Testing Coroutines

The kotlinx-coroutines-test library makes coroutine code fast and deterministic to test by replacing real time with virtual time.

runTest

Wrap a suspend test body in runTest. Calls to delay are skipped instantly — a test that “waits” 10 seconds finishes in milliseconds:

import kotlinx.coroutines.test.runTest
import kotlinx.coroutines.delay
import kotlin.test.assertEquals

@Test
fun fetchesUser() = runTest {
    val user = fetchUser()       // any delay() inside is auto-advanced
    assertEquals("Ada", user)
}

Add the dependency:

testImplementation("org.jetbrains.kotlinx:kotlinx-coroutines-test:1.9.0")

Virtual time control

Inside runTest you have a TestScope with a scheduler you can advance manually:

@Test
fun advancesTime() = runTest {
    var done = false
    launch {
        delay(1000)
        done = true
    }
    assertEquals(false, done)
    advanceTimeBy(1001)          // move virtual time forward
    advanceUntilIdle()           // run everything pending
    assertEquals(true, done)
}

Injecting dispatchers

Production code should not hard-code dispatchers — inject them so tests can swap in a test dispatcher:

class UserRepository(private val io: CoroutineDispatcher = Dispatchers.IO) {
    suspend fun load(): String = withContext(io) { /* ... */ "Ada" }
}

@Test
fun loads() = runTest {
    val repo = UserRepository(StandardTestDispatcher(testScheduler))
    assertEquals("Ada", repo.load())
}

This dependency-injection pattern is what makes ViewModels testable in Part 8.

Testing flows

Collect a flow into a list inside runTest:

@Test
fun emitsThreeValues() = runTest {
    val values = numbers().toList()
    assertEquals(listOf(1, 2, 3), values)
}

Exercises

1. Write a runTest that calls a suspend function containing delay(5000) and verify it completes essentially instantly.

2. Test that a Flow<Int> of 1..3 collects to [1, 2, 3].

This solution is in examples/core/part6/ and is tested by CI.

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