Integrate RevenueCat billing SDK with KMP architecture

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Summary

Integrate RevenueCat's purchases-kmp SDK (v2.4.0+17.30.0) as a feature package in :composeApp.

• Problem: No billing infrastructure for in-app purchases
• Solution: Add billing/ feature with domain/data/presentation layers, platform-specific initialization
• Type: [x] Feature [ ] Bugfix [ ] Refactor [ ] Chore [ ] Docs

Context / Links

• Design / spec / ticket: RevenueCat KMP integration following existing feature patterns
• Related PRs: N/A

Implementation details

Architecture: Feature package in :composeApp (not separate module), matching timer/, session/, settings/ structure.

Domain layer (billing/domain/):

• BillingService interface: observeBillingState(), refreshCustomerInfo(), purchase(), restorePurchases()
• BillingState sealed interface: Loading | Ready(isPro, activeEntitlements) | Unavailable | Error
• EntitlementInfo data class

Data layer (billing/data/):

• RevenueCatBillingService (commonMain, internal): wraps Purchases.sharedInstance, maps CustomerInfo to state, tracks "pro" entitlement
• BillingInitializer (expect/actual): platform-specific SDK config
  • Android: Purchases.configure(REVENUECAT_GOOGLE_API_KEY)
  • iOS: Purchases.configure(REVENUECAT_APPLE_API_KEY)
  • JVM: no-op
• NoOpBillingService (jvmMain): returns Unavailable state (RevenueCat doesn't support Desktop)

Presentation layer:

• BillingViewModel: exposes StateFlow<BillingState>, delegates to service

DI wiring:

// sharedModule
singleOf(::RevenueCatBillingService).bind<BillingService>()
viewModelOf(::BillingViewModel)

// platformModule (jvmMain override)
single<BillingService> { NoOpBillingService() }

Initialization: added initializeBilling() calls to HushTimerApplication.onCreate() (Android) and MainViewController configure block (iOS).

Security: API keys via BuildKonfig with empty string fallbacks—no hardcoded secrets. All implementations internal, only interfaces/ViewModel public.

Trade-offs:

• iOS requires manual SPM addition of PurchasesHybridCommon v17.30.0 (KMP toolchain limitation)
• Desktop gets no-op stub rather than compile-time exclusion for consistent DI graph

Screenshots / API changes (if applicable)

N/A (infrastructure only, no UI changes)

Testing

Requires API keys in local.properties for runtime testing:

REVENUECAT_GOOGLE_API_KEY=<key>
REVENUECAT_APPLE_API_KEY=<key>

• Unit tests (future: mock BillingService)
• Integration / e2e tests (future: verify DI wiring, platform initialization)
• Manual tests (code compiles, DI graph validated)

Note: Repository has pre-existing AGP 9.0.0 build issue preventing ./gradlew execution. Unrelated to this PR.

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Original prompt

Overview

Integrate RevenueCat billing into HushTimer using the official purchases-kmp SDK as a feature package inside the existing :composeApp KMP module. This follows the same data/ / domain/ / presentation/ structure used by all other features (timer/, session/, settings/, config/).

Reference documentation: https://www.revenuecat.com/docs/getting-started/installation/kotlin-multiplatform

Architecture Decisions

• No separate Gradle module — billing lives as a package in :composeApp alongside other features, consistent with the project's established pattern.
• KMP-first — the purchases-kmp SDK supports Android and iOS from commonMain. Desktop/JVM gets a no-op stub.
• Interface-based — a BillingService interface allows swapping the implementation later.
• internal visibility on implementation details for a soft boundary.

Implementation Plan

Phase 1: Gradle & Dependency Setup

1.1 — gradle/libs.versions.toml
Add entries in alphabetical order (per AGENTS.md):

# [versions]
purchases-kmp = "2.4.0+17.30.0"

# [libraries]
purchases-kmp-core = { module = "com.revenuecat.purchases:purchases-kmp-core", version.ref = "purchases-kmp" }
purchases-kmp-result = { module = "com.revenuecat.purchases:purchases-kmp-result", version.ref = "purchases-kmp" }

1.2 — composeApp/build.gradle.kts
Add to commonMain.dependencies:

implementation(libs.purchases.kmp.core)
implementation(libs.purchases.kmp.result)

1.3 — API key management via BuildKonfig
In the buildkonfig { defaultConfigs { } } block in composeApp/build.gradle.kts, add:

buildConfigField(STRING, "REVENUECAT_APPLE_API_KEY", getSecret("REVENUECAT_APPLE_API_KEY", ""))
buildConfigField(STRING, "REVENUECAT_GOOGLE_API_KEY", getSecret("REVENUECAT_GOOGLE_API_KEY", ""))

Phase 2: Domain Layer — billing/domain/

Create these files under composeApp/src/commonMain/kotlin/com/rickyhu/hushtimer/billing/domain/:

2.1 — EntitlementInfo.kt

package com.rickyhu.hushtimer.billing.domain

data class EntitlementInfo(
    val id: String,
    val isActive: Boolean,
    val expiresAtEpochMillis: Long?,
)

2.2 — BillingState.kt

package com.rickyhu.hushtimer.billing.domain

sealed interface BillingState {
    data object Loading : BillingState
    data class Ready(
        val isPro: Boolean,
        val activeEntitlements: List<EntitlementInfo>,
    ) : BillingState
    data object Unavailable : BillingState
    data class Error(val message: String) : BillingState
}

2.3 — BillingService.kt

package com.rickyhu.hushtimer.billing.domain

import kotlinx.coroutines.flow.Flow

interface BillingService {
    fun observeBillingState(): Flow<BillingState>
    suspend fun refreshCustomerInfo()
    suspend fun purchase(packageId: String): Result<Unit>
    suspend fun restorePurchases(): Result<Unit>
}

Phase 3: Data Layer — billing/data/

3.1 — RevenueCatBillingService.kt in commonMain
An internal class implementing BillingService that wraps RevenueCat's Purchases KMP API:

• Uses Purchases.sharedInstance to get customer info, offerings, make purchases, and restore
• Maps CustomerInfo to BillingState.Ready
• Exposes state via MutableStateFlow
• Constant PRO_ENTITLEMENT_ID = "pro" (matching RevenueCat dashboard)

3.2 — BillingInitializer.kt — expect/actual for platform-specific SDK configuration:

• commonMain: expect fun initializeBilling()
• androidMain: calls Purchases.configure(apiKey = BuildKonfig.REVENUECAT_GOOGLE_API_KEY) — on Android the SDK gets context automatically via its ContentProvider
• iosMain: calls Purchases.configure(apiKey = BuildKonfig.REVENUECAT_APPLE_API_KEY)
• jvmMain: no-op (RevenueCat doesn't support Desktop)

Phase 4: Desktop No-Op

4.1 — NoOpBillingService.kt in jvmMain

internal class NoOpBillingService : BillingService {
    override fun observeBillingState() = flowOf(BillingState.Unavailable)
    override suspend fun refreshCustomerInfo() { /* no-op */ }
    override suspend fun purchase(packageId: String) =
        Result.failure<Unit>(UnsupportedOperationException("Billing not available on Desktop"))
    override suspend fun restorePurchases() =
        Result.failure<Unit>(UnsupportedOperationException("Billing not available on Desktop"))
}

Phase 5: Presentation Layer

5.1 — BillingViewModel.kt in commonMain

class BillingViewModel(
    private val billingService: BillingService,
) : ViewModel() {
    val billingState: StateFlow<BillingState> = billingService.observeBillingState()
        .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5_000), BillingState.Loading)

    init { viewModelScope.launch { billingService.refreshCustomerInfo() } }

    fun purchase(packageId: String) {
        viewModelScope.launch { billingService.purchase(packageId) }
    }

...

</details>



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