How to mock a ProducerScope from callbackFlow builder in Kotlin Flow? - kotlin

Question

I'd like to test a function where I use the scope of a callbackFlow builder. Assuming I have a function inside the flow builder like this:
fun items(): Flow<Items> = callbackFlow {
getItems(this) {
trySend(it)
}
awaitClose()
}
In getItems function, I received data from websockets. The scope of ProducerScope is used to either launch a new coroutine with a delay and do something or to close the scope if an error happens. So it might call scope.launch { } or scope.close().
For example, this could do something as follows:
fun getItems(scope: ProducerScope<Items>, callback: (Items) -> Unit) {
if (something) {
scope.launch { ... }
}
if (somethingElse) {
...
scope.close(error)
}
...
callback(items)
}
The callbackFlow's block uses a ProducerScope, extension of CoroutineScope and SendChannel, I tried to mock it using Mockk:
val scope: ProducerScope<Items> = mockk()
Unfortunately, I end up with:
java.lang.ClassCastException: class kotlin.coroutines.CoroutineContext$Element$Subclass6 cannot be cast to class kotlin.coroutines.ContinuationInterceptor
How can I mock a ProducerScope?
How do I unit test getItems above when scope can be either a CoroutineScope and a SendChannel?
Thanks in advance.

Answer 1

After many tries, I cannot do this easily without expecting strange behaviors. So I refactored my function to use a Channel and a CoroutineScope separately. Thanks to the CoroutineScope plus extension, I can create a new scope from the flow builder. This is now testable!
Therefore, the flow builder became:
fun items(): Flow<Items> = callbackFlow {
val channel = this.channel
val scope = this.plus(this.coroutineContext)
getItems(channel, scope) {
...
}
...
}
My function still uses both but gets them separately:
fun getItems(
channel: SendChannel<Items>,
scope: CoroutineScope,
callback: (Items) -> Unit
) {
if (something) {
scope.launch { ... } // <-- use scope
}
if (somethingElse) {
...
channel.close(error) // <-- use channel
}
...
callback(items)
}
Then, I can now test using a Channel with the same requirements than the one in callbackFlow and the scope from runTest:
#Test
fun `get items and succeed`() = runTest {
val channel = Channel<Any>(Channel.BUFFERED, BufferOverflow.SUSPEND)
...
service.getItems(channel, this#runTest, callback)
...
}