Subscription Billing in Android Apps

class BillingRepository(
    context: Context,
    private val backend: EntitlementApi,
    private val scope: CoroutineScope,
) : PurchasesUpdatedListener {

    private val client = BillingClient.newBuilder(context)
        .setListener(this)
        .enablePendingPurchases(PendingPurchasesParams.newBuilder().enableOneTimeProducts().build())
        .build()

    suspend fun connect(): Boolean = suspendCancellableCoroutine { cont ->
        client.startConnection(object : BillingClientStateListener {
            override fun onBillingSetupFinished(result: BillingResult) {
                if (cont.isActive) cont.resume(result.responseCode == BillingClient.BillingResponseCode.OK)
            }
            override fun onBillingServiceDisconnected() { /* reconnect on next call */ }
        })
    }

    suspend fun premiumDetails(): ProductDetails? {
        val params = QueryProductDetailsParams.newBuilder().setProductList(listOf(
            QueryProductDetailsParams.Product.newBuilder()
                .setProductId("premium").setProductType(BillingClient.ProductType.SUBS).build()
        )).build()
        return client.queryProductDetails(params).productDetailsList?.firstOrNull()   // billing-ktx suspend API
    }

    fun launch(activity: Activity, details: ProductDetails, offerToken: String) {
        val flowParams = BillingFlowParams.newBuilder().setProductDetailsParamsList(listOf(
            BillingFlowParams.ProductDetailsParams.newBuilder()
                .setProductDetails(details).setOfferToken(offerToken).build()
        )).build()
        client.launchBillingFlow(activity, flowParams)
    }

    override fun onPurchasesUpdated(result: BillingResult, purchases: MutableList<Purchase>?) {
        if (result.responseCode != BillingClient.BillingResponseCode.OK) return   // cancelled / error → show message
        purchases.orEmpty()
            .filter { it.purchaseState == Purchase.PurchaseState.PURCHASED }
            .forEach { purchase ->
                // Server verifies with Google, acknowledges, and grants the entitlement.
                scope.launch { backend.verifyAndGrant(purchase.purchaseToken, purchase.products) }
            }
    }
}