Add unit tests and comments

core/domain/src/main/java/com/google/samples/apps/nowinandroid/core/domain/GetUserNewsResourcesUseCase.kt

@@ -63,7 +63,6 @@ class GetFollowedUserNewsResourcesUseCase @Inject constructor(
      * Returns a list of UserNewsResources for topics which the user is following
      */
     operator fun invoke(): Flow<List<UserNewsResource>> =
-
         /**
          * This sequence of flow transformation functions does the following:
          *
@@ -72,9 +71,9 @@ class GetFollowedUserNewsResourcesUseCase @Inject constructor(
          * - distinctUntilChanged: will only emit a set of followed topic IDs if it's changed. This
          * avoids calling potentially expensive operations (like setting up a new flow) when nothing
          * has changed.
-         * - flatMapLatest: getUserNewsResources returns a flow, so we end up with a flow inside a
-         * flow. flatMapLatest solves this and cancels any previous flows created by
-         * getUserNewsResources. 
+         * - flatMapLatest: getUserNewsResources returns a flow, so we have a flow inside a
+         * flow. flatMapLatest moves the inner flow (the one we want to return) to the outer flow
+         * and cancels any previous flows created by getUserNewsResources.
          */
         userDataRepository.userData
             .map { userData ->

core/domain/src/test/java/com/google/samples/apps/nowinandroid/core/domain/GetUserNewsResourcesUseCaseTest.kt

@@ -83,6 +83,55 @@ class GetUserNewsResourcesUseCaseTest {
     }
 }
 
+class GetFollowedUserNewsResourcesUseCaseTest {
+
+    @get:Rule
+    val mainDispatcherRule = MainDispatcherRule()
+
+    private val newsRepository = TestNewsRepository()
+    private val userDataRepository = TestUserDataRepository()
+    private val getUserNewsResourcesUseCase =
+        GetUserNewsResourcesUseCase(newsRepository, userDataRepository)
+
+    val useCase =
+        GetFollowedUserNewsResourcesUseCase(userDataRepository, getUserNewsResourcesUseCase)
+
+    @Test
+    fun whenOnboardingShownAndNoTopicsFollowed_emptyListIsReturned() = runTest {
+        val followedNewsResources = useCase()
+
+        // Send some news resources and empty user data
+        newsRepository.sendNewsResources(sampleNewsResources)
+        userDataRepository.setUserData(emptyUserData)
+
+        // Check that an empty list is returned
+        assertEquals(
+            emptyList(),
+            followedNewsResources.first(),
+        )
+    }
+
+    @Test
+    fun whenTopicsAreFollowed_correctNewsResourcesAreReturned() = runTest {
+        val followedNewsResources = useCase()
+
+        // Send some news resources and user data with a followed topic
+        newsRepository.sendNewsResources(sampleNewsResources)
+
+        val userData = emptyUserData.copy(
+            followedTopics = setOf(sampleTopic1.id),
+        )
+        userDataRepository.setUserData(userData)
+
+        assertEquals(
+            sampleNewsResources
+                .filter { it.topics.contains(sampleTopic1) }
+                .mapToUserNewsResources(userData),
+            followedNewsResources.first(),
+        )
+    }
+}
+
 private val sampleTopic1 = Topic(
     id = "Topic1",
     name = "Headlines",