Changing a MutableState inside a coroutine seemingly leads to skipped emissions

Repro of issue submitted at: cashapp/molecule#249

Author: Marco Romano

Diff for: features/leaveroom/impl/build.gradle.kts

@@ -41,6 +41,7 @@ dependencies {
     testImplementation(libs.molecule.runtime)
     testImplementation(libs.test.truth)
     testImplementation(libs.test.turbine)
+    testImplementation(kotlin("test"))
     testImplementation(projects.libraries.matrix.test)
     testImplementation(projects.tests.testutils)
 }

Diff for: features/leaveroom/impl/src/test/kotlin/io/element/android/features/leaveroom/impl/MoleculeTestCase.kt

@@ -0,0 +1,83 @@
+/*
+ * Copyright (c) 2023 New Vector Ltd
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package example.test
+
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.MutableState
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.rememberCoroutineScope
+import app.cash.molecule.RecompositionClock
+import app.cash.molecule.moleculeFlow
+import app.cash.turbine.test
+import kotlinx.coroutines.launch
+import kotlinx.coroutines.test.runTest
+import org.junit.Test
+import kotlin.test.assertEquals
+
+data class MyState(
+    val anInt: Int,
+    val eventSink: (MyEvent) -> Unit,
+)
+
+sealed interface MyEvent {
+    object Increment : MyEvent
+    object IncrementSuspending : MyEvent
+}
+
+@Composable
+fun myPresenter(): MyState {
+    val scope = rememberCoroutineScope()
+    val anInt: MutableState<Int> = remember { mutableStateOf(0) }
+    return MyState(anInt.value) {
+        when (it) {
+            MyEvent.Increment -> {
+                anInt.value++
+                anInt.value++
+            }
+            MyEvent.IncrementSuspending -> scope.launch {
+                anInt.value++
+                anInt.value++
+            }
+        }
+    }
+}
+
+class MoleculeTestCase {
+    @Test
+    fun `process Increment event`() = runTest {
+        moleculeFlow(RecompositionClock.Immediate) { myPresenter() }.test {
+            awaitItem().apply {
+                assertEquals(0, anInt)
+                eventSink(MyEvent.Increment)
+            }
+            assertEquals(1, awaitItem().anInt)
+            assertEquals(2, awaitItem().anInt)
+        }
+    }
+
+    @Test
+    fun `process IncrementSuspending event`() = runTest {
+        moleculeFlow(RecompositionClock.Immediate) { myPresenter() }.test {
+            awaitItem().apply {
+                assertEquals(0, anInt)
+                eventSink(MyEvent.IncrementSuspending)
+            }
+            assertEquals(2, awaitItem().anInt)
+        }
+    }
+}