Show loading states in purchasing flow a minimum amount of time

Author: Pururun

android/app/src/main/kotlin/net/mullvad/mullvadvpn/usecase/PaymentUseCase.kt

@@ -1,6 +1,7 @@
 package net.mullvad.mullvadvpn.usecase
 
 import android.app.Activity
+import kotlinx.coroutines.FlowPreview
 import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.flow.MutableStateFlow
 import kotlinx.coroutines.flow.asStateFlow
@@ -12,6 +13,7 @@ import net.mullvad.mullvadvpn.lib.payment.model.PaymentAvailability
 import net.mullvad.mullvadvpn.lib.payment.model.ProductId
 import net.mullvad.mullvadvpn.lib.payment.model.PurchaseResult
 import net.mullvad.mullvadvpn.lib.payment.model.VerificationResult
+import net.mullvad.mullvadvpn.util.delayAtLeast
 import net.mullvad.mullvadvpn.util.retryWithExponentialBackOff
 
 interface PaymentUseCase {
@@ -34,8 +36,18 @@ class PlayPaymentUseCase(private val paymentRepository: PaymentRepository) : Pay
     override val paymentAvailability = _paymentAvailability.asStateFlow()
     override val purchaseResult = _purchaseResult.asStateFlow()
 
+    @OptIn(FlowPreview::class)
     override suspend fun purchaseProduct(productId: ProductId, activityProvider: () -> Activity) {
-        paymentRepository.purchaseProduct(productId, activityProvider).collect(_purchaseResult)
+        paymentRepository
+            .purchaseProduct(productId, activityProvider)
+            .delayAtLeast {
+                if (purchaseResult.value.shouldDelayLoading()) {
+                    MINIMUM_LOADING_DELAY_MS
+                } else {
+                    0
+                }
+            }
+            .collect(_purchaseResult)
     }
 
     override suspend fun queryPaymentAvailability() {
@@ -64,6 +76,13 @@ class PlayPaymentUseCase(private val paymentRepository: PaymentRepository) : Pay
                 }
             }
     }
+
+    private fun PurchaseResult?.shouldDelayLoading() =
+        this is PurchaseResult.FetchingProducts || this is PurchaseResult.VerificationStarted
+
+    companion object {
+        const val MINIMUM_LOADING_DELAY_MS = 500L
+    }
 }
 
 class EmptyPaymentUseCase : PaymentUseCase {

android/app/src/main/kotlin/net/mullvad/mullvadvpn/util/FlowUtils.kt

@@ -10,6 +10,7 @@ import kotlinx.coroutines.flow.callbackFlow
 import kotlinx.coroutines.flow.catch
 import kotlinx.coroutines.flow.firstOrNull
 import kotlinx.coroutines.flow.flatMapLatest
+import kotlinx.coroutines.flow.flow
 import kotlinx.coroutines.flow.map
 import kotlinx.coroutines.flow.retryWhen
 import kotlinx.coroutines.withTimeoutOrNull
@@ -154,3 +155,16 @@ class ExceptionWrapper(val item: Any) : Throwable()
 suspend fun <T> Flow<T>.firstOrNullWithTimeout(timeMillis: Long): T? {
     return withTimeoutOrNull(timeMillis) { firstOrNull() }
 }
+
+suspend fun <T> Flow<T>.delayAtLeast(timeMillis: (T) -> Long) = flow {
+    var lastEmittedTime = System.currentTimeMillis()
+    collect {
+        val timeSinceLastEmit = System.currentTimeMillis() - lastEmittedTime
+        val minimumTime = timeMillis(it)
+        if (timeSinceLastEmit < minimumTime) {
+            delay(minimumTime - timeSinceLastEmit)
+        }
+        lastEmittedTime = System.currentTimeMillis()
+        emit(it)
+    }
+}