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Giorgos Neokleous

Kotlin's Coroutines Testing Tips


Do you love Kotlin’s Coroutines? So do I!

Kotlin’s Coroutines offer many benefits such as:

  • Readability: the following code can be read line by line and understand the order of execution without going into callbacks (at least on what the developers write).
suspend fun runTwoExpensiveOperations() {
      // Running First Operation 
      val resultOfFirstExpensiveOperation = withContext(Dispatchers.IO) {        

      // Running Second Operation
     val resultOfSecondExpensiveOperation = withContext(Dispatchers.IO) {

      return resultOfSecondExpensiveOperation
  • Reactive capabilities: Flows, Channels, StateFlow come with a lot of operators which provide an API for hot and cold streams.
  • Supported as a language feature

I found unit testing with Kotlin’s Coroutines easy, especially when testing things like withContext or suspend functions.

Time-master - runBlockingTest

runBlockingTest (link) provides a scope which can be used to control time, such as advancing time, pausing and resuming dispatchers.

Within the body of the runBlockingTest you can run suspending operations and manipulate time. You can forward time using the TestCoroutineScope which can be accessed directly from within the runBlockingTest.

Let’s see an example and then break it down.

fun `Advancing Time with TestCoroutineScope`() = runBlockingTest {
    val delay = 100.milliseconds
    val flow = operationForTimeController(delay = delay, rounds = 5)

    // observing flow and storing emissions
    val results = mutableListOf<Int>()
    launch { flow.collect { results.add(it) } }

    // assert that results is empty
    Assert.assertThat(results.isEmpty(), CoreMatchers.equalTo(true))

    // forward one emission with `delay

    // assert first item is correct
    Assert.assertThat(results.first(), CoreMatchers.equalTo(0))

    // advanced at the end of the flow

    // assert emissions
    Assert.assertThat(results, CoreMatchers.equalTo(mutableListOf(0, 1, 2, 3, 4)))

The unit test above observes a flow of integers. In order to assert the behaviour on flow, we are launching a coroutine which adds into results the emissions for inspection at a later stage.

Then we do the following to test emissions:

  1. Assert that the list of results is empty
  2. Forward time by delay (100ms) for one emission using advancedTimeBy
  3. Assert that the list’s first item is 0
  4. Forward until idle (i.e. until there are no running jobs) using advanceUntilIdle
  5. Assert list contains all items 0, 1, 2, 3, 4

All of the above are achieved without waiting 500ms for the delays to run!

runBlockingTest under the hood calls cleanupTestCoroutines to make sure that coroutines are properly cleaned up.

Let’s see the implemenation of the function, emited logic for simplification (source code):

 val (safeContext, dispatcher) = context.checkArguments()
    val deferred = scope.async {

The cleanupTestCoroutines will call the same method from the UncaughtExceptionCaptor and DelayController. What does that mean for us?

  • The UncaughtExceptionCaptor throws the first uncaught exception + prints all exceptions' stacktraces.
  • The DelayController tries to complete all pending tasks.

Exception Master - TestCoroutineExceptionHandler

TestCoroutineExceptionHandler (link)is a special version of the CoroutineExceptionHandler. You can use a CoroutineExceptionHandler to be able to handle exception from within coroutines.

Personal favourite articles about Coroutine’s exception handling:

Let’s see how to leverage TestCoroutineExceptionHandler to test exceptions.

fun `Assert Exception with Coroutines`() = runBlockingTest {
    // handler will catch Exceptions
    val exceptionHandler = TestCoroutineExceptionHandler()

    launch(exceptionHandler) { operation().collect() }

    // asserting that first uncaught exception is CustomException

The unit test above observes a Flow and asserts the first exception. The operation flow throws a immediately.

As you can see I am passing to launch an exceptionHandler as extra context which will catch exceptions. The exceptions can be observed using the .uncaughtExceptions which returns all exceptions thrown from within the enclosed job.

Today we explored how to:

  1. Collect Flow’s emissions, by storing the emissions into a collection.
  2. Forward time by a fixed time using advancedTimeBy (link)
  3. Forward time until idle using advanceUntilIdle (link) to execute all pending tasks.
  4. Catch and inspect uncaught exceptions by using the TestCoroutineExceptionHandler

You can find the repository with the examples at my personal github account.

Feel free to ping me on Twitter.

Till next time! 👋

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