Reduce test flakiness by warming up molecule tests (#1226)

tests/testutils/build.gradle.kts

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 plugins {
-    id("io.element.android-library")
+    id("io.element.android-compose-library")
     alias(libs.plugins.ksp)
 }
 
@@ -31,4 +31,5 @@ dependencies {
     implementation(libs.coroutines.test)
     implementation(projects.libraries.core)
     implementation(libs.test.turbine)
+    implementation(libs.molecule.runtime)
 }

tests/testutils/src/main/kotlin/io/element/android/tests/testutils/WarmUpRule.kt

@@ -0,0 +1,48 @@
+/*
+ * 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 io.element.android.tests.testutils
+
+import app.cash.molecule.RecompositionMode
+import app.cash.molecule.moleculeFlow
+import app.cash.turbine.test
+import kotlinx.coroutines.test.runTest
+import org.junit.rules.TestRule
+import org.junit.runner.Description
+import org.junit.runners.model.Statement
+import kotlin.time.Duration.Companion.seconds
+
+/**
+ * moleculeFlow can take time to initialise during the first test of any given
+ * test class.
+ *
+ * Applying this test rule ensures that the slow initialisation is not done
+ * inside runTest which has a short default timeout.
+ */
+class WarmUpRule: TestRule {
+    override fun apply(base: Statement, description: Description): Statement = object: Statement() {
+        override fun evaluate() {
+            runTest(timeout = 60.seconds) {
+                moleculeFlow(RecompositionMode.Immediate) {
+                    // Do nothing
+                }.test {
+                    awaitItem() // Await a Unit composition
+                }
+            }
+            base.evaluate()
+        }
+    }
+}