ExampleTest.kt

import com.dropbox.android.external.store4.Fetcher
import com.dropbox.android.external.store4.Store
import com.dropbox.android.external.store4.StoreBuilder
import com.dropbox.android.external.store4.fresh
import com.dropbox.android.sample.Graph
import com.dropbox.android.sample.data.model.Post
import io.mockk.coEvery
import io.mockk.every
import io.mockk.mockk
import kotlinx.coroutines.ExperimentalCoroutinesApi
import kotlinx.coroutines.FlowPreview
import kotlinx.coroutines.test.TestCoroutineScope
import kotlinx.coroutines.test.runBlockingTest
import org.junit.Before
import org.junit.Test
import org.junit.runner.RunWith
import org.junit.runners.JUnit4
import kotlin.time.ExperimentalTime

@OptIn(
        FlowPreview::class,
        ExperimentalCoroutinesApi::class,
        ExperimentalTime::class,
        ExperimentalStdlibApi::class
)
class StoreProvider() {

    fun getStore() =
            StoreBuilder.from(Fetcher.of { key: String ->
                Graph.provideRetrofit().fetchSubreddit(key, 10).data.children.map(Graph::toPosts)
            }).build()

}

class MyTestClass(val provider: StoreProvider) {

    suspend fun testFetching(): List<Post> {
        val store = provider.getStore()
        return store.fresh("asd")
    }

}

@ExperimentalCoroutinesApi
@FlowPreview
@RunWith(JUnit4::class)
class MyTestClassTest {
    private val testScope = TestCoroutineScope()
    private val storeProvider: StoreProvider = mockk()
    private val store: Store<String, List<Post>> = mockk()
    private lateinit var testClass: MyTestClass

    @Before
    fun setUp() {
        every { storeProvider.getStore() } returns store
        testClass = MyTestClass(storeProvider)
    }


    @Test
    fun failingTest() = testScope.runBlockingTest {
        coEvery { store.fresh(any()) } returns mockk() // crashes here NoSuchElementException: Expected at least one element
        val result = testClass.testFetching()
    }
}


/*

Line 62 throws:

java.util.NoSuchElementException: Expected at least one element

	at kotlinx.coroutines.flow.FlowKt__ReduceKt.first(Reduce.kt:89)
	at kotlinx.coroutines.flow.FlowKt.first(Unknown Source)
	at com.dropbox.android.external.store4.StoreKt.fresh(Store.kt:84)
	at com.dropbox.android.sample.ui.reddit.MyTestClassTest$failingTest$1$1.invokeSuspend(ExampleTest.kt:72)
	at com.dropbox.android.sample.ui.reddit.MyTestClassTest$failingTest$1$1.invoke(ExampleTest.kt)
	at io.mockk.impl.eval.RecordedBlockEvaluator$record$block$2$1.invokeSuspend(RecordedBlockEvaluator.kt:26)
	at io.mockk.impl.eval.RecordedBlockEvaluator$record$block$2$1.invoke(RecordedBlockEvaluator.kt)
	at io.mockk.InternalPlatformDsl$runCoroutine$1.invokeSuspend(InternalPlatformDsl.kt:20)
	at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
	at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:56)
	at kotlinx.coroutines.EventLoopImplBase.processNextEvent(EventLoop.common.kt:274)
	at kotlinx.coroutines.BlockingCoroutine.joinBlocking(Builders.kt:84)
	at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking(Builders.kt:59)
	at kotlinx.coroutines.BuildersKt.runBlocking(Unknown Source)
	at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking$default(Builders.kt:38)
	at kotlinx.coroutines.BuildersKt.runBlocking$default(Unknown Source)
	at io.mockk.InternalPlatformDsl.runCoroutine(InternalPlatformDsl.kt:19)
	at io.mockk.impl.eval.RecordedBlockEvaluator$record$block$2.invoke(RecordedBlockEvaluator.kt:26)
	at io.mockk.impl.eval.RecordedBlockEvaluator$enhanceWithNPERethrow$1.invoke(RecordedBlockEvaluator.kt:74)
	at io.mockk.impl.recording.JvmAutoHinter.autoHint(JvmAutoHinter.kt:23)
	at io.mockk.impl.eval.RecordedBlockEvaluator.record(RecordedBlockEvaluator.kt:36)
	at io.mockk.impl.eval.EveryBlockEvaluator.every(EveryBlockEvaluator.kt:30)
	at io.mockk.MockKDsl.internalCoEvery(API.kt:98)
	at io.mockk.MockKKt.coEvery(MockK.kt:115)
	at com.dropbox.android.sample.ui.reddit.MyTestClassTest$failingTest$1.invokeSuspend(ExampleTest.kt:72)
	at com.dropbox.android.sample.ui.reddit.MyTestClassTest$failingTest$1.invoke(ExampleTest.kt)
	at kotlinx.coroutines.test.TestBuildersKt$runBlockingTest$deferred$1.invokeSuspend(TestBuilders.kt:50)
	at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
	at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:56)
	at kotlinx.coroutines.test.TestCoroutineDispatcher.dispatch(TestCoroutineDispatcher.kt:50)
	at kotlinx.coroutines.DispatchedContinuationKt.resumeCancellableWith(DispatchedContinuation.kt:288)
	at kotlinx.coroutines.intrinsics.CancellableKt.startCoroutineCancellable(Cancellable.kt:26)
	at kotlinx.coroutines.CoroutineStart.invoke(CoroutineStart.kt:109)
	at kotlinx.coroutines.AbstractCoroutine.start(AbstractCoroutine.kt:158)
	at kotlinx.coroutines.BuildersKt__Builders_commonKt.async(Builders.common.kt:91)
	at kotlinx.coroutines.BuildersKt.async(Unknown Source)
	at kotlinx.coroutines.BuildersKt__Builders_commonKt.async$default(Builders.common.kt:84)
	at kotlinx.coroutines.BuildersKt.async$default(Unknown Source)
	at kotlinx.coroutines.test.TestBuildersKt.runBlockingTest(TestBuilders.kt:49)
	at kotlinx.coroutines.test.TestBuildersKt.runBlockingTest(TestBuilders.kt:73)
	at com.dropbox.android.sample.ui.reddit.MyTestClassTest.failingTest(ExampleTest.kt:71)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:59)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:56)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
	at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
	at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
	at org.junit.runners.BlockJUnit4ClassRunner$1.evaluate(BlockJUnit4ClassRunner.java:100)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:366)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:103)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:63)
	at org.junit.runners.ParentRunner$4.run(ParentRunner.java:331)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:79)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:329)
	at org.junit.runners.ParentRunner.access$100(ParentRunner.java:66)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:293)
	at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:413)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
	at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
	at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:33)
	at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:230)
	at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:58)
*/