JUnit extensions for LiveData(InstantExecutor), RxJava2, and Coroutines

CoroutineExtension.kt

import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.ExperimentalCoroutinesApi
import kotlinx.coroutines.test.TestCoroutineDispatcher
import kotlinx.coroutines.test.TestCoroutineScope
import kotlinx.coroutines.test.resetMain
import kotlinx.coroutines.test.setMain
import org.junit.jupiter.api.extension.AfterEachCallback
import org.junit.jupiter.api.extension.BeforeEachCallback
import org.junit.jupiter.api.extension.ExtensionContext

@ExperimentalCoroutinesApi
class CoroutineExtension(
    private val dispatcher: TestCoroutineDispatcher = TestCoroutineDispatcher()
) : BeforeEachCallback, AfterEachCallback, TestCoroutineScope by TestCoroutineScope(dispatcher) {

    override fun beforeEach(context: ExtensionContext?) {
        Dispatchers.setMain(dispatcher)
    }

    override fun afterEach(context: ExtensionContext?) {
        cleanupTestCoroutines()
        Dispatchers.resetMain()
    }
}

InstantExecutorExtension.kt

import android.annotation.SuppressLint
import androidx.arch.core.executor.ArchTaskExecutor
import androidx.arch.core.executor.TaskExecutor
import org.junit.jupiter.api.extension.AfterEachCallback
import org.junit.jupiter.api.extension.BeforeEachCallback
import org.junit.jupiter.api.extension.ExtensionContext

class InstantExecutorExtension : BeforeEachCallback, AfterEachCallback {

    @SuppressLint("RestrictedApi")
    override fun beforeEach(context: ExtensionContext?) {
        ArchTaskExecutor.getInstance()
            .setDelegate(object : TaskExecutor() {
                override fun executeOnDiskIO(runnable: Runnable) = runnable.run()
                override fun postToMainThread(runnable: Runnable) = runnable.run()
                override fun isMainThread(): Boolean = true
            })
    }

    @SuppressLint("RestrictedApi")
    override fun afterEach(context: ExtensionContext?) {
        ArchTaskExecutor.getInstance().setDelegate(null)
    }

}

RxSchedulerExtension.kt

    class TestSchedulerExtension : BeforeTestExecutionCallback, AfterTestExecutionCallback {

        override fun beforeTestExecution(context: ExtensionContext?) {
            RxAndroidPlugins.setInitMainThreadSchedulerHandler { Schedulers.trampoline() }
            RxJavaPlugins.setIoSchedulerHandler { Schedulers.trampoline() }
            RxJavaPlugins.setComputationSchedulerHandler { Schedulers.trampoline() }
            RxJavaPlugins.setNewThreadSchedulerHandler { Schedulers.trampoline() }
            RxAndroidPlugins.setMainThreadSchedulerHandler { Schedulers.trampoline() }
        }

        override fun afterTestExecution(context: ExtensionContext?) {
            RxJavaPlugins.reset()
            RxAndroidPlugins.reset()
        }

    }

SampleViewModelTest.kt

@ExperimentalCoroutinesApi
@ExtendWith(value = [InstantExecutorExtension::class, RxSchedulerExtension::class, CoroutineExtension::class])
class SampleViewModelTest {
  ....
}

the CoroutineExtension is outdated and there's probably no need to use it with Coroutines 1.6. See https://medium.com/androiddevelopers/migrating-to-the-new-coroutines-1-6-test-apis-b99f7fc47774 for a great rundown of how to migrate to the new Coroutines 1.6 unit test APIs

Repo instant-task-executor-extension provides a packaged version of this InstantExecutorExtension.
No need to copy the code anymore, now include it with just one line:

dependencies {
    testImplementation 'io.github.neboskreb:instant-task-executor-extension:1.0.0'
}