - JVM only
        api(libs.url.detector)            // URL extraction - JVM only
        implementation(libs.okhttp)       // HTTP client - JVM only
    }
}

// Both depend on jvmAndroid
jvmMain { dependsOn(jvmAndroid) }
androidMain { dependsOn(jvmAndroid) }

Why Jackson in jvmAndroid, not commonMain?

• Jackson is JVM-specific library
• Works on Android (runs on JVM)
• Works on Desktop (runs on JVM)
• Does NOT work on iOS (not JVM) or web (not JVM)

Web/wasm consideration: For future web support, consider migrating from Jackson → kotlinx.serialization (see Target-Specific Guidance).

What to Abstract vs Keep Platform-Specific

Quick decision guidelines based on codebase patterns:

Always Abstract

• Crypto (Secp256k1, encryption, signing)
• Core protocol logic (Nostr events, NIPs)
• Why: Needed everywhere, platform security APIs vary

Often Abstract

• I/O operations (file reading, caching)
• Logging (platform logging systems differ)
• Serialization (if using kotlinx.serialization)
• Why: Commonly reused, platform implementations available

Sometimes Abstract

• Business logic: YES - state machines, data processing
• ViewModels: YES - state + business logic shareable (StateFlow/SharedFlow)
• Screen layouts: YES - shared in commonsUI; they adapt to window size, not platform
• Why: One UI for Android (phones to laptops) and Desktop; only entry points differ

Rarely Abstract

• Complex UI components (composables with heavy platform dependencies)
• Why: Platform paradigms can differ significantly

Never Abstract

• Process / window entry points (Activity, Service, Window, tray)
• Permissions (Android vs iOS APIs incompatible)
• Platform UX patterns
• Why: Too platform-specific, abstraction creates leaky APIs

Evidence from shared-ui-analysis.md

expect/actual Mechanics

When to use: Code needed by 2+ platforms, varies by platform.

Pattern Categories from Codebase

Objects (singletons):

// 24 expect declarations found, common pattern:
expect object Secp256k1Instance { ... }
expect object Log { ... }
expect object LibSodiumInstance { ... }

Classes (instantiable):

expect class AESCBC { ... }
expect class DigestInstance { ... }

Functions (utilities):

expect fun platform(): String
expect fun currentTimeSeconds(): Long

See references/expect-actual-catalog.md for complete catalog with rationale.

Target-Specific Guidance

Android, JVM (Desktop), iOS - Current Primary Targets

Status: Mature patterns, stable APIs

Android (androidMain):

• Uses Android framework (Activity, Context, etc.)
• secp256k1-kmp-jni-android (0.23.0 in libs.versions.toml) for crypto
• AndroidX libraries

Desktop JVM (jvmMain):

• Uses Compose Desktop (Window, MenuBar, etc.)
• secp256k1-kmp-jni-jvm (same 0.23.0 line) for crypto
• Pure JVM libraries

iOS (iosMain):

• Mature target — actively built and tested
• Architecture targets: iosArm64, iosSimulatorArm64, iosX64 (plus macosArm64 for host tooling)
• Platform APIs via platform.posix, Security framework

Web, wasm - Future Targets

Status: Not yet implemented, consider for future-proofing

Constraints to know:

• ❌ No platform.posix (file I/O different)
• ❌ No JVM libraries (Jackson, OkHttp won't work)
• ❌ Different async model (JS event loop vs threads)

Future-proofing tips:

1. Prefer pure Kotlin in commonMain
2. Use kotlinx.* libraries:
   • kotlinx.serialization instead of Jackson
   • ktor instead of OkHttp (ktor supports web)
   • kotlinx.datetime instead of custom date handling
3. Avoid platform.posix for file operations
4. Test abstractions work without JVM assumptions

Example migration path:

// Current: jvmAndroid (JVM-only)
api(libs.jackson.module.kotlin)

// Future: commonMain (all platforms)
api(libs.kotlinx.serialization.json)

Integration: When to Invoke Other Skills

Invoke gradle-expert

Trigger gradle-expert skill when encountering:

• Dependency conflicts (e.g., secp256k1-android vs secp256k1-jvm version mismatch)
• Build errors related to source sets
• Version catalog issues (libs.versions.toml)
• "Duplicate class" errors
• Performance/build time issues

Example trigger:

Error: Duplicate class found: fr.acinq.secp256k1.Secp256k1

→ Invoke gradle-expert for dependency conflict resolution.

Flags to Raise

Platform code in commonMain:

// ❌ INCORRECT - Android API in commonMain
expect fun getContext(): Context  // Context is Android-only!

→ Flag: "Android API in commonMain won't compile on other platforms"

Duplicated business logic:

// ❌ INCORRECT - Same logic in both
// androidMain/.../CryptoUtils.kt
fun validateSignature(...) { ... }

// jvmMain/.../CryptoUtils.kt
fun validateSignature(...) { ... }  // Duplicated!

→ Flag: "Business logic duplicated, should be in commonMain or expect/actual"

Reinventing wheel - suggest KMP alternatives:

• Custom date/time → kotlinx.datetime
• OkHttp → ktor (supports web)
• Jackson → kotlinx.serialization
• Custom UUID → kotlinx.uuid (when stable)

Common Pitfalls

1. Over-Abstraction

Problem: Creating expect/actual for UI components

// ❌ BAD
expect fun NavigationComponent(...)

Why: It forks the whole navigation UI per platform. Fix: Write the navigation once in commonsUI; put only the platform leaf (an Activity result, a file picker) behind expect/actual or a slot

2. Under-Sharing

Problem: Duplicating business logic across platforms

// ❌ BAD - duplicated in androidMain and jvmMain
fun parseNostrEvent(json: String): Event { ... }

Why: Bug fixes need to be applied twice, tests duplicated Fix: Move to commonMain (pure Kotlin) or create expect/actual

3. Leaky Abstractions

Problem: Platform code in commonMain

// commonMain - ❌ BAD
import android.content.Context  // Won't compile on iOS!

Fix: Use expect/actual or dependency injection

4. Premature Abstraction

Problem: Creating expect/actual before second platform needs it

// ❌ BAD - only used on Android currently
expect fun showNotification(...)

Why: Wrong abstraction boundaries, wasted effort Fix: Wait until iOS actually needs it, then abstract

5. Wrong Source Set

Problem: JVM libraries in commonMain

// commonMain - ❌ BAD
import com.fasterxml.jackson.databind.ObjectMapper

Why: Jackson won't compile on iOS/web Fix: Move to jvmAndroid or migrate to kotlinx.serialization

Quick Reference

See Also

• references/abstraction-examples.md - Good/bad abstraction examples with rationale
• references/source-set-hierarchy.md - Visual hierarchy with Amethyst examples
• references/expect-actual-catalog.md - All 24 expect/actual pairs with "why abstracted"
• references/target-compatibility.md - Platform constraints and future-proofing

Scripts

• scripts/validate-kmp-structure.sh - Detect incorrect placements, validate source sets
• scripts/suggest-kmp-dependency.sh - Suggest KMP library alternatives (ktor, kotlinx.serialization, etc.)