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Version: 5.5 🚧

Test Coroutine Dispatcher

A TestDispatcher is a special CoroutineDispatcher provided by the kotlinx-coroutines-test module that allows developers to control its virtual clock and skip delays.

A TestDispatcher supports the following operations:

  • currentTime gets the current virtual time.
  • runCurrent() runs the tasks that are scheduled at this point of virtual time.
  • advanceUntilIdle() runs all enqueued tasks until there are no more.
  • advanceTimeBy(timeDelta) runs the enqueued tasks until the current virtual time advances by timeDelta.

To use a TestDispatcher for a test, you can enable coroutineTestScope in test config:

class TestDispatcherTest : FunSpec() {
init {
test("foo").config(coroutineTestScope = true) {
// this test will run with a test dispatcher
}
}
}

Inside this test, can you retrieve a handle to the scheduler through the extension val testCoroutineScheduler. Using this scheduler, you can then manipulate the time:

import io.kotest.core.test.testCoroutineScheduler

class TestDispatcherTest : FunSpec() {
init {
test("advance time").config(coroutineTestScope = true) {
val duration = 1.days
// launch a coroutine that would normally sleep for 1 day
launch {
delay(duration.inWholeMilliseconds)
}
// move the clock on and the delay in the above coroutine will finish immediately.
testCoroutineScheduler.advanceTimeBy(duration.inWholeMilliseconds)
val currentTime = testCoroutineScheduler.currentTime
}
}
}

You can enable a test dispatcher for all tests in a spec by setting coroutineTestScope to true at the spec level:

class TestDispatcherTest : FunSpec() {
init {
coroutineTestScope = true
test("this test uses a test dispatcher") {
}
test("and so does this test!") {
}
}
}

Finally, you can enable test dispatchers for all tests in a module by using ProjectConfig:

class ProjectConfig : AbstractProjectConfig() {
override var testCoroutineDispatcher = true
}