DelayEmitter

DelayEmitterTest.kt

package com.example.delayemitter

import kotlinx.coroutines.*
import kotlinx.coroutines.flow.*
import kotlinx.coroutines.test.UnconfinedTestDispatcher
import kotlinx.coroutines.test.runTest
import org.junit.Assert.assertEquals
import org.junit.Test
import java.lang.Thread.sleep
import java.util.*

/**
 * Example local unit test, which will execute on the development machine (host).
 *
 * See [testing documentation](http://d.android.com/tools/testing).
 */
class ExampleUnitTest {

    private val _emitter = MutableStateFlow<Unit?>(null)
    private var delayEmitter: Flow<Unit?> =
        _emitter.filterNotNull().onStart {
            delay(1000)
        }.flowOn(Dispatchers.IO)

    suspend fun start() {
        _emitter.emit(Unit)
    }
    var job: Job? = null
    fun cancel() {
        job?.cancel()
    }

    @OptIn(ExperimentalCoroutinesApi::class)
    @Test
    fun addition_isCorrect() = runTest(UnconfinedTestDispatcher()) {


        val values = mutableListOf<Unit?>()

        job = launch {
            delayEmitter.collect {
                values.add(it)
                println("${Date()} collect value $values")
            }
        }

        job?.start()
        start()
        println("${Date()} value $values")
        assertEquals(0, values.size)

        sleep(3100)
        println("${Date()} value $values")
        assertEquals(1, values.size)
        job?.cancel()
    }
}