Test your Paging implementation

Implementing the Paging library in your app should be paired with a robust testing strategy. You should test data loading components such as PagingSource and RemoteMediator to ensure that they work as expected. You should also write end-to-end tests to verify that all of the components in your Paging implementation work correctly together without unexpected side effects.

This guide explains how to test the Paging library in the different architecture layers of your app as well as how to write end-to-end tests for your entire Paging implementation.

UI layer tests

Data fetched with the Paging library is consumed in the UI as a Flow<PagingData<Value>>. To write tests to verify the data in the UI is as you expect, include the paging-testing dependency. It contains the asSnapshot() extension on a Flow<PagingData<Value>>. This lets you assert the data paged through contains the expected elements you using a standard List<Value>. This is illustrated in the following snippet:

fun test_items_contain_one_to_ten() = runTest {
  // Get the Flow of PagingData from the ViewModel under test
  val items: Flow<PagingData<String>> = viewModel.items

  val itemsSnapshot: List<String> = items.asSnapshot(
    coroutineScope = this
  ) {
    // Each operation inside the lambda waits for the data to settle before continuing
    scrollTo(index = 50)

    // While you can’t view the items within the asSnapshot call,
    // you can continuously scroll in a direction while some condition is true
    // i.e., in this case until you hit the first header.
    appendScrollWhile {  item: String -> item != "Header 1" }
  }

  // With the asSnapshot complete, you can now verify that the snapshot
  // has the expected values
  assertEquals(
    expected = (0..50).map(Int::toString),
    actual = itemsSnapshot
  )  
}

Testing transformations

You should also write unit tests that cover any transformations you apply to the PagingData stream. To write these tests:

  • Use the asPagingSourceFactory extension on a Flow<List<Value>>.
  • Use the returned PagingSourceFactory in a fake for your Repository.
  • Pass that Repository to your ViewModel.

The ViewModel can then be tested as in covered in the previous section. Consider the following ViewModel:

class MyViewModel(
  myRepository: myRepository
) {
  val items = Pager(
    config: PagingConfig,
    initialKey = null,
    pagingSourceFactory = { myRepository.pagingSource() }
  )
  .flow
  .map { pagingData ->
    pagingData.insertSeparators<String, String> { before, _ ->
      when {
        // Add a dashed String separator if the prior item is a multiple of 10
        before.last() == '0' -> "---------"
        // Return null to avoid adding a separator between two items.
        else -> null
      }
  }
}

To test the transformation in MyViewModel, supply a fake instance of MyRepository that delegates to a Flow representing the data to be transformed as shown in the following snippet:

class FakeMyRepository(
  testScope: CoroutineScope
): MyRepository {
    private val itemsFlow = flowOf((0..100).map(Any::toString))

    private val pagingSourceFactory = itemsFlow.asPagingSourceFactory(
      coroutineScope = testScope
    )

    val pagingSource = pagingSourceFactory()
}

You can then write a test for the separator logic as in the following snippet:

fun test_separators_are_added_every_10_items() = runTest {
  // Create your ViewModel
  val viewModel = MyViewModel(
    myRepository = FakeMyRepository()
  )
  // Get the Flow of PagingData from the ViewModel with the separator transformations applied
  val items: Flow<PagingData<String>> = viewModel.items
                  
  val snapshot: List<String> = items.asSnapshot(coroutineScope = this) {}

  // With the asSnapshot complete, you can now verify that the snapshot
  // has the expected separators  
}

Data layer tests

Write unit tests for the components in your data layer to ensure that they load the data from your data sources appropriately. Provide mock versions of dependencies to verify that the components under test function correctly in isolation. The main components that you need to test in the repository layer are the PagingSource and the RemoteMediator. The examples in the sections to follow are based on the Paging with Network Sample.

PagingSource tests

Unit tests for your PagingSource classes involve setting up the PagingSource instance and verifying that the load() function returns the correct paged data based on a LoadParams argument. Provide mock data to the PagingSource constructor so that you have control over the data in your tests. You can pass in a default value for primitive data types but you need to pass in a mock version for other objects, such as a database or an API implementation. This gives you full control over the output of the mock data source when the PagingSource under test interacts with it.

The constructor parameters for your specific PagingSource implementation dictate what type of test data you need to pass. In the following example, the PagingSource implementation requires a RedditApi object as well as a String for the subreddit name. You can pass a default value for the String parameter, but for the RedditApi parameter you must create a mock implementation for tests.

Kotlin

class ItemKeyedSubredditPagingSource(
  private val redditApi: RedditApi,
  private val subredditName: String
) : PagingSource<String, RedditPost>() {
  ...
}

Java

public class ItemKeyedSubredditPagingSource
  extends RxPagingSource<String, RedditPost> {

  @NonNull
  private RedditApi redditApi;
  @NonNull
  private String subredditName;

  ItemKeyedSubredditPagingSource(
    @NotNull RedditApi redditApi,
    @NotNull String subredditName
  ) {
    this.redditApi = redditApi;
    this.subredditName = subredditName;
    ...
  }
}

Java

public class ItemKeyedSubredditPagingSource
  extends ListenableFuturePagingSource<String, RedditPost> {

  @NonNull
  private RedditApi redditApi;
  @NonNull
  private String subredditName;
  @NonNull
  private Executor bgExecutor;

  public ItemKeyedSubredditPagingSource(
    @NotNull RedditApi redditApi,
    @NotNull String subredditName,
    @NonNull Executor bgExecutor
  ) {
    this.redditApi = redditApi;
    this.subredditName = subredditName;
    this.bgExecutor = bgExecutor;
    ...
  }
}

In this example, the RedditApi parameter is a Retrofit interface that defines the server requests as well as the response classes. A mock version can implement the interface, override any required functions, and provide convenience methods to configure how the mock object should react in tests.

After the mock objects are in place, set up the dependencies and initialize the PagingSource object in the test. The following example demonstrates initializing the MockRedditApi object with a list of test posts:

Kotlin

@OptIn(ExperimentalCoroutinesApi::class)
class SubredditPagingSourceTest {
  private val postFactory = PostFactory()
  private val mockPosts = listOf(
    postFactory.createRedditPost(DEFAULT_SUBREDDIT),
    postFactory.createRedditPost(DEFAULT_SUBREDDIT),
    postFactory.createRedditPost(DEFAULT_SUBREDDIT)
  )
  private val mockApi = MockRedditApi().apply {
    mockPosts.forEach { post -> addPost(post) }
  }

  @Test
  fun loadReturnsPageWhenOnSuccessfulLoadOfItemKeyedData() = runTest {
    val pagingSource = ItemKeyedSubredditPagingSource(mockApi, DEFAULT_SUBREDDIT)
  }
}

Java

public class SubredditPagingSourceTest {
  static PostFactory postFactory = new PostFactory();
  static List<RedditPost> mockPosts = new ArrayList<>();
  static MockRedditApi mockApi = new MockRedditApi();

  static {
    for (int i=0; i<3; i++) {
      RedditPost post = postFactory.createRedditPost(DEFAULT_SUBREDDIT);
      mockPosts.add(post);
      mockApi.addPost(post);
    }
  }

  @After
  public void tearDown() {
    // Clear the failure message after each test run.
    mockApi.setFailureMsg(null);
  }

  @Test
  public void loadReturnsPageWhenOnSuccessfulLoadOfItemKeyedData()
    throws InterruptedException {

    ItemKeyedSubredditPagingSource pagingSource =
      new ItemKeyedSubredditPagingSource(mockApi, DEFAULT_SUBREDDIT);
  }
}

Java

public class SubredditPagingSourceTest {
  static PostFactory postFactory = new PostFactory();
  static List<RedditPost> mockPosts = new ArrayList<>();
  static MockRedditApi mockApi = new MockRedditApi();

  static {
    for (int i=0; i<3; i++) {
      RedditPost post = postFactory.createRedditPost(DEFAULT_SUBREDDIT);
      mockPosts.add(post);
      mockApi.addPost(post);
    }
  }

  @After
  public void tearDown() {
    // Clear the failure message after each test run.
    mockApi.setFailureMsg(null);
  }

  @Test
  public void loadReturnsPageWhenOnSuccessfulLoadOfItemKeyedData()
    throws ExecutionException, InterruptedException {

    ItemKeyedSubredditPagingSource pagingSource =
      new ItemKeyedSubredditPagingSource(
        mockApi,
        DEFAULT_SUBREDDIT,
        new CurrentThreadExecutor()
      );

  }
}

The next step is to test the PagingSource object. The load() method is the main focus of the tests. The following example demonstrates an assertion verifying that the load() method returns the correct data, previous key, and next key for a given LoadParams parameter:

Kotlin

@Test
// Since load is a suspend function, runTest is used to ensure that it
// runs on the test thread.
fun loadReturnsPageWhenOnSuccessfulLoadOfItemKeyedData() = runTest {
  val pagingSource = ItemKeyedSubredditPagingSource(mockApi, DEFAULT_SUBREDDIT)
  assertEquals(
    expected = Page(
      data = listOf(mockPosts[0], mockPosts[1]),
      prevKey = mockPosts[0].name,
      nextKey = mockPosts[1].name
    ),
    actual = pagingSource.load(
      Refresh(
        key = null,
        loadSize = 2,
        placeholdersEnabled = false
      )
    ),
  )
}

Java

@Test
public void loadReturnsPageWhenOnSuccessfulLoadOfItemKeyedData()
  throws InterruptedException {

  ItemKeyedSubredditPagingSource pagingSource =
    new ItemKeyedSubredditPagingSource(mockApi, DEFAULT_SUBREDDIT);

  LoadParams.Refresh<String> refreshRequest =
    new LoadParams.Refresh<>(null, 2, false);

  pagingSource.loadSingle(refreshRequest)
    .test()
    .await()
    .assertValueCount(1)
    .assertValue(new LoadResult.Page<>(
      mockPosts.subList(0, 2),
      mockPosts.get(0).getName(),
      mockPosts.get(1).getName()
    ));
}

Java

@Test
public void loadReturnsPageWhenOnSuccessfulLoadOfItemKeyedData()
  throws ExecutionException, InterruptedException {

  ItemKeyedSubredditPagingSource pagingSource = new ItemKeyedSubredditPagingSource(
    mockApi,
    DEFAULT_SUBREDDIT,
    new CurrentThreadExecutor()
  );

  PagingSource.LoadParams.Refresh<String> refreshRequest =
    new PagingSource.LoadParams.Refresh<>(null, 2, false);

  PagingSource.LoadResult<String, RedditPost> result =
    pagingSource.loadFuture(refreshRequest).get();

  PagingSource.LoadResult<String, RedditPost> expected =
    new PagingSource.LoadResult.Page<>(
      mockPosts.subList(0, 2),
      mockPosts.get(0).getName(),
      mockPosts.get(1).getName()
    );

  assertThat(result, equalTo(expected));
}

The previous example demonstrates testing a PagingSource that uses item-keyed paging. If the data source that you are using is page-keyed instead, then the PagingSource tests will be different. The main difference is the previous and next keys that are expected from the load() method.

Kotlin

@Test
fun loadReturnsPageWhenOnSuccessfulLoadOfPageKeyedData() = runTest {
  val pagingSource = PageKeyedSubredditPagingSource(mockApi, DEFAULT_SUBREDDIT)
  assertEquals(
    expected = Page(
      data = listOf(mockPosts[0], mockPosts[1]),
      prevKey = null,
      nextKey = mockPosts[1].id
    ),
    actual = pagingSource.load(
      Refresh(
        key = null,
        loadSize = 2,
        placeholdersEnabled = false
      )
    ),
  )
}

Java

@Test
public void loadReturnsPageWhenOnSuccessfulLoadOfPageKeyedData()
  throws InterruptedException {

  PageKeyedSubredditPagingSource pagingSource =
    new PageKeyedSubredditPagingSource(mockApi, DEFAULT_SUBREDDIT);

  LoadParams.Refresh<String> refreshRequest =
    new LoadParams.Refresh<>(null, 2, false);

  pagingSource.loadSingle(refreshRequest)
    .test()
    .await()
    .assertValueCount(1)
    .assertValue(new LoadResult.Page<>(
      mockPosts.subList(0, 2),
      null,
      mockPosts.get(1).getId()
    ));
}

Java

@Test
public void loadReturnsPageWhenOnSuccessfulLoadOfPageKeyedData()
  throws InterruptedException, ExecutionException {

  PageKeyedSubredditPagingSource pagingSource =
    new PageKeyedSubredditPagingSource(
      mockApi,
      DEFAULT_SUBREDDIT,
      new CurrentThreadExecutor()
    );

  PagingSource.LoadParams.Refresh<String> refreshRequest =
    new PagingSource.LoadParams.Refresh<>(null, 2, false);

  PagingSource.LoadResult<String, RedditPost> result =
    pagingSource.loadFuture(refreshRequest).get();

  PagingSource.LoadResult<String, RedditPost> expected =
    new PagingSource.LoadResult.Page<>(
      mockPosts.subList(0, 2),
      null,
      mockPosts.get(1).getId()
    );

  assertThat(result, equalTo(expected));
}

RemoteMediator tests

The goal of the RemoteMediator unit tests is to verify that the load() function returns the correct MediatorResult. Tests for side effects, such as data being inserted into the database, are better suited for integration tests.

The first step is to determine what dependencies your RemoteMediator implementation needs. The following example demonstrates a RemoteMediator implementation that requires a Room database, a Retrofit interface, and a search string:

Kotlin

@OptIn(ExperimentalPagingApi::class)
class PageKeyedRemoteMediator(
  private val db: RedditDb,
  private val redditApi: RedditApi,
  private val subredditName: String
) : RemoteMediator<Int, RedditPost>() {
  ...
}

Java

public class PageKeyedRemoteMediator
  extends RxRemoteMediator<Integer, RedditPost> {

  @NonNull
  private RedditDb db;
  @NonNull
  private RedditPostDao postDao;
  @NonNull
  private SubredditRemoteKeyDao remoteKeyDao;
  @NonNull
  private RedditApi redditApi;
  @NonNull
  private String subredditName;

  public PageKeyedRemoteMediator(
    @NonNull RedditDb db,
    @NonNull RedditApi redditApi,
    @NonNull String subredditName
  ) {
      this.db = db;
      this.postDao = db.posts();
      this.remoteKeyDao = db.remoteKeys();
      this.redditApi = redditApi;
      this.subredditName = subredditName;
      ...
  }
}

Java

public class PageKeyedRemoteMediator
  extends ListenableFutureRemoteMediator<Integer, RedditPost> {

  @NonNull
  private RedditDb db;
  @NonNull
  private RedditPostDao postDao;
  @NonNull
  private SubredditRemoteKeyDao remoteKeyDao;
  @NonNull
  private RedditApi redditApi;
  @NonNull
  private String subredditName;
  @NonNull
  private Executor bgExecutor;

  public PageKeyedRemoteMediator(
    @NonNull RedditDb db,
    @NonNull RedditApi redditApi,
    @NonNull String subredditName,
    @NonNull Executor bgExecutor
  ) {
    this.db = db;
    this.postDao = db.posts();
    this.remoteKeyDao = db.remoteKeys();
    this.redditApi = redditApi;
    this.subredditName = subredditName;
    this.bgExecutor = bgExecutor;
    ...
  }
}

You can provide the Retrofit interface and the search string as demonstrated in the PagingSource tests section. Providing a mock version of the Room database is very involved, so it can be easier to provide an in-memory implementation of the database instead of a full mock version. Because creating a Room database requires a Context object, you must place this RemoteMediator test in the androidTest directory and execute it with the AndroidJUnit4 test runner so that it has access to a test application context. For more information about instrumented tests, see Build instrumented unit tests.

Define tear-down functions to ensure that state does not leak between test functions. This ensures consistent results between test runs.

Kotlin

@ExperimentalPagingApi
@OptIn(ExperimentalCoroutinesApi::class)
@RunWith(AndroidJUnit4::class)
class PageKeyedRemoteMediatorTest {
  private val postFactory = PostFactory()
  private val mockPosts = listOf(
    postFactory.createRedditPost(SubRedditViewModel.DEFAULT_SUBREDDIT),
    postFactory.createRedditPost(SubRedditViewModel.DEFAULT_SUBREDDIT),
    postFactory.createRedditPost(SubRedditViewModel.DEFAULT_SUBREDDIT)
  )
  private val mockApi = mockRedditApi()

  private val mockDb = RedditDb.create(
    ApplicationProvider.getApplicationContext(),
    useInMemory = true
  )

  @After
  fun tearDown() {
    mockDb.clearAllTables()
    // Clear out failure message to default to the successful response.
    mockApi.failureMsg = null
    // Clear out posts after each test run.
    mockApi.clearPosts()
  }
}

Java

@RunWith(AndroidJUnit4.class)
public class PageKeyedRemoteMediatorTest {
  static PostFactory postFactory = new PostFactory();
  static List<RedditPost> mockPosts = new ArrayList<>();
  static MockRedditApi mockApi = new MockRedditApi();
  private RedditDb mockDb = RedditDb.Companion.create(
    ApplicationProvider.getApplicationContext(),
    true
  );

  static {
    for (int i=0; i<3; i++) {
      RedditPost post = postFactory.createRedditPost(DEFAULT_SUBREDDIT);
      mockPosts.add(post);
    }
  }

  @After
  public void tearDown() {
    mockDb.clearAllTables();
    // Clear the failure message after each test run.
    mockApi.setFailureMsg(null);
    // Clear out posts after each test run.
    mockApi.clearPosts();
  }
}

Java

@RunWith(AndroidJUnit4.class)
public class PageKeyedRemoteMediatorTest {
  static PostFactory postFactory = new PostFactory();
  static List<RedditPost> mockPosts = new ArrayList<>();
  static MockRedditApi mockApi = new MockRedditApi();

  private RedditDb mockDb = RedditDb.Companion.create(
    ApplicationProvider.getApplicationContext(),
    true
  );

  static {
    for (int i=0; i<3; i++) {
      RedditPost post = postFactory.createRedditPost(DEFAULT_SUBREDDIT);
      mockPosts.add(post);
    }
  }

  @After
  public void tearDown() {
    mockDb.clearAllTables();
    // Clear the failure message after each test run.
    mockApi.setFailureMsg(null);
    // Clear out posts after each test run.
    mockApi.clearPosts();
  }
}

The next step is to test the load() function. In this example, there are three cases to test:

  • The first case is when mockApi returns valid data. The load() function should return MediatorResult.Success, and the endOfPaginationReached property should be false.
  • The second case is when mockApi returns a successful response, but the returned data is empty. The load() function should return MediatorResult.Success, and the endOfPaginationReached property should be true.
  • The third case is when mockApi throws an exception when fetching the data. The load() function should return MediatorResult.Error.

Follow these steps to test the first case:

  1. Set up the mockApi with the post data to return.
  2. Initialize the RemoteMediator object.
  3. Test the load() function.

Kotlin

@Test
fun refreshLoadReturnsSuccessResultWhenMoreDataIsPresent() = runTest {
  // Add mock results for the API to return.
  mockPosts.forEach { post -> mockApi.addPost(post) }
  val remoteMediator = PageKeyedRemoteMediator(
    mockDb,
    mockApi,
    SubRedditViewModel.DEFAULT_SUBREDDIT
  )
  val pagingState = PagingState<Int, RedditPost>(
    listOf(),
    null,
    PagingConfig(10),
    10
  )
  val result = remoteMediator.load(LoadType.REFRESH, pagingState)
  assertTrue { result is MediatorResult.Success }
  assertFalse { (result as MediatorResult.Success).endOfPaginationReached }
}

Java

@Test
public void refreshLoadReturnsSuccessResultWhenMoreDataIsPresent()
  throws InterruptedException {

  // Add mock results for the API to return.
  for (RedditPost post: mockPosts) {
    mockApi.addPost(post);
  }

  PageKeyedRemoteMediator remoteMediator = new PageKeyedRemoteMediator(
    mockDb,
    mockApi,
    SubRedditViewModel.DEFAULT_SUBREDDIT
  );
  PagingState<Integer, RedditPost> pagingState = new PagingState<>(
    new ArrayList(),
    null,
    new PagingConfig(10),
    10
  );
  remoteMediator.loadSingle(LoadType.REFRESH, pagingState)
    .test()
    .await()
    .assertValueCount(1)
    .assertValue(value -> value instanceof RemoteMediator.MediatorResult.Success &&
      ((RemoteMediator.MediatorResult.Success) value).endOfPaginationReached() == false);
}

Java

@Test
public void refreshLoadReturnsSuccessResultWhenMoreDataIsPresent()
  throws InterruptedException, ExecutionException {

  // Add mock results for the API to return.
  for (RedditPost post: mockPosts) {
    mockApi.addPost(post);
  }

  PageKeyedRemoteMediator remoteMediator = new PageKeyedRemoteMediator(
    mockDb,
    mockApi,
    SubRedditViewModel.DEFAULT_SUBREDDIT,
    new CurrentThreadExecutor()
  );
  PagingState<Integer, RedditPost> pagingState = new PagingState<>(
    new ArrayList(),
    null,
    new PagingConfig(10),
    10
  );

  RemoteMediator.MediatorResult result =
    remoteMediator.loadFuture(LoadType.REFRESH, pagingState).get();

  assertThat(result, instanceOf(RemoteMediator.MediatorResult.Success.class));
  assertFalse(((RemoteMediator.MediatorResult.Success) result).endOfPaginationReached());
}

The second test requires the mockApi to return an empty result. Because you clear the data from the mockApi after each test run, it will return an empty result by default.

Kotlin

@Test
fun refreshLoadSuccessAndEndOfPaginationWhenNoMoreData() = runTest {
  // To test endOfPaginationReached, don't set up the mockApi to return post
  // data here.
  val remoteMediator = PageKeyedRemoteMediator(
    mockDb,
    mockApi,
    SubRedditViewModel.DEFAULT_SUBREDDIT
  )
  val pagingState = PagingState<Int, RedditPost>(
    listOf(),
    null,
    PagingConfig(10),
    10
  )
  val result = remoteMediator.load(LoadType.REFRESH, pagingState)
  assertTrue { result is MediatorResult.Success }
  assertTrue { (result as MediatorResult.Success).endOfPaginationReached }
}

Java

@Test
public void refreshLoadSuccessAndEndOfPaginationWhenNoMoreData()
  throws InterruptedException() {

  // To test endOfPaginationReached, don't set up the mockApi to return post
  // data here.
  PageKeyedRemoteMediator remoteMediator = new PageKeyedRemoteMediator(
    mockDb,
    mockApi,
    SubRedditViewModel.DEFAULT_SUBREDDIT
  );
  PagingState<Integer, RedditPost> pagingState = new PagingState<>(
    new ArrayList(),
    null,
    new PagingConfig(10),
    10
  );
  remoteMediator.loadSingle(LoadType.REFRESH, pagingState)
    .test()
    .await()
    .assertValueCount(1)
    .assertValue(value -> value instanceof RemoteMediator.MediatorResult.Success &&
      ((RemoteMediator.MediatorResult.Success) value).endOfPaginationReached() == true);
}

Java

@Test
public void refreshLoadSuccessAndEndOfPaginationWhenNoMoreData()
  throws InterruptedException, ExecutionException {

  // To test endOfPaginationReached, don't set up the mockApi to return post
  // data here.
  PageKeyedRemoteMediator remoteMediator = new PageKeyedRemoteMediator(
    mockDb,
    mockApi,
    SubRedditViewModel.DEFAULT_SUBREDDIT,
    new CurrentThreadExecutor()
  );
  PagingState<Integer, RedditPost> pagingState = new PagingState<>(
    new ArrayList(),
    null,
    new PagingConfig(10),
    10
  );

  RemoteMediator.MediatorResult result =
    remoteMediator.loadFuture(LoadType.REFRESH, pagingState).get();

  assertThat(result, instanceOf(RemoteMediator.MediatorResult.Success.class));
  assertTrue(((RemoteMediator.MediatorResult.Success) result).endOfPaginationReached());
}

The final test requires the mockApi to throw an exception so that the test can verify that the load() function correctly returns MediatorResult.Error.

Kotlin

@Test
fun refreshLoadReturnsErrorResultWhenErrorOccurs() = runTest {
  // Set up failure message to throw exception from the mock API.
  mockApi.failureMsg = "Throw test failure"
  val remoteMediator = PageKeyedRemoteMediator(
    mockDb,
    mockApi,
    SubRedditViewModel.DEFAULT_SUBREDDIT
  )
  val pagingState = PagingState<Int, RedditPost>(
    listOf(),
    null,
    PagingConfig(10),
    10
  )
  val result = remoteMediator.load(LoadType.REFRESH, pagingState)
  assertTrue {result is MediatorResult.Error }
}

Java

@Test
public void refreshLoadReturnsErrorResultWhenErrorOccurs()
  throws InterruptedException {

  // Set up failure message to throw exception from the mock API.
  mockApi.setFailureMsg("Throw test failure");
  PageKeyedRemoteMediator remoteMediator = new PageKeyedRemoteMediator(
    mockDb,
    mockApi,
    SubRedditViewModel.DEFAULT_SUBREDDIT
  );
  PagingState<Integer, RedditPost> pagingState = new PagingState<>(
    new ArrayList(),
    null,
    new PagingConfig(10),
    10
  );
  remoteMediator.loadSingle(LoadType.REFRESH, pagingState)
    .test()
    .await()
    .assertValueCount(1)
    .assertValue(value -> value instanceof RemoteMediator.MediatorResult.Error);
}

Java

@Test
public void refreshLoadReturnsErrorResultWhenErrorOccurs()
  throws InterruptedException, ExecutionException {

  // Set up failure message to throw exception from the mock API.
  mockApi.setFailureMsg("Throw test failure");
  PageKeyedRemoteMediator remoteMediator = new PageKeyedRemoteMediator(
    mockDb,
    mockApi,
    SubRedditViewModel.DEFAULT_SUBREDDIT,
    new CurrentThreadExecutor()
  );
  PagingState<Integer, RedditPost> pagingState = new PagingState<>(
    new ArrayList(),
    null,
    new PagingConfig(10),
    10
  );
  RemoteMediator.MediatorResult result =
    remoteMediator.loadFuture(LoadType.REFRESH, pagingState).get();

  assertThat(result, instanceOf(RemoteMediator.MediatorResult.Error.class));
}

End-to-end tests

Unit tests provide assurance that individual Paging components work in isolation, but end-to-end tests provide more confidence that the application works as a whole. These tests will still need some mock dependencies, but generally they will cover most of your app code.

The example in this section uses a mock API dependency to avoid using the network in tests. The mock API is configured to return a consistent set of test data, resulting in repeatable tests. Decide which dependencies to swap out for mock implementations based on what each dependency does, how consistent its output is, and how much fidelity you need from your tests.

Write your code in a way that allows you to easily swap in mock versions of your dependencies. The following example uses a basic service locator implementation to provide and change dependencies as needed. In larger apps, using a dependency injection library like Hilt can help manage more-complex dependency graphs.

Kotlin

class RedditActivityTest {

  companion object {
    private const val TEST_SUBREDDIT = "test"
  }

  private val postFactory = PostFactory()
  private val mockApi = MockRedditApi().apply {
    addPost(postFactory.createRedditPost(DEFAULT_SUBREDDIT))
    addPost(postFactory.createRedditPost(TEST_SUBREDDIT))
    addPost(postFactory.createRedditPost(TEST_SUBREDDIT))
  }

  @Before
  fun init() {
    val app = ApplicationProvider.getApplicationContext<Application>()
    // Use a controlled service locator with a mock API.
    ServiceLocator.swap(
      object : DefaultServiceLocator(app = app, useInMemoryDb = true) {
        override fun getRedditApi(): RedditApi = mockApi
      }
    )
  }
}

Java

public class RedditActivityTest {

  public static final String TEST_SUBREDDIT = "test";

  private static PostFactory postFactory = new PostFactory();
  private static MockRedditApi mockApi = new MockRedditApi();

  static {
    mockApi.addPost(postFactory.createRedditPost(DEFAULT_SUBREDDIT));
    mockApi.addPost(postFactory.createRedditPost(TEST_SUBREDDIT));
    mockApi.addPost(postFactory.createRedditPost(TEST_SUBREDDIT));
  }

  @Before
  public void setup() {
    Application app = ApplicationProvider.getApplicationContext();
    // Use a controlled service locator with a mock API.
    ServiceLocator.Companion.swap(
      new DefaultServiceLocator(app, true) {
        @NotNull
        @Override
        public RedditApi getRedditApi() {
          return mockApi;
        }
      }
    );
  }
}

Java

public class RedditActivityTest {

  public static final String TEST_SUBREDDIT = "test";

  private static PostFactory postFactory = new PostFactory();
  private static MockRedditApi mockApi = new MockRedditApi();

  static {
    mockApi.addPost(postFactory.createRedditPost(DEFAULT_SUBREDDIT));
    mockApi.addPost(postFactory.createRedditPost(TEST_SUBREDDIT));
    mockApi.addPost(postFactory.createRedditPost(TEST_SUBREDDIT));
  }

  @Before
  public void setup() {
    Application app = ApplicationProvider.getApplicationContext();
    // Use a controlled service locator with a mock API.
    ServiceLocator.Companion.swap(
      new DefaultServiceLocator(app, true) {
        @NotNull
        @Override
        public RedditApi getRedditApi() {
          return mockApi;
        }
      }
    );
  }
}

After you set up the test structure, the next step is to verify that the data returned by the Pager implementation is correct. One test should ensure that the Pager object loads the default data when the page first loads, and another test should verify that the Pager object correctly loads additional data based on user input. In the following example, the test verifies that the Pager object updates the RecyclerView.Adapter with the correct number of items returned from the API when the user enters a different subreddit to search.

Kotlin

@Test
fun loadsTheDefaultResults() {
    ActivityScenario.launch(RedditActivity::class.java)

    onView(withId(R.id.list)).check { view, noViewFoundException ->
        if (noViewFoundException != null) {
            throw noViewFoundException
        }

        val recyclerView = view as RecyclerView
        assertEquals(1, recyclerView.adapter?.itemCount)
    }
}

@Test
// Verify that the default data is swapped out when the user searches for a
// different subreddit.
fun loadsTheTestResultsWhenSearchingForSubreddit() {
  ActivityScenario.launch(RedditActivity::class.java )

  onView(withId(R.id.list)).check { view, noViewFoundException ->
    if (noViewFoundException != null) {
      throw noViewFoundException
    }

    val recyclerView = view as RecyclerView
    // Verify that it loads the default data first.
    assertEquals(1, recyclerView.adapter?.itemCount)
  }

  // Search for test subreddit instead of default to trigger new data load.
  onView(withId(R.id.input)).perform(
    replaceText(TEST_SUBREDDIT),
    pressKey(KeyEvent.KEYCODE_ENTER)
  )

  onView(withId(R.id.list)).check { view, noViewFoundException ->
    if (noViewFoundException != null) {
      throw noViewFoundException
    }

    val recyclerView = view as RecyclerView
    assertEquals(2, recyclerView.adapter?.itemCount)
  }
}

Java

@Test
public void loadsTheDefaultResults() {
  ActivityScenario.launch(RedditActivity.class);

  onView(withId(R.id.list)).check((view, noViewFoundException) -> {
    if (noViewFoundException != null) {
      throw noViewFoundException;
    }

    RecyclerView recyclerView = (RecyclerView) view;
    assertEquals(1, recyclerView.getAdapter().getItemCount());
  });
}

@Test
// Verify that the default data is swapped out when the user searches for a
// different subreddit.
public void loadsTheTestResultsWhenSearchingForSubreddit() {
  ActivityScenario.launch(RedditActivity.class);

  onView(withId(R.id.list)).check((view, noViewFoundException) -> {
    if (noViewFoundException != null) {
      throw noViewFoundException;
    }

    RecyclerView recyclerView = (RecyclerView) view;
    // Verify that it loads the default data first.
    assertEquals(1, recyclerView.getAdapter().getItemCount());
  });

  // Search for test subreddit instead of default to trigger new data load.
  onView(withId(R.id.input)).perform(
    replaceText(TEST_SUBREDDIT),
    pressKey(KeyEvent.KEYCODE_ENTER)
  );

  onView(withId(R.id.list)).check((view, noViewFoundException) -> {
    if (noViewFoundException != null) {
      throw noViewFoundException;
    }

    RecyclerView recyclerView = (RecyclerView) view;
    assertEquals(2, recyclerView.getAdapter().getItemCount());
  });
}

Java

@Test
public void loadsTheDefaultResults() {
  ActivityScenario.launch(RedditActivity.class);

  onView(withId(R.id.list)).check((view, noViewFoundException) -> {
    if (noViewFoundException != null) {
      throw noViewFoundException;
    }

    RecyclerView recyclerView = (RecyclerView) view;
    assertEquals(1, recyclerView.getAdapter().getItemCount());
  });
}

@Test
// Verify that the default data is swapped out when the user searches for a
// different subreddit.
public void loadsTheTestResultsWhenSearchingForSubreddit() {
  ActivityScenario.launch(RedditActivity.class);

  onView(withId(R.id.list)).check((view, noViewFoundException) -> {
    if (noViewFoundException != null) {
      throw noViewFoundException;
    }

    RecyclerView recyclerView = (RecyclerView) view;
    // Verify that it loads the default data first.
    assertEquals(1, recyclerView.getAdapter().getItemCount());
  });

  // Search for test subreddit instead of default to trigger new data load.
  onView(withId(R.id.input)).perform(
    replaceText(TEST_SUBREDDIT),
    pressKey(KeyEvent.KEYCODE_ENTER)
  );

  onView(withId(R.id.list)).check((view, noViewFoundException) -> {
    if (noViewFoundException != null) {
      throw noViewFoundException;
    }

    RecyclerView recyclerView = (RecyclerView) view;
    assertEquals(2, recyclerView.getAdapter().getItemCount());
  });
}

Instrumented tests should verify that the data displays correctly in the UI. Do this either by verifying that the correct number of items exists in the RecyclerView.Adapter, or by iterating through the individual row views and verifying that the data is formatted correctly.