Anyone knows why a mutableSharedFlow.collect might...

# coroutines

[Guillermo Alcantara]

Anyone knows why a mutableSharedFlow.collect might not be called during a Unit Test?

@Test
    fun testFoo() = runBlocking {
        val testCoroutineScope = TestCoroutineScope().apply {
            pauseDispatcher() // This needs to be here because the actual test, handles time.
        }
        val sharedFlow = MutableSharedFlow<Int>()
        val values = mutableListOf<Int>()
        println("before launch")
        val job = testCoroutineScope.launch {
            println("before collect")
            sharedFlow.collect {
                println("before adding $it")
                values.add(it)
            }
        }

        println("before emits")
        sharedFlow.emit(1)
        sharedFlow.emit(2)
        testCoroutineScope.runCurrent()

        assertEquals(mutableListOf(1, 2), values)

        job.cancel()
    }

[Guillermo Alcantara]

The string "before adding XXX" is never called

[FunkyMuse]

Because collect is blocking the execution

[Guillermo Alcantara]

I don't understand what you mean, the collect is inside a launch. The string "before emits" does print.

[tateisu]

Does SharedFlow store previously emitted values?

[tateisu]

public fun <T> MutableSharedFlow( replay: Int = 0, ...)

[tateisu]

default value of replay is 0.

[tateisu]

also, you might use job.join() before assertEquals

[tateisu]

( i don't know what condition is used to stop collect ... )

[Guillermo Alcantara]

Thanks Tate but having a replay value would change the business logic. (It also to my knowledge makes no difference.) I also tried having extraBufferCapacity, this doesn't seem to help either. I added logic to have a job.join and cancel the collect but again doesn't seem to help.

[tateisu]

Do you need to emit first in in this test code ?

[tateisu]

I don't know the correctness of the test that emits before collect for SharedFlow that doesn't have replayCache.

[tateisu]

The order I think reasonable. Launch the coroutine to collect first. Then emit. The test code waits until the collect side finishes receiving the value.

[Guillermo Alcantara]

I want to emit the values after the collect is set up. But even putting a big delay doesn't seem to make a difference.

[tateisu]

I think I've answered at least your question well. I will not intervene in your design.

[Guillermo Alcantara]

Sorry, I'm not following. Could you tell me how can I emit values after the collect is set up without a replay value?

[Guillermo Alcantara]

Btw, the test passes if I comment pauseDispatcher()

[tateisu]

that case collect is start before emit

[Guillermo Alcantara]

ahh, I think I see what's happening. I need to add a bunch of testCoroutineScope.runCurrent()

[tateisu]

https://gist.github.com/tateisu/f1cfd32a28c672e88009f7bd16669159

[Guillermo Alcantara]

For posterity this is how it's working for me:

@Test
    fun testFoo() = runBlocking {
        val testCoroutineScope = TestCoroutineScope().apply {
            pauseDispatcher()
        }
        val sharedFlow = MutableSharedFlow<Int>(
            extraBufferCapacity = 2 // Without it, sharedFlow.emit won't have a space to save data. It will be collected
                                    // next time there's a testCoroutineScope.runCurrent()
        )
        val values = mutableListOf<Int>()
        println("before launch")
        val job = testCoroutineScope.launch {
            println("before collect")
            sharedFlow.collect {
                println("before adding $it")
                values.add(it)
            }
        }
        testCoroutineScope.runCurrent() // Allows the previous launch to start collecting

        println("before emits")
        sharedFlow.emit(1)
        sharedFlow.emit(2)
        testCoroutineScope.runCurrent()

        assertEquals(mutableListOf(1, 2), values)
        job.cancel()
    }

[Guillermo Alcantara]

Also added the question to https://stackoverflow.com/questions/66018309/how-can-i-collect-values-from-a-mutablesharedflow-in-a-unit-test/66018310#66018310 in case someone needs it.

[Guillermo Alcantara]

Thanks for the help!