Kotlin flows junit test freezes when to test collect method - kotlin

Question

I'm trying to write a component which uses different datasources of data.
Then data is combined and emitted in the different resulting flow.
class TaskControlComponent(
private val diskCacheDataSource: DiskCacheDataSource,
private val debugDataSource: DebugDataSource
) {
private val _localTasks = MutableStateFlow<Map<String, TaskItem>>(emptyMap())
val localTasks: StateFlow<Map<String, TaskItem>> = _localTasks
suspend fun loadLocal() {
flowOf(
diskCacheDataSource.defaultFeatures,
diskCacheDataSource.localFeatures,
debugDataSource.debugFeatures
).flattenMerge().collect {
computeLocalTasks()
}
}
private suspend fun computeLocalTasks() {
val resultTasks = HashMap<String, TaskItem>(64)
listOf(
diskCacheDataSource.defaultTasks,
diskCacheDataSource.localTasks,
debugDataSource.debugTasks
).forEach { tasksMap ->
tasksMap.value.forEach { entry ->
resultTasks[entry.key] = entry.value
}
}
_localTasks.emit(resultTasks)
}
}
DataSource
interface DiskCacheDataSource {
val defaultTasks: StateFlow<Map<String, TaskItem>>
val localTasks: StateFlow<Map<String, TaskItem>>
}
It works, but how to write junit test for that?
class TaskControlImplTest {
private lateinit var taskControl: TaskControlComponent
#Mock
lateinit var diskCacheDataSource: DiskCacheDataSource
#Mock
lateinit var debugDataSource: DebugDataSource
#Before
fun setup() {
MockitoAnnotations.initMocks(this)
taskControl = TaskControlComponent(diskCacheDataSource, debugDataSource)
}
#Test
fun testFeatureControl() {
whenever(diskCacheDataSource.defaultTasks).thenReturn(
MutableStateFlow(
mapOf(
"1" to TaskItem(
"1",
TaskStatus.On
)
)
)
)
whenever(diskCacheDataSource.localTasks).thenReturn(MutableStateFlow(emptyMap()))
whenever(debugDataSource.debugTasks).thenReturn(MutableStateFlow(emptyMap()))
runBlocking {
taskControl.loadLocal()
}
runBlocking {
taskControl.localTasks.collect {
Assert.assertEquals(it.size, 1)
}
}
}
}
In case of the following sequence of commands
runBlocking {
taskControl.loadLocal()
}
runBlocking {
taskControl.localTasks.collect {
Assert.assertEquals(it.size, 1)
}
}
test freezes, and runs forewer.
When I swap the pieces of code, first instead of second and the contrary
runBlocking {
featureControl.localFeatures.collect {
Assert.assertEquals(it.size, 1)
}
}
runBlocking {
featureControl.loadLocal()
}
Tests finishes with warning
expected:<0> but was:<1>
Expected :0
Actual :1
Is it possible to write test for such usecase? What should be investigated or done in order to make test workable?

Answer 1

The reason the order matters here is because StateFlow is hot, unlike normal Flow, meaning it starts running with data immediately not when it is collected
I test with the turbine library but you don't need it. I don't remember the exact setup of not using turbine but it was a bit more complicated so I chose to use turbine
https://github.com/cashapp/turbine