Migration to the new kotlinx-coroutines-test API

In version 1.6.0, the API of the test module changed significantly. This is a guide for gradually adapting the existing test code to the new API. This guide is written step-by-step; the idea is to separate the migration into several sets of small changes.

Remove custom UncaughtExceptionCaptor, DelayController, and TestCoroutineScope implementations

We couldn‘t find any code that defined new implementations of these interfaces, so they are deprecated. It’s likely that you don't need to do anything for this section.

UncaughtExceptionCaptor

If the code base has an UncaughtExceptionCaptor, its special behavior as opposed to just CoroutineExceptionHandler was that, at the end of runBlockingTest or cleanupTestCoroutines (or both), its cleanupTestCoroutines procedure was called.

We currently don't provide a replacement for this. However, runTest follows structured concurrency better than runBlockingTest did, so exceptions from child coroutines are propagated structurally, which makes uncaught exception handlers less useful.

If you have a use case for this, please tell us about it at the issue tracker. Meanwhile, it should be possible to use a custom exception captor, which should only implement CoroutineExceptionHandler now, like this:

@Test
fun testFoo() = runTest {
    val customCaptor = MyUncaughtExceptionCaptor()
    launch(customCaptor) {
        // ...
    }
    advanceUntilIdle()
    customCaptor.cleanupTestCoroutines()
}

DelayController

We don't provide a way to define custom dispatching strategies that support virtual time. That said, we significantly enhanced this mechanism:

• Using multiple test dispatchers simultaneously is supported. For the dispatchers to have a shared knowledge of the virtual time, either the same TestCoroutineScheduler should be passed to each of them, or all of them should be constructed after Dispatchers.setMain is called with some test dispatcher.
• Both a simple StandardTestDispatcher that is always paused, and unconfined UnconfinedTestDispatcher are provided.

If you have a use case for DelayController that's not covered by what we provide, please tell us about it in the issue tracker.

TestCoroutineScope

This scope couldn‘t be meaningfully used in tandem with runBlockingTest: according to the definition of TestCoroutineScope.runBlockingTest, only the scope’s coroutineContext is used. So, there could be two reasons for defining a custom implementation:

• Avoiding the restrictions on placed coroutineContext in the TestCoroutineScope constructor function. These restrictions consisted of requirements for CoroutineExceptionHandler being an UncaughtExceptionCaptor, and ContinuationInterceptor being a DelayController, so it is also possible to fulfill these restrictions by defining conforming instances. In this case, follow the instructions about replacing them.
• Using without runBlockingTest. In this case, you don't even need to implement TestCoroutineScope: nothing else accepts a TestCoroutineScope specifically as an argument.

Remove usages of TestCoroutineExceptionHandler and TestCoroutineScope.uncaughtExceptions

It is already illegal to use a TestCoroutineScope without performing cleanupTestCoroutines, so the valid uses of TestCoroutineExceptionHandler include:

• Accessing uncaughtExceptions in the middle of the test to make sure that there weren‘t any uncaught exceptions yet. If there are any, they will be thrown by the cleanup procedure anyway. We don’t support this use case, given how comparatively rare it is, but it can be handled in the same way as the following one.
• Accessing uncaughtExceptions when the uncaught exceptions are actually expected. In this case, cleanupTestCoroutines will fail with an exception that is being caught later. It would be better in this case to use a custom CoroutineExceptionHandler so that actual problems that could be found by the cleanup procedure are not superseded by the exceptions that are expected. An example is shown below.

val exceptions = mutableListOf<Throwable>()
val customCaptor = CoroutineExceptionHandler { ctx, throwable ->
    exceptions.add(throwable) // add proper synchronization if the test is multithreaded
}

@Test
fun testFoo() = runTest {
    launch(customCaptor) {
        // ...
    }
    advanceUntilIdle()
    // check the list of the caught exceptions
}

Auto-replace TestCoroutineScope constructor function with createTestCoroutineScope

This should not break anything, as TestCoroutineScope is now defined in terms of createTestCoroutineScope. If it does break something, it means that you already supplied a TestCoroutineScheduler to some scope; in this case, also pass this scheduler as the argument to the dispatcher.

Replace usages of pauseDispatcher and resumeDispatcher with a StandardTestDispatcher

• In places where pauseDispatcher in its block form is called, replace it with a call to withContext(StandardTestDispatcher(testScheduler)) (testScheduler is available as a field of TestCoroutineScope, or scheduler is available as a field of TestCoroutineDispatcher), followed by advanceUntilIdle(). This is not an automatic replacement, as there can be tricky situations where the test dispatcher is already paused when pauseDispatcher { X } is called. In such cases, simply replace pauseDispatcher { X } with X.
• Often, pauseDispatcher() in a non-block form is used at the start of the test. Then, attempt to remove TestCoroutineDispatcher from the arguments to createTestCoroutineScope, if a standalone TestCoroutineScope or the scope.runBlockingTest form is used, or pass a StandardTestDispatcher as an argument to runBlockingTest. This will lead to the test using a StandardTestDispatcher, which does not allow pausing and resuming, instead of the deprecated TestCoroutineDispatcher.
• Sometimes, pauseDispatcher() and resumeDispatcher() are employed used throughout the test. In this case, attempt to wrap everything until the next resumeDispatcher() in a withContext(StandardTestDispatcher(testScheduler)) block, or try using some other combinations of StandardTestDispatcher (where dispatches are needed) and UnconfinedTestDispatcher (where it isn't important where execution happens).

Replace advanceTimeBy(n) with advanceTimeBy(n); runCurrent()

For TestCoroutineScope and DelayController, the advanceTimeBy method is deprecated. It is not deprecated for TestCoroutineScheduler and TestScope, but has a different meaning: it does not run the tasks scheduled at currentTime + n.

There is an automatic replacement for this deprecation, which produces correct but inelegant code.

Alternatively, you can wait until replacing TestCoroutineScope with TestScope: it's possible that you will not encounter this edge case.

Replace runBlockingTest with runTest(UnconfinedTestDispatcher())

This is a major change, affecting many things, and can be done in parallel with replacing TestCoroutineScope with TestScope.

Significant differences of runTest from runBlockingTest are each given a section below.

It works properly with other dispatchers and asynchronous completions.

No action on your part is required, other than replacing runBlocking with runTest as well.

It uses StandardTestDispatcher by default, not TestCoroutineDispatcher.

By now, calls to pauseDispatcher and resumeDispatcher should be purged from the code base, so only the unpaused variant of TestCoroutineDispatcher should be used. This version of the dispatcher has the property of eagerly entering launch and async blocks: code until the first suspension is executed without dispatching.

There are two common ways in which this property is useful.

TestCoroutineDispatcher for the top-level coroutine

Some tests that rely on launch and async blocks being entered immediately have a form similar to this:

runTest(TestCoroutineDispatcher()) {
    launch {
        updateSomething()
    }
    checkThatSomethingWasUpdated()
    launch {
        updateSomethingElse()
    }
    checkThatSomethingElseWasUpdated()
}

If the TestCoroutineDispatcher() is simply removed, StandardTestDispatcher() will be used, which will cause the test to fail.

In these cases, UnconfinedTestDispatcher() should be used. We ensured that, when run with an UnconfinedTestDispatcher, runTest also eagerly enters launch and async blocks.

Note though that this only works at the top level: if a child coroutine also called launch or async, we don't provide any guarantees about their dispatching order.

TestCoroutineDispatcher for testing intermediate emissions

Some code tests StateFlow or channels in a manner similar to this:

@Test
fun testAllEmissions() = runTest(TestCoroutineDispatcher()) {
    val values = mutableListOf<Int>()
    val stateFlow = MutableStateFlow(0)
    val job = launch {
        stateFlow.collect {
            values.add(it)
        }
    }
    stateFlow.value = 1
    stateFlow.value = 2
    stateFlow.value = 3
    job.cancel()
    // each assignment will immediately resume the collecting child coroutine,
    // so no values will be skipped.
    assertEquals(listOf(0, 1, 2, 3), values)
}

Such code will fail when TestCoroutineDispatcher() is not used: not every emission will be listed. In this particular case, none will be listed at all.

The reason for this is that setting stateFlow.value (as is sending to a channel, as are some other things) wakes up the coroutine waiting for the new value, but typically does not immediately run the collecting code, instead simply dispatching it. The exceptions are the coroutines running in dispatchers that don't (always) go through a dispatch, Dispatchers.Unconfined, Dispatchers.Main.immediate, UnconfinedTestDispatcher, or TestCoroutineDispatcher in the unpaused state.

Therefore, a solution is to launch the collection in an unconfined dispatcher:

@Test
fun testAllEmissions() = runTest {
    val values = mutableListOf<Int>()
    val stateFlow = MutableStateFlow(0)
    val job = launch(UnconfinedTestDispatcher(testScheduler)) { // <------
        stateFlow.collect {
            values.add(it)
        }
    }
    stateFlow.value = 1
    stateFlow.value = 2
    stateFlow.value = 3
    job.cancel()
    // each assignment will immediately resume the collecting child coroutine,
    // so no values will be skipped.
    assertEquals(listOf(0, 1, 2, 3), values)
}

Note that testScheduler is passed so that the unconfined dispatcher is linked to runTest. Also, note that UnconfinedTestDispatcher is not passed to runTest. This is due to the fact that, inside the UnconfinedTestDispatcher, there are no execution order guarantees, so it would not be guaranteed that setting stateFlow.value would immediately run the collecting code (though in this case, it does).

Other considerations

Using UnconfinedTestDispatcher as an argument to runTest will probably lead to the test being executed as it did, but it's still possible that the test relies on the specific dispatching order of TestCoroutineDispatcher, so it will need to be tweaked.

If some code is expected to have run at some point, but it hasn't, use runCurrent to force the tasks scheduled at this moment of time to run. For example, the StateFlow example above can also be forced to succeed by doing this:

@Test
fun testAllEmissions() = runTest {
    val values = mutableListOf<Int>()
    val stateFlow = MutableStateFlow(0)
    val job = launch {
        stateFlow.collect {
            values.add(it)
        }
    }
    runCurrent()
    stateFlow.value = 1
    runCurrent()
    stateFlow.value = 2
    runCurrent()
    stateFlow.value = 3
    runCurrent()
    job.cancel()
    // each assignment will immediately resume the collecting child coroutine,
    // so no values will be skipped.
    assertEquals(listOf(0, 1, 2, 3), values)
}

Be wary though of this approach: using runCurrent, advanceTimeBy, or advanceUntilIdle is, essentially, simulating some particular execution order, which is not guaranteed to happen in production code. For example, using UnconfinedTestDispatcher to fix this test reflects how, in production code, one could use Dispatchers.Unconfined to observe all emitted values without conflation, but the runCurrent() approach only states that the behavior would be observed if a dispatch were to happen at some chosen points. It is, therefore, recommended to structure tests in a way that does not rely on a particular interleaving, unless that is the intention.

The job hierarchy is completely different.

• Structured concurrency is used, with the scope provided as the receiver of runTest actually being the scope of the created coroutine.
• Not SupervisorJob but a normal Job is used for the TestCoroutineScope.
• The job passed as an argument is used as a parent job.

Most tests should not be affected by this. In case your test is, try explicitly launching a child coroutine with a SupervisorJob; this should make the job hierarchy resemble what it used to be.

@Test
fun testFoo() = runTest {
    val deferred = async(SupervisorJob()) {
        // test code
    }
    advanceUntilIdle()
    deferred.getCompletionExceptionOrNull()?.let {
      throw it
    }
}

Only a single call to runTest is permitted per test.

In order to work on JS, only a single call to runTest must happen during one test, and its result must be returned immediately:

@Test
fun testFoo(): TestResult {
    // arbitrary code here
    return runTest {
        // ...
    }
}

When used only on the JVM, runTest will work when called repeatedly, but this is not supported. Please only call runTest once per test, and if for some reason you can't, please tell us about in on the issue tracker.

It uses TestScope, not TestCoroutineScope, by default.

There is a runTestWithLegacyScope method that allows migrating from runBlockingTest to runTest before migrating from TestCoroutineScope to TestScope, if exactly the TestCoroutineScope needs to be passed somewhere else and TestScope will not suffice.

Replace TestCoroutineScope.cleanupTestCoroutines with runTest

Likely can be done together with the next step.

Remove all calls to TestCoroutineScope.cleanupTestCoroutines from the code base. Instead, as the last step of each test, do return scope.runTest; if possible, the whole test body should go inside the runTest block.

The cleanup procedure in runTest will not check that the virtual time doesn't advance during cleanup. If a test must check that no other delays are remaining after it has finished, the following form may help:

runTest {
    testBody()
    val timeAfterTest = currentTime()
    advanceUntilIdle() // run the remaining tasks
    assertEquals(timeAfterTest, currentTime()) // will fail if there were tasks scheduled at a later moment
}

Note that this will report time advancement even if the job scheduled at a later point was cancelled.

It may be the case that cleanupTestCoroutines must be executed after de-initialization in @AfterTest, which happens outside the test itself. In this case, we propose that you write a wrapper of the form:

fun runTestAndCleanup(body: TestScope.() -> Unit) = runTest {
    try {
        body()
    } finally {
        // the usual cleanup procedures that used to happen before `cleanupTestCoroutines`
    }
}

Replace runBlockingTest with runBlockingTestOnTestScope, createTestCoroutineScope with TestScope

Also, replace runTestWithLegacyScope with just runTest. All of this can be done in parallel with replacing runBlockingTest with runTest.

This step should remove all uses of TestCoroutineScope, explicit or implicit.

Replacing runTestWithLegacyScope and runBlockingTest with runTest and runBlockingTestOnTestScope should be straightforward if there is no more code left that requires passing exactly TestCoroutineScope to it. Some tests may fail because TestCoroutineScope.cleanupTestCoroutines and the cleanup procedure in runTest handle cancelled tasks differently: if there are cancelled jobs pending at the moment of TestCoroutineScope.cleanupTestCoroutines, they are ignored, whereas runTest will report them.

Of all the methods supported by TestCoroutineScope, only cleanupTestCoroutines is not provided on TestScope, and its usages should have been removed during the previous step.

Replace runBlocking with runTest

Now that runTest works properly with asynchronous completions, runBlocking is only occasionally useful. As is, most uses of runBlocking in tests come from the need to interact with dispatchers that execute on other threads, like Dispatchers.IO or Dispatchers.Default.

Replace TestCoroutineDispatcher with UnconfinedTestDispatcher and StandardTestDispatcher

TestCoroutineDispatcher is a dispatcher with two modes:

• (“unpaused”) Almost (but not quite) unconfined, with the ability to eagerly enter launch and async blocks.
• (“paused”) Behaving like a StandardTestDispatcher.

In one of the earlier steps, we replaced pauseDispatcher with StandardTestDispatcher usage, and replaced the implicit TestCoroutineScope dispatcher in runBlockingTest with UnconfinedTestDispatcher during migration to runTest.

Now, the rest of the usages should be replaced with whichever dispatcher is most appropriate.

Simplify code by removing unneeded entities

Likely, now some code has the form

val dispatcher = StandardTestDispatcher()
val scope = TestScope(dispatcher)

@BeforeTest
fun setUp() {
    Dispatchers.setMain(dispatcher)
}

@AfterTest
fun tearDown() {
    Dispatchers.resetMain()
}

@Test
fun testFoo() = scope.runTest {
    // ...
}

The point of this pattern is to ensure that the test runs with the same TestCoroutineScheduler as the one used for Dispatchers.Main.

However, now this can be simplified to just

@BeforeTest
fun setUp() {
  Dispatchers.setMain(StandardTestDispatcher())
}

@AfterTest
fun tearDown() {
  Dispatchers.resetMain()
}

@Test
fun testFoo() = runTest {
  // ...
}

The reason this works is that all entities that depend on TestCoroutineScheduler will attempt to acquire one from the current Dispatchers.Main.