Testing Kotlin coroutines on Android

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class="devsite-breadcrumb-item "> <div class="devsite-breadcrumb-guillemet material-icons" aria-hidden="true"></div> <a href="https://developer.android.com/kotlin" class="devsite-breadcrumb-link gc-analytics-event" data-category="Site-Wide Custom Events" data-label="Breadcrumbs" data-value="3" track-type="globalNav" track-name="breadcrumb" track-metadata-position="3" track-metadata-eventdetail="Kotlin" > Kotlin </a> </li> <li class="devsite-breadcrumb-item "> <div class="devsite-breadcrumb-guillemet material-icons" aria-hidden="true"></div> <a href="https://developer.android.com/kotlin/first" class="devsite-breadcrumb-link gc-analytics-event" data-category="Site-Wide Custom Events" data-label="Breadcrumbs" data-value="4" track-type="globalNav" track-name="breadcrumb" track-metadata-position="4" track-metadata-eventdetail="" > Guides </a> </li> </ul> <devsite-thumb-rating position="header"> </devsite-thumb-rating> </div> <h1 class="devsite-page-title" tabindex="-1"> Testing Kotlin coroutines on Android </h1> <devsite-feature-tooltip ack-key="AckCollectionsBookmarkTooltipDismiss" analytics-category="Site-Wide Custom Events" analytics-action-show="Callout Profile displayed" analytics-action-close="Callout Profile dismissed" analytics-label="Create Collection Callout" class="devsite-page-bookmark-tooltip nocontent" dismiss-button="true" id="devsite-collections-dropdown" dismiss-button-text="Dismiss" close-button-text="Got it"> <devsite-bookmark></devsite-bookmark> Stay organized with collections Save and categorize content based on your preferences. </devsite-feature-tooltip> <div class="devsite-page-title-meta"><devsite-view-release-notes></devsite-view-release-notes></div> <devsite-toc class="devsite-nav" depth="2" devsite-toc-embedded > </devsite-toc> <devsite-recommendations-dropdown class="nocontent"></devsite-recommendations-dropdown> <div class="devsite-article-body clearfix "> Unit testing code that uses <a href="/kotlin/coroutines">coroutines</a> requires some extra attention, as their execution can be asynchronous and happen across multiple threads. This guide covers how suspending functions can be tested, the testing constructs you need to be familiar with, and how to make your code that uses coroutines testable. The APIs used in this guide are part of the <a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-test/" class="external">kotlinx.coroutines.test</a> library. Make sure to <a href="https://github.com/Kotlin/kotlinx.coroutines/tree/master/kotlinx-coroutines-test#using-in-your-project" class="external">add the artifact</a> as a test dependency to your project to have access to these APIs. <div></div><devsite-code><pre class="devsite-click-to-copy" translate="no" dir="ltr" is-upgraded syntax="Kotlin"><code translate="no" dir="ltr">dependencies { testImplementation "org.jetbrains.kotlinx:kotlinx-coroutines-test:$coroutines_version" } </code></pre></devsite-code><aside class="note">Note: This guide uses the coroutine testing APIs introduced in kotlinx.coroutines.test 1.6. If you’re looking to migrate from an earlier version, use the <a href="https://github.com/Kotlin/kotlinx.coroutines/blob/master/kotlinx-coroutines-test/MIGRATION.md" class="external">migration guide</a>.</aside> <h2 id="invoking-suspending-functions" data-text="Invoking suspending functions in tests" tabindex="-1">Invoking suspending functions in tests</h2> To call suspending functions in tests, you need to be in a coroutine. As JUnit test functions themselves aren't suspending functions, you need to call a coroutine builder inside your tests to start a new coroutine. <a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-test/kotlinx.coroutines.test/run-test.html" class="external"><code translate="no" dir="ltr">runTest</code></a> is a coroutine builder designed for testing. Use this to wrap any tests that include coroutines. Note that coroutines can be started not only directly in the test body, but also by the objects being used in the test. <div></div><devsite-code><pre class="devsite-click-to-copy" translate="no" dir="ltr" is-upgraded syntax="Kotlin">suspend fun fetchData(): String { delay(1000L) return "Hello world" } @Test fun dataShouldBeHelloWorld() = runTest { val data = fetchData() assertEquals("Hello world", data) }<div class="devsite-github-link nocode no-select"><a target="_top" href="https://github.com/android/snippets/blob/f595a0d317fd8aca6b33d4344bf87696cd45c481/kotlin/src/test/kotlin/com/example/android/coroutines/testing/SuspendingFunctionTests.kt#L26-L35" class="gc-analytics-event" data-category="github_link" data-label="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/SuspendingFunctionTests.kt#coroutine_test_runtest" data-code-snippet="true" data-github-path="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/SuspendingFunctionTests.kt" data-region-tag="coroutine_test_runtest">SuspendingFunctionTests.kt</a></div></pre></devsite-code> In general, you should have one invocation of <code translate="no" dir="ltr">runTest</code> per test, and using an <a href="https://kotlinlang.org/docs/functions.html#single-expression-functions" class="external">expression body</a> is recommended. <aside class="key-point">Key Point: Use <a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-test/kotlinx.coroutines.test/run-test.html" class="external"><code translate="no" dir="ltr">runTest</code></a> for tests that use coroutines.</aside> Wrapping your test’s code in <code translate="no" dir="ltr">runTest</code> will work for testing basic suspending functions, and it will automatically skip any delays in coroutines, making the test above complete much faster than one second. However, there are additional considerations to make, depending on what happens in your code under test: <ul> <li>When your code creates new coroutines other than the top-level test coroutine that <code translate="no" dir="ltr">runTest</code> creates, you’ll need to control how those new coroutines are scheduled by <a href="#testdispatchers">choosing the appropriate <code translate="no" dir="ltr">TestDispatcher</code></a>.</li> <li>If your code moves the coroutine execution to other dispatchers (for example, by using <a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/with-context.html" class="external"><code translate="no" dir="ltr">withContext</code></a>), <code translate="no" dir="ltr">runTest</code> will still generally work, but delays will no longer be skipped, and tests will be less predictable as code runs on multiple threads. For these reasons, in tests you should <a href="#injecting-test-dispatchers">inject test dispatchers</a> to replace real dispatchers.</li> </ul> <h2 id="testdispatchers" data-text="TestDispatchers" tabindex="-1">TestDispatchers</h2> <a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-test/kotlinx.coroutines.test/-test-dispatcher/index.html" class="external"><code translate="no" dir="ltr">TestDispatchers</code></a> are <a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-coroutine-dispatcher/index.html" class="external"><code translate="no" dir="ltr">CoroutineDispatcher</code></a> implementations for testing purposes. You’ll need to use <code translate="no" dir="ltr">TestDispatchers</code> if new coroutines are created during the test to make the execution of the new coroutines predictable. <aside class="note">Note: New coroutines can be created directly in the test body, but also in any of the code you’re calling in your test (for example, inside the objects you’re testing).</aside> There are two available implementations of <code translate="no" dir="ltr">TestDispatcher</code>: <a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-test/kotlinx.coroutines.test/-standard-test-dispatcher.html" class="external"><code translate="no" dir="ltr">StandardTestDispatcher</code></a> and <a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-test/kotlinx.coroutines.test/-unconfined-test-dispatcher.html" class="external"><code translate="no" dir="ltr">UnconfinedTestDispatcher</code></a>, which perform different scheduling of newly-started coroutines. These both use a <a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-test/kotlinx.coroutines.test/-test-coroutine-scheduler/index.html" class="external"><code translate="no" dir="ltr">TestCoroutineScheduler</code></a> to control virtual time and manage running coroutines within a test. There should only be one scheduler instance used in a test, shared between all <code translate="no" dir="ltr">TestDispatchers</code>. See <a href="#injecting-test-dispatchers">Injecting TestDispatchers</a> to learn about sharing schedulers. To start the top-level test coroutine, <code translate="no" dir="ltr">runTest</code> creates a <a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-test/kotlinx.coroutines.test/-test-scope.html" class="external"><code translate="no" dir="ltr">TestScope</code></a>, which is an implementation of <a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-coroutine-scope/" class="external"><code translate="no" dir="ltr">CoroutineScope</code></a> that will always use a <code translate="no" dir="ltr">TestDispatcher</code>. If not specified, a <code translate="no" dir="ltr">TestScope</code> will create a <code translate="no" dir="ltr">StandardTestDispatcher</code> by default, and use that to run the top-level test coroutine. <code translate="no" dir="ltr">runTest</code> keeps track of the coroutines that are queued on the scheduler used by the dispatcher of its <code translate="no" dir="ltr">TestScope</code>, and will not return as long as there’s pending work on that scheduler. <aside class="key-point">Key Point: <code translate="no" dir="ltr">runTest</code> runs the test coroutine in a <code translate="no" dir="ltr">TestScope</code>, using a <code translate="no" dir="ltr">TestDispatcher. TestDispatchers</code> use a <code translate="no" dir="ltr">TestCoroutineScheduler</code> to control virtual time and schedule new coroutines in a test. All <code translate="no" dir="ltr">TestDispatchers</code> in a test must use the same scheduler instance.</aside> <h3 id="standardtestdispatcher" data-text="StandardTestDispatcher" tabindex="-1">StandardTestDispatcher</h3> When you start new coroutines on a <code translate="no" dir="ltr">StandardTestDispatcher</code>, they are queued up on the underlying scheduler, to be run whenever the test thread is free to use. To let these new coroutines run, you need to yield the test thread (free it up for other coroutines to use). This queueing behavior gives you precise control over how new coroutines run during the test, and it resembles the scheduling of coroutines in production code. <aside class="note">Note: For simplicity, the examples here show new coroutines started directly within <code translate="no" dir="ltr">runTest</code>, as <code translate="no" dir="ltr">runTest</code> uses a <code translate="no" dir="ltr">StandardTestDispatcher</code> by default. However, the scheduling behavior shown here applies to any coroutine started on a <code translate="no" dir="ltr">StandardTestDispatcher</code>, including ones <a href="#injecting-test-dispatchers">injected</a> into classes under test.</aside> If the test thread is never yielded during the execution of the top-level test coroutine, any new coroutines will only run after the test coroutine is done (but before <code translate="no" dir="ltr">runTest</code> returns): <div></div><devsite-code><pre class="devsite-click-to-copy" translate="no" dir="ltr" is-upgraded syntax="Kotlin">@Test fun standardTest() = runTest { val userRepo = UserRepository() launch { userRepo.register("Alice") } launch { userRepo.register("Bob") } assertEquals(listOf("Alice", "Bob"), userRepo.getAllUsers()) // ❌ Fails }<div class="devsite-github-link nocode no-select"><a target="_top" href="https://github.com/android/snippets/blob/f595a0d317fd8aca6b33d4344bf87696cd45c481/kotlin/src/test/kotlin/com/example/android/coroutines/testing/StandardTestDispatcherTest.kt#L29-L37" class="gc-analytics-event" data-category="github_link" data-label="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/StandardTestDispatcherTest.kt#coroutine_test_standard_failing" data-code-snippet="true" data-github-path="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/StandardTestDispatcherTest.kt" data-region-tag="coroutine_test_standard_failing">StandardTestDispatcherTest.kt</a></div></pre></devsite-code> There are several ways to yield the test coroutine to let queued-up coroutines run. All of these calls let other coroutines run on the test thread before returning: <ul> <li><a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-test/kotlinx.coroutines.test/-test-coroutine-scheduler/advance-until-idle.html" class="external"><code translate="no" dir="ltr">advanceUntilIdle</code></a>: Runs all other coroutines on the scheduler until there is nothing left in the queue. This is a good default choice to let all pending coroutines run, and it will work in most test scenarios.</li> <li><a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-test/kotlinx.coroutines.test/-test-coroutine-scheduler/advance-time-by.html" class="external"><code translate="no" dir="ltr">advanceTimeBy</code></a>: Advances virtual time by the given amount and runs any coroutines scheduled to run before that point in virtual time.</li> <li><a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-test/kotlinx.coroutines.test/-test-coroutine-scheduler/run-current.html" class="external"><code translate="no" dir="ltr">runCurrent</code></a>: Runs coroutines that are scheduled at the current virtual time.</li> </ul> To fix the previous test, <code translate="no" dir="ltr">advanceUntilIdle</code> can be used to let the two pending coroutines perform their work before continuing to the assertion: <div></div><devsite-code><pre class="devsite-click-to-copy" translate="no" dir="ltr" is-upgraded syntax="Kotlin">@Test fun standardTest() = runTest { val userRepo = UserRepository() launch { userRepo.register("Alice") } launch { userRepo.register("Bob") } advanceUntilIdle() // Yields to perform the registrations assertEquals(listOf("Alice", "Bob"), userRepo.getAllUsers()) // ✅ Passes }<div class="devsite-github-link nocode no-select"><a target="_top" href="https://github.com/android/snippets/blob/f595a0d317fd8aca6b33d4344bf87696cd45c481/kotlin/src/test/kotlin/com/example/android/coroutines/testing/StandardTestDispatcherTest.kt#L43-L52" class="gc-analytics-event" data-category="github_link" data-label="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/StandardTestDispatcherTest.kt#coroutine_test_standard_fixed" data-code-snippet="true" data-github-path="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/StandardTestDispatcherTest.kt" data-region-tag="coroutine_test_standard_fixed">StandardTestDispatcherTest.kt</a></div></pre></devsite-code> <aside class="note">Note: You could also <a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-job/join.html" class="external"><code translate="no" dir="ltr">join</code></a> the <a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-job/index.html" class="external"><code translate="no" dir="ltr">Job</code></a> instances returned by the <code translate="no" dir="ltr">launch</code> calls to make sure that the new coroutines are done before performing the assertion.</aside><aside class="key-point">Key Point: <code translate="no" dir="ltr">StandardTestDispatcher</code> offers precise control over the execution of coroutines, which is useful in complex test scenarios. <code translate="no" dir="ltr">runTest</code> uses a <code translate="no" dir="ltr">StandardTestDispatcher</code> by default.</aside> <h3 id="unconfinedtestdispatcher" data-text="UnconfinedTestDispatcher" tabindex="-1">UnconfinedTestDispatcher</h3> When new coroutines are started on an <code translate="no" dir="ltr">UnconfinedTestDispatcher</code>, they are started eagerly on the current thread. This means that they’ll start running immediately, without waiting for their coroutine builder to return. In many cases, this dispatching behavior results in simpler test code, as you don’t need to manually yield the test thread to let new coroutines run. However, this behavior is different from what you’ll see in production with non-test dispatchers. If your test focuses on concurrency, prefer using <code translate="no" dir="ltr">StandardTestDispatcher</code> instead. <aside class="key-point">Key Point: <code translate="no" dir="ltr">UnconfinedTestDispatcher</code> executes new coroutines eagerly, and it can be a good choice for simple tests with coroutines.</aside> To use this dispatcher for the top-level test coroutine in <code translate="no" dir="ltr">runTest</code> instead of the default one, create an instance and pass it in as a parameter. This will make new coroutines created within <code translate="no" dir="ltr">runTest</code> execute eagerly, as they inherit the dispatcher from the <code translate="no" dir="ltr">TestScope</code>. <div></div><devsite-code><pre class="devsite-click-to-copy" translate="no" dir="ltr" is-upgraded syntax="Kotlin">@Test fun unconfinedTest() = runTest(UnconfinedTestDispatcher()) { val userRepo = UserRepository() launch { userRepo.register("Alice") } launch { userRepo.register("Bob") } assertEquals(listOf("Alice", "Bob"), userRepo.getAllUsers()) // ✅ Passes }<div class="devsite-github-link nocode no-select"><a target="_top" href="https://github.com/android/snippets/blob/f595a0d317fd8aca6b33d4344bf87696cd45c481/kotlin/src/test/kotlin/com/example/android/coroutines/testing/UnconfinedTestDispatcherTest.kt#L29-L37" class="gc-analytics-event" data-category="github_link" data-label="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/UnconfinedTestDispatcherTest.kt#coroutine_test_unconfined" data-code-snippet="true" data-github-path="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/UnconfinedTestDispatcherTest.kt" data-region-tag="coroutine_test_unconfined">UnconfinedTestDispatcherTest.kt</a></div></pre></devsite-code> In this example, the launch calls will start their new coroutines eagerly on the <code translate="no" dir="ltr">UnconfinedTestDispatcher</code>, which means that each call to launch will only return after the registration is completed. <aside class="key-point">Key Point: You can pass in a <code translate="no" dir="ltr">TestDispatcher</code> to <code translate="no" dir="ltr">runTest</code> to control the dispatching behavior in the top-level test coroutine.</aside><aside class="note">Note: Using <code translate="no" dir="ltr">runTest</code> with an <code translate="no" dir="ltr">UnconfinedTestDispatcher</code> provides the same scheduling behavior as <a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-test/kotlinx.coroutines.test/run-blocking-test.html" class="external"><code translate="no" dir="ltr">runBlockingTest</code></a>, which was part of the coroutine testing APIs before version 1.6.</aside> Remember that <code translate="no" dir="ltr">UnconfinedTestDispatcher</code> starts new coroutines eagerly, but this doesn’t mean that it’ll run them to completion eagerly as well. If the new coroutine suspends, other coroutines will resume executing. For example, the new coroutine launched within this test will register Alice, but then it suspends when <a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/delay.html" class="external"><code translate="no" dir="ltr">delay</code></a> is called. This lets the top-level coroutine proceed with the assertion, and the test fails as Bob is not registered yet: <div></div><devsite-code><pre class="devsite-click-to-copy" translate="no" dir="ltr" is-upgraded syntax="Kotlin">@Test fun yieldingTest() = runTest(UnconfinedTestDispatcher()) { val userRepo = UserRepository() launch { userRepo.register("Alice") delay(10L) userRepo.register("Bob") } assertEquals(listOf("Alice", "Bob"), userRepo.getAllUsers()) // ❌ Fails }<div class="devsite-github-link nocode no-select"><a target="_top" href="https://github.com/android/snippets/blob/f595a0d317fd8aca6b33d4344bf87696cd45c481/kotlin/src/test/kotlin/com/example/android/coroutines/testing/UnconfinedTestDispatcherTest.kt#L42-L53" class="gc-analytics-event" data-category="github_link" data-label="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/UnconfinedTestDispatcherTest.kt#coroutine_test_unconfined_yielding" data-code-snippet="true" data-github-path="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/UnconfinedTestDispatcherTest.kt" data-region-tag="coroutine_test_unconfined_yielding">UnconfinedTestDispatcherTest.kt</a></div></pre></devsite-code> <h2 id="injecting-test-dispatchers" data-text="Injecting test dispatchers" tabindex="-1">Injecting test dispatchers</h2> Code under test might use dispatchers to switch threads (using <a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/with-context.html" class="external"><code translate="no" dir="ltr">withContext</code></a>) or to start new coroutines. When code is executed on multiple threads in parallel, tests can become flaky. It can be difficult to perform assertions at the correct time or to wait for tasks to complete if they’re running on background threads that you have no control over. In tests, replace these dispatchers with instances of <code translate="no" dir="ltr">TestDispatchers</code>. This has several benefits: <ul> <li>The code will run on the single test thread, making tests more deterministic</li> <li>You can control how new coroutines are scheduled and executed</li> <li>TestDispatchers use a scheduler for virtual time, which skips delays automatically and lets you advance time manually</li> </ul> Using <a href="/training/dependency-injection">dependency injection</a> to provide dispatchers to your classes makes it easy to replace the real dispatchers in tests. In these examples, we’ll inject a <code translate="no" dir="ltr">CoroutineDispatcher</code>, but you can also inject the broader <a href="https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.coroutines/-coroutine-context/" class="external"><code translate="no" dir="ltr">CoroutineContext</code></a> type, which allows for even more flexibility during tests. For classes that start coroutines, you can also inject a <code translate="no" dir="ltr">CoroutineScope</code> instead of a dispatcher, as detailed in the <a href="#inject-scope">Injecting a scope</a> section. <aside class="key-point">Key Point: In tests, replace real dispatchers with instances of <code translate="no" dir="ltr">TestDispatchers</code> to ensure that all code runs on the single test thread.</aside><aside class="note">Note: See the <a href="#setting-main-dispatcher">Setting the Main dispatcher</a> section to learn how to replace the Main dispatcher in tests.</aside> <code translate="no" dir="ltr">TestDispatchers</code> will, by default, create a new scheduler when they’re instantiated. Inside <code translate="no" dir="ltr">runTest</code>, you can access the <code translate="no" dir="ltr">testScheduler</code> property of the <code translate="no" dir="ltr">TestScope</code> and pass it in to any newly created <code translate="no" dir="ltr">TestDispatchers</code>. This will share their understanding of virtual time, and methods like <code translate="no" dir="ltr">advanceUntilIdle</code> will run coroutines on all test dispatchers to completion. <aside class="caution">Caution: You can create and use any number of <code translate="no" dir="ltr">TestDispatchers</code> within a test, however, they must all share the same scheduler. Be careful not to create multiple schedulers.</aside> In the following example, you can see a <code translate="no" dir="ltr">Repository</code> class that creates a new coroutine using the <code translate="no" dir="ltr">IO</code> dispatcher in its <code translate="no" dir="ltr">initialize</code> method and switches the caller to the <code translate="no" dir="ltr">IO</code> dispatcher in its <code translate="no" dir="ltr">fetchData</code> method: <div></div><devsite-code><pre class="devsite-click-to-copy" translate="no" dir="ltr" is-upgraded syntax="Kotlin">// Example class demonstrating dispatcher use cases class Repository(private val ioDispatcher: CoroutineDispatcher = Dispatchers.IO) { private val scope = CoroutineScope(ioDispatcher) val initialized = AtomicBoolean(false) // A function that starts a new coroutine on the IO dispatcher fun initialize() { scope.launch { initialized.set(true) } } // A suspending function that switches to the IO dispatcher suspend fun fetchData(): String = withContext(ioDispatcher) { require(initialized.get()) { "Repository should be initialized first" } delay(500L) "Hello world" } }<div class="devsite-github-link nocode no-select"><a target="_top" href="https://github.com/android/snippets/blob/f595a0d317fd8aca6b33d4344bf87696cd45c481/kotlin/src/main/kotlin/com/example/android/coroutines/testing/Repository.kt#L29-L47" class="gc-analytics-event" data-category="github_link" data-label="android/snippets/kotlin/src/main/kotlin/com/example/android/coroutines/testing/Repository.kt#coroutine_test_repo_dispatcher_injection" data-code-snippet="true" data-github-path="android/snippets/kotlin/src/main/kotlin/com/example/android/coroutines/testing/Repository.kt" data-region-tag="coroutine_test_repo_dispatcher_injection">Repository.kt</a></div></pre></devsite-code> In tests, you can inject a <code translate="no" dir="ltr">TestDispatcher</code> implementation to replace the <code translate="no" dir="ltr">IO</code> dispatcher. In the example below, we inject a <code translate="no" dir="ltr">StandardTestDispatcher</code> into the repository, and use <code translate="no" dir="ltr">advanceUntilIdle</code> to make sure that the new coroutine started in <code translate="no" dir="ltr">initialize</code> completes before proceeding. <aside class="caution">Caution: Advancing until the new coroutine is done is only possible because the new coroutine uses a <code translate="no" dir="ltr">TestDispatcher</code>. Having to do this reveals that the <code translate="no" dir="ltr">initialize</code> method in the example above is not a well-designed API. It starts asynchronous work that callers need to wait for, but has no way to notify callers when that work is done.</aside> <code translate="no" dir="ltr">fetchData</code> will also benefit from running on a <code translate="no" dir="ltr">TestDispatcher</code>, as it will run on the test thread and skip the delay it contains during the test. <div></div><devsite-code><pre class="devsite-click-to-copy" translate="no" dir="ltr" is-upgraded syntax="Kotlin">class RepositoryTest { @Test fun repoInitWorksAndDataIsHelloWorld() = runTest { val dispatcher = StandardTestDispatcher(testScheduler) val repository = Repository(dispatcher) repository.initialize() advanceUntilIdle() // Runs the new coroutine assertEquals(true, repository.initialized.get()) val data = repository.fetchData() // No thread switch, delay is skipped assertEquals("Hello world", data) } }<div class="devsite-github-link nocode no-select"><a target="_top" href="https://github.com/android/snippets/blob/f595a0d317fd8aca6b33d4344bf87696cd45c481/kotlin/src/test/kotlin/com/example/android/coroutines/testing/RepositoryTest.kt#L10-L23" class="gc-analytics-event" data-category="github_link" data-label="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/RepositoryTest.kt#coroutine_test_repo_dispatcher_injection_test" data-code-snippet="true" data-github-path="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/RepositoryTest.kt" data-region-tag="coroutine_test_repo_dispatcher_injection_test">RepositoryTest.kt</a></div></pre></devsite-code> New coroutines started on a <code translate="no" dir="ltr">TestDispatcher</code> can be advanced manually as shown above with <code translate="no" dir="ltr">initialize</code>. Note, however, that this would not be possible or desirable in production code. Instead, this method should be redesigned to be either suspending (for sequential execution), or to return a <code translate="no" dir="ltr">Deferred</code> value (for concurrent execution). For example, you can use <a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/async.html" class="external"><code translate="no" dir="ltr">async</code></a> to start a new coroutine and create a <a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-deferred/index.html" class="external"><code translate="no" dir="ltr">Deferred</code></a>: <div></div><devsite-code><pre class="devsite-click-to-copy" translate="no" dir="ltr" is-upgraded syntax="Kotlin">class BetterRepository(private val ioDispatcher: CoroutineDispatcher = Dispatchers.IO) { private val scope = CoroutineScope(ioDispatcher) fun initialize() = scope.async { // ... } }<div class="devsite-github-link nocode no-select"><a target="_top" href="https://github.com/android/snippets/blob/f595a0d317fd8aca6b33d4344bf87696cd45c481/kotlin/src/main/kotlin/com/example/android/coroutines/testing/Repository.kt#L51-L69" class="gc-analytics-event" data-category="github_link" data-label="android/snippets/kotlin/src/main/kotlin/com/example/android/coroutines/testing/Repository.kt#coroutine_test_repo_dispatcher_injection_better" data-code-snippet="true" data-github-path="android/snippets/kotlin/src/main/kotlin/com/example/android/coroutines/testing/Repository.kt" data-region-tag="coroutine_test_repo_dispatcher_injection_better">Repository.kt</a></div></pre></devsite-code> This lets you safely <code translate="no" dir="ltr">await</code> the completion of this code in both tests and production code: <div></div><devsite-code><pre class="devsite-click-to-copy" translate="no" dir="ltr" is-upgraded syntax="Kotlin">@Test fun repoInitWorks() = runTest { val dispatcher = StandardTestDispatcher(testScheduler) val repository = BetterRepository(dispatcher) repository.initialize().await() // Suspends until the new coroutine is done assertEquals(true, repository.initialized.get()) // ... }<div class="devsite-github-link nocode no-select"><a target="_top" href="https://github.com/android/snippets/blob/f595a0d317fd8aca6b33d4344bf87696cd45c481/kotlin/src/test/kotlin/com/example/android/coroutines/testing/RepositoryTest.kt#L28-L40" class="gc-analytics-event" data-category="github_link" data-label="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/RepositoryTest.kt#coroutine_test_repo_dispatcher_injection_test_better" data-code-snippet="true" data-github-path="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/RepositoryTest.kt" data-region-tag="coroutine_test_repo_dispatcher_injection_test_better">RepositoryTest.kt</a></div></pre></devsite-code> <aside class="note">Note: Similarly, you can create a new coroutine with <code translate="no" dir="ltr">launch</code> and then call <a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-job/join.html" class="external"><code translate="no" dir="ltr">join</code></a> on the <a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-job/index.html" class="external"><code translate="no" dir="ltr">Job</code></a> that it returns to wait for its completion.</aside> <code translate="no" dir="ltr">runTest</code> will wait for pending coroutines to complete before returning if the coroutines are on a <code translate="no" dir="ltr">TestDispatcher</code> that it shares a scheduler with. It will also wait for coroutines that are children of the top-level test coroutine, even if they’re on other dispatchers (up to a timeout specified by the <code translate="no" dir="ltr">dispatchTimeoutMs</code> parameter, which is 60 seconds by default). <h2 id="setting-main-dispatcher" data-text="Setting the Main dispatcher" tabindex="-1">Setting the Main dispatcher</h2> In <a href="/training/testing/local-tests">local unit tests</a>, the <code translate="no" dir="ltr">Main</code> dispatcher that wraps the Android UI thread will be unavailable, as these tests are executed on a local JVM and not an Android device. If your code under test references the main thread, it’ll throw an exception during unit tests. <aside class="note">Note: This only applies to local unit tests. You should not replace the <code translate="no" dir="ltr">Main</code> dispatcher in <a href="/training/testing/instrumented-tests">instrumented tests</a> where the real UI thread is available.</aside> In some cases, you can inject the <code translate="no" dir="ltr">Main</code> dispatcher the same way as other dispatchers, as described in <a href="#injecting-test-dispatchers">the previous section</a>, allowing you to replace it with a <code translate="no" dir="ltr">TestDispatcher</code> in tests. However, some APIs such as <a href="/topic/libraries/architecture/coroutines#viewmodelscope"><code translate="no" dir="ltr">viewModelScope</code></a> use a hardcoded <code translate="no" dir="ltr">Main</code> dispatcher under the hood. Here’s an example of a <a href="/topic/libraries/architecture/viewmodel"><code translate="no" dir="ltr">ViewModel</code></a> implementation that uses <code translate="no" dir="ltr">viewModelScope</code> to launch a coroutine that loads data: <div></div><devsite-code><pre class="devsite-click-to-copy" translate="no" dir="ltr" is-upgraded syntax="Kotlin">class HomeViewModel : ViewModel() { private val _message = MutableStateFlow("") val message: StateFlow<String> get() = _message fun loadMessage() { viewModelScope.launch { _message.value = "Greetings!" } } }<div class="devsite-github-link nocode no-select"><a target="_top" href="https://github.com/android/snippets/blob/f595a0d317fd8aca6b33d4344bf87696cd45c481/kotlin/src/main/kotlin/com/example/android/coroutines/testing/HomeViewModel.kt#L26-L35" class="gc-analytics-event" data-category="github_link" data-label="android/snippets/kotlin/src/main/kotlin/com/example/android/coroutines/testing/HomeViewModel.kt#coroutine_test_homeviewmodel" data-code-snippet="true" data-github-path="android/snippets/kotlin/src/main/kotlin/com/example/android/coroutines/testing/HomeViewModel.kt" data-region-tag="coroutine_test_homeviewmodel">HomeViewModel.kt</a></div></pre></devsite-code> To replace the <code translate="no" dir="ltr">Main</code> dispatcher with a <code translate="no" dir="ltr">TestDispatcher</code> in all cases, use the <a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-test/kotlinx.coroutines.test/set-main.html" class="external"><code translate="no" dir="ltr">Dispatchers.setMain</code></a> and <a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-test/kotlinx.coroutines.test/reset-main.html" class="external"><code translate="no" dir="ltr">Dispatchers.resetMain</code></a> functions. <div></div><devsite-code><pre class="devsite-click-to-copy" translate="no" dir="ltr" is-upgraded syntax="Kotlin">class HomeViewModelTest { @Test fun settingMainDispatcher() = runTest { val testDispatcher = UnconfinedTestDispatcher(testScheduler) Dispatchers.setMain(testDispatcher) try { val viewModel = HomeViewModel() viewModel.loadMessage() // Uses testDispatcher, runs its coroutine eagerly assertEquals("Greetings!", viewModel.message.value) } finally { Dispatchers.resetMain() } } }<div class="devsite-github-link nocode no-select"><a target="_top" href="https://github.com/android/snippets/blob/f595a0d317fd8aca6b33d4344bf87696cd45c481/kotlin/src/test/kotlin/com/example/android/coroutines/testing/HomeViewModelTest.kt#L28-L42" class="gc-analytics-event" data-category="github_link" data-label="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/HomeViewModelTest.kt#coroutine_test_homeviewmodel_test" data-code-snippet="true" data-github-path="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/HomeViewModelTest.kt" data-region-tag="coroutine_test_homeviewmodel_test">HomeViewModelTest.kt</a></div></pre></devsite-code> If the <code translate="no" dir="ltr">Main</code> dispatcher has been replaced with a <code translate="no" dir="ltr">TestDispatcher</code>, any newly-created <code translate="no" dir="ltr">TestDispatchers</code> will automatically use the scheduler from the <code translate="no" dir="ltr">Main</code> dispatcher, including the <code translate="no" dir="ltr">StandardTestDispatcher</code> created by <code translate="no" dir="ltr">runTest</code> if no other dispatcher is passed to it. This makes it easier to ensure that there is only a single scheduler in use during the test. For this to work, make sure to create all other <code translate="no" dir="ltr">TestDispatcher</code> instances after calling <code translate="no" dir="ltr">Dispatchers.setMain</code>. <aside class="key-point">Key Point: The <code translate="no" dir="ltr">Main</code> dispatcher should be replaced by a <code translate="no" dir="ltr">TestDispatcher</code> in local unit tests. Replacing the <code translate="no" dir="ltr">Main</code> dispatcher also simplifies sharing schedulers between <code translate="no" dir="ltr">TestDispatchers</code>.</aside> A common pattern to avoid duplicating the code that replaces the <code translate="no" dir="ltr">Main</code> dispatcher in each test is to extract it into a JUnit <a href="https://junit.org/junit4/javadoc/4.12/org/junit/rules/TestRule.html" class="external">test rule</a>: <div></div><devsite-code><pre class="devsite-click-to-copy" translate="no" dir="ltr" is-upgraded syntax="Kotlin">// Reusable JUnit4 TestRule to override the Main dispatcher class MainDispatcherRule( val testDispatcher: TestDispatcher = UnconfinedTestDispatcher(), ) : TestWatcher() { override fun starting(description: Description) { Dispatchers.setMain(testDispatcher) } override fun finished(description: Description) { Dispatchers.resetMain() } } class HomeViewModelTestUsingRule { @get:Rule val mainDispatcherRule = MainDispatcherRule() @Test fun settingMainDispatcher() = runTest { // Uses Main’s scheduler val viewModel = HomeViewModel() viewModel.loadMessage() assertEquals("Greetings!", viewModel.message.value) } }<div class="devsite-github-link nocode no-select"><a target="_top" href="https://github.com/android/snippets/blob/f595a0d317fd8aca6b33d4344bf87696cd45c481/kotlin/src/test/kotlin/com/example/android/coroutines/testing/HomeViewModelTestUsingRule.kt#L32-L55" class="gc-analytics-event" data-category="github_link" data-label="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/HomeViewModelTestUsingRule.kt#coroutine_test_homeviewmodeltest_with_rule" data-code-snippet="true" data-github-path="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/HomeViewModelTestUsingRule.kt" data-region-tag="coroutine_test_homeviewmodeltest_with_rule">HomeViewModelTestUsingRule.kt</a></div></pre></devsite-code> This rule implementation uses an <code translate="no" dir="ltr">UnconfinedTestDispatcher</code> by default, but a <code translate="no" dir="ltr">StandardTestDispatcher</code> can be passed in as a parameter if the <code translate="no" dir="ltr">Main</code> dispatcher shouldn’t execute eagerly in a given test class. When you need a <code translate="no" dir="ltr">TestDispatcher</code> instance in the test body, you can reuse the <code translate="no" dir="ltr">testDispatcher</code> from the rule, as long as it’s the desired type. If you want to be explicit about the type of <code translate="no" dir="ltr">TestDispatcher</code> used in the test, or if you need a <code translate="no" dir="ltr">TestDispatcher</code> that’s a different type than the one used for <code translate="no" dir="ltr">Main</code>, you can create a new <code translate="no" dir="ltr">TestDispatcher</code> within <code translate="no" dir="ltr">runTest</code>. As the <code translate="no" dir="ltr">Main</code> dispatcher is set to a <code translate="no" dir="ltr">TestDispatcher</code>, any newly created <code translate="no" dir="ltr">TestDispatchers</code> will share its scheduler automatically. <div></div><devsite-code><pre class="devsite-click-to-copy" translate="no" dir="ltr" is-upgraded syntax="Kotlin">class DispatcherTypesTest { @get:Rule val mainDispatcherRule = MainDispatcherRule() @Test fun injectingTestDispatchers() = runTest { // Uses Main’s scheduler // Use the UnconfinedTestDispatcher from the Main dispatcher val unconfinedRepo = Repository(mainDispatcherRule.testDispatcher) // Create a new StandardTestDispatcher (uses Main’s scheduler) val standardRepo = Repository(StandardTestDispatcher()) } }<div class="devsite-github-link nocode no-select"><a target="_top" href="https://github.com/android/snippets/blob/f595a0d317fd8aca6b33d4344bf87696cd45c481/kotlin/src/test/kotlin/com/example/android/coroutines/testing/DispatcherTypesTest.kt#L25-L37" class="gc-analytics-event" data-category="github_link" data-label="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/DispatcherTypesTest.kt#coroutine_test_dispatcher_types" data-code-snippet="true" data-github-path="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/DispatcherTypesTest.kt" data-region-tag="coroutine_test_dispatcher_types">DispatcherTypesTest.kt</a></div></pre></devsite-code> <h2 id="creating-dispatchers" data-text="Creating dispatchers outside a test" tabindex="-1">Creating dispatchers outside a test</h2> In some cases, you might need a <code translate="no" dir="ltr">TestDispatcher</code> to be available outside the test method. For example, during the initialization of a property in the test class: <div></div><devsite-code><pre class="devsite-click-to-copy" translate="no" dir="ltr" is-upgraded syntax="Kotlin">class ExampleRepository(private val ioDispatcher: CoroutineDispatcher) { /* ... */ } class RepositoryTestWithRule { private val repository = ExampleRepository(/* What TestDispatcher? */) @get:Rule val mainDispatcherRule = MainDispatcherRule() @Test fun someRepositoryTest() = runTest { // Test the repository... // ... } }<div class="devsite-github-link nocode no-select"><a target="_top" href="https://github.com/android/snippets/blob/f595a0d317fd8aca6b33d4344bf87696cd45c481/kotlin/src/test/kotlin/com/example/android/coroutines/testing/DispatchersOutsideTests.kt#L37-L52" class="gc-analytics-event" data-category="github_link" data-label="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/DispatchersOutsideTests.kt#coroutine_test_repo_with_rule_blank" data-code-snippet="true" data-github-path="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/DispatchersOutsideTests.kt" data-region-tag="coroutine_test_repo_with_rule_blank">DispatchersOutsideTests.kt</a></div></pre></devsite-code> If you’re replacing the <code translate="no" dir="ltr">Main</code> dispatcher as shown in the previous section, <code translate="no" dir="ltr">TestDispatchers</code> created after the <code translate="no" dir="ltr">Main</code> dispatcher has been replaced will automatically share its scheduler. This isn’t the case, however, for <code translate="no" dir="ltr">TestDispatchers</code> created as properties of the test class or <code translate="no" dir="ltr">TestDispatchers</code> created during the initialization of properties in the test class. These are initialized before the <code translate="no" dir="ltr">Main</code> dispatcher is replaced. Therefore, they would create new schedulers. <aside class="caution">Caution: If you create a <code translate="no" dir="ltr">TestDispatcher</code> as a property of a test class, it won’t take its scheduler from the <code translate="no" dir="ltr">Main</code> dispatcher, as the <code translate="no" dir="ltr">Main</code> dispatcher is only replaced before each test method is executed.</aside> To make sure that there’s only one scheduler in your test, create the <code translate="no" dir="ltr">MainDispatcherRule</code> property first. Then reuse its dispatcher (or its scheduler, if you need a <code translate="no" dir="ltr">TestDispatcher</code> of a different type) in the initializers of other class-level properties as needed. <div></div><devsite-code><pre class="devsite-click-to-copy" translate="no" dir="ltr" is-upgraded syntax="Kotlin">class RepositoryTestWithRule { @get:Rule val mainDispatcherRule = MainDispatcherRule() private val repository = ExampleRepository(mainDispatcherRule.testDispatcher) @Test fun someRepositoryTest() = runTest { // Takes scheduler from Main // Any TestDispatcher created here also takes the scheduler from Main val newTestDispatcher = StandardTestDispatcher() // Test the repository... } }<div class="devsite-github-link nocode no-select"><a target="_top" href="https://github.com/android/snippets/blob/f595a0d317fd8aca6b33d4344bf87696cd45c481/kotlin/src/test/kotlin/com/example/android/coroutines/testing/DispatchersOutsideTests.kt#L58-L71" class="gc-analytics-event" data-category="github_link" data-label="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/DispatchersOutsideTests.kt#coroutine_test_repo_with_rule" data-code-snippet="true" data-github-path="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/DispatchersOutsideTests.kt" data-region-tag="coroutine_test_repo_with_rule">DispatchersOutsideTests.kt</a></div></pre></devsite-code> Note that both <code translate="no" dir="ltr">runTest</code> and <code translate="no" dir="ltr">TestDispatchers</code> created within the test will still automatically share the scheduler of the <code translate="no" dir="ltr">Main</code> dispatcher. If you’re not replacing the <code translate="no" dir="ltr">Main</code> dispatcher, create your first <code translate="no" dir="ltr">TestDispatcher</code> (which creates a new scheduler) as a property of the class. Then, manually pass that scheduler to each <code translate="no" dir="ltr">runTest</code> invocation and each new <code translate="no" dir="ltr">TestDispatcher</code> created, both as properties and within the test: <div></div><devsite-code><pre class="devsite-click-to-copy" translate="no" dir="ltr" is-upgraded syntax="Kotlin">class RepositoryTest { // Creates the single test scheduler private val testDispatcher = UnconfinedTestDispatcher() private val repository = ExampleRepository(testDispatcher) @Test fun someRepositoryTest() = runTest(testDispatcher.scheduler) { // Take the scheduler from the TestScope val newTestDispatcher = UnconfinedTestDispatcher(this.testScheduler) // Or take the scheduler from the first dispatcher, they’re the same val anotherTestDispatcher = UnconfinedTestDispatcher(testDispatcher.scheduler) // Test the repository... } }<div class="devsite-github-link nocode no-select"><a target="_top" href="https://github.com/android/snippets/blob/f595a0d317fd8aca6b33d4344bf87696cd45c481/kotlin/src/test/kotlin/com/example/android/coroutines/testing/DispatchersOutsideTests.kt#L75-L89" class="gc-analytics-event" data-category="github_link" data-label="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/DispatchersOutsideTests.kt#coroutine_test_repo_without_rule" data-code-snippet="true" data-github-path="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/DispatchersOutsideTests.kt" data-region-tag="coroutine_test_repo_without_rule">DispatchersOutsideTests.kt</a></div></pre></devsite-code> In this sample, the scheduler from the first dispatcher is passed to <code translate="no" dir="ltr">runTest</code>. This will create a new <code translate="no" dir="ltr">StandardTestDispatcher</code> for the <code translate="no" dir="ltr">TestScope</code> using that scheduler. You could also pass in the dispatcher to <code translate="no" dir="ltr">runTest</code> directly to run the test coroutine on that dispatcher. <h2 id="creating-your-own-testscope" data-text="Creating your own TestScope" tabindex="-1">Creating your own TestScope</h2> Like with <code translate="no" dir="ltr">TestDispatchers</code>, you might need to access a <code translate="no" dir="ltr">TestScope</code> outside the test body. While <code translate="no" dir="ltr">runTest</code> creates a <code translate="no" dir="ltr">TestScope</code> under the hood automatically, you can also create your own <code translate="no" dir="ltr">TestScope</code> to use with <code translate="no" dir="ltr">runTest</code>. When doing this, make sure to call <code translate="no" dir="ltr">runTest</code> on the <code translate="no" dir="ltr">TestScope</code> you’ve created: <div></div><devsite-code><pre class="devsite-click-to-copy" translate="no" dir="ltr" is-upgraded syntax="Kotlin">class SimpleExampleTest { val testScope = TestScope() // Creates a StandardTestDispatcher @Test fun someTest() = testScope.runTest { // ... } }<div class="devsite-github-link nocode no-select"><a target="_top" href="https://github.com/android/snippets/blob/f595a0d317fd8aca6b33d4344bf87696cd45c481/kotlin/src/test/kotlin/com/example/android/coroutines/testing/CreatingYourOwn.kt#L27-L34" class="gc-analytics-event" data-category="github_link" data-label="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/CreatingYourOwn.kt#coroutine_test_your_own_scope" data-code-snippet="true" data-github-path="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/CreatingYourOwn.kt" data-region-tag="coroutine_test_your_own_scope">CreatingYourOwn.kt</a></div></pre></devsite-code> <aside class="caution">Caution: If you create your own <code translate="no" dir="ltr">TestScope</code>, you must call <code translate="no" dir="ltr">runTest</code> on that scope within your test. There can only be one <code translate="no" dir="ltr">TestScope</code> instance in a test.</aside> The code above creates a <code translate="no" dir="ltr">StandardTestDispatcher</code> for the <code translate="no" dir="ltr">TestScope</code> implicitly, as well as a new scheduler. These objects can all also be created explicitly. This can be useful if you need to integrate it with dependency injection setups. <div></div><devsite-code><pre class="devsite-click-to-copy" translate="no" dir="ltr" is-upgraded syntax="Kotlin">class ExampleTest { val testScheduler = TestCoroutineScheduler() val testDispatcher = StandardTestDispatcher(testScheduler) val testScope = TestScope(testDispatcher) @Test fun someTest() = testScope.runTest { // ... } }<div class="devsite-github-link nocode no-select"><a target="_top" href="https://github.com/android/snippets/blob/f595a0d317fd8aca6b33d4344bf87696cd45c481/kotlin/src/test/kotlin/com/example/android/coroutines/testing/CreatingYourOwn.kt#L38-L47" class="gc-analytics-event" data-category="github_link" data-label="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/CreatingYourOwn.kt#coroutine_test_your_own_everything" data-code-snippet="true" data-github-path="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/CreatingYourOwn.kt" data-region-tag="coroutine_test_your_own_everything">CreatingYourOwn.kt</a></div></pre></devsite-code> <aside class="key-point">Key Point: You can create all the coroutine testing constructs outside of tests if your project setup requires it.</aside> <h2 id="inject-scope" data-text="Injecting a scope" tabindex="-1">Injecting a scope</h2> If you have a class that creates coroutines that you need to control during tests, you can inject a coroutine scope into that class, replacing it with a <code translate="no" dir="ltr">TestScope</code> in tests. In the following example, the <code translate="no" dir="ltr">UserState</code> class depends on a <code translate="no" dir="ltr">UserRepository</code> to register new users and to fetch the list of registered users. As these calls to <code translate="no" dir="ltr">UserRepository</code> are suspending function calls, <code translate="no" dir="ltr">UserState</code> uses the injected <code translate="no" dir="ltr">CoroutineScope</code> to start a new coroutine inside its <code translate="no" dir="ltr">registerUser</code> function. <div></div><devsite-code><pre class="devsite-click-to-copy" translate="no" dir="ltr" is-upgraded syntax="Kotlin">class UserState( private val userRepository: UserRepository, private val scope: CoroutineScope, ) { private val _users = MutableStateFlow(emptyList<String>()) val users: StateFlow<List<String>> = _users.asStateFlow() fun registerUser(name: String) { scope.launch { userRepository.register(name) _users.update { userRepository.getAllUsers() } } } }<div class="devsite-github-link nocode no-select"><a target="_top" href="https://github.com/android/snippets/blob/f595a0d317fd8aca6b33d4344bf87696cd45c481/kotlin/src/main/kotlin/com/example/android/coroutines/testing/UserState.kt#L29-L42" class="gc-analytics-event" data-category="github_link" data-label="android/snippets/kotlin/src/main/kotlin/com/example/android/coroutines/testing/UserState.kt#android_coroutine_test_user_state" data-code-snippet="true" data-github-path="android/snippets/kotlin/src/main/kotlin/com/example/android/coroutines/testing/UserState.kt" data-region-tag="android_coroutine_test_user_state">UserState.kt</a></div></pre></devsite-code> To test this class, you can pass in the <code translate="no" dir="ltr">TestScope</code> from <code translate="no" dir="ltr">runTest</code> when creating the <code translate="no" dir="ltr">UserState</code> object: <div></div><devsite-code><pre class="devsite-click-to-copy" translate="no" dir="ltr" is-upgraded syntax="Kotlin">class UserStateTest { @Test fun addUserTest() = runTest { // this: TestScope val repository = FakeUserRepository() val userState = UserState(repository, scope = this) userState.registerUser("Mona") advanceUntilIdle() // Let the coroutine complete and changes propagate assertEquals(listOf("Mona"), userState.users.value) } }<div class="devsite-github-link nocode no-select"><a target="_top" href="https://github.com/android/snippets/blob/f595a0d317fd8aca6b33d4344bf87696cd45c481/kotlin/src/test/kotlin/com/example/android/coroutines/testing/UserStateTest.kt#L14-L25" class="gc-analytics-event" data-category="github_link" data-label="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/UserStateTest.kt#android_coroutine_test_user_state_test" data-code-snippet="true" data-github-path="android/snippets/kotlin/src/test/kotlin/com/example/android/coroutines/testing/UserStateTest.kt" data-region-tag="android_coroutine_test_user_state_test">UserStateTest.kt</a></div></pre></devsite-code> <aside class="tip">Tip: If your class creates coroutines that don't complete on their own and should be canceled at the end of the test, you can inject <a href="https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-test/kotlinx.coroutines.test/-test-scope/background-scope.html" class="external"><code translate="no" dir="ltr">TestScope.backgroundScope</code></a> instead of the <code translate="no" dir="ltr">TestScope</code> itself.</aside> To inject a scope outside of the test function, for example into an object under test that's created as a property in the test class, see <a href="#creating-your-own-testscope">Creating your own TestScope</a>. <h2 id="additional-resources" data-text="Additional resources" tabindex="-1">Additional resources</h2> <ul> <li><a href="/kotlin/flow/test">Testing Kotlin flows on Android</a></li> <li><a href="https://github.com/Kotlin/kotlinx.coroutines/tree/master/kotlinx-coroutines-test" class="external">kotlinx.coroutines.test</a> on GitHub</li> </ul> <devsite-hats-survey class="nocontent" hats-id="VxqvKSur40kxBYCLVTd0SSGykbno" listnr-id="5207477"></devsite-hats-survey> </div> <devsite-recommendations display="in-page" hidden yield> </devsite-recommendations> <devsite-thumb-rating position="footer"> </devsite-thumb-rating> <devsite-recommendations id="recommendations-link" yield></devsite-recommendations> <div class="devsite-floating-action-buttons"> </div> </article> <devsite-content-footer class="nocontent"> Content and code samples on this page are subject to the licenses described in the <a href="/license">Content License</a>. 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