Kotlin Multiplatform

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
   → Keep platform-specific
   Example: Navigation (Activity vs Window too different)

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 → platform-specific:

• Android: MainActivity (Activity + Compose Navigation)
• Desktop: Window + sidebar + MenuBar

Why: UI paradigms fundamentally different.

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: NO - platform-native (Window vs Activity)
• Why: ViewModels contain platform-agnostic state; Screens render differently per platform

Rarely Abstract

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

Never Abstract

• Navigation (Activity vs Window fundamentally different)
• 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)