Example of Kotlin Coroutine Flow with multiple subscribers

FooService.kt

package com.example.atsukofukui.fooexample

import com.google.common.truth.Truth.assertThat
import kotlinx.coroutines.CoroutineDispatcher
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.channels.ConflatedBroadcastChannel
import kotlinx.coroutines.flow.asFlow
import kotlinx.coroutines.flow.collect
import kotlinx.coroutines.flow.flowOn
import kotlinx.coroutines.flow.map
import kotlinx.coroutines.launch
import kotlinx.coroutines.test.TestCoroutineDispatcher
import kotlinx.coroutines.test.TestCoroutineScope
import kotlinx.coroutines.test.runBlockingTest
import org.junit.Test

import org.junit.Before

class ExampleUnitTest {

    @OptIn(kotlinx.coroutines.ExperimentalCoroutinesApi::class)
    private val testScope = TestCoroutineScope()

    @OptIn(kotlinx.coroutines.ExperimentalCoroutinesApi::class)
    private val dispatcher = TestCoroutineDispatcher()

    private lateinit var fooService: FooService

    @Before
    fun setup() {
        fooService = FooService(dispatcher)
    }

    @OptIn(kotlinx.coroutines.ExperimentalCoroutinesApi::class)
    @Test
    fun `requestFoo() should trigger getting foo and fooFlow emit foo`() {
        testScope.runBlockingTest {
            val foo = mutableListOf<String>()
            val job = launch {
                fooService.fooFlow.collect { foo.add(it) }
            }

            // ここではまだfooは空
            assertThat(foo).isEmpty()

            fooService.requestFoo()

            assertThat(foo).isEqualTo(listOf("result"))

            job.cancel()
        }
    }
}

class FooService(private val dispatcher: CoroutineDispatcher = Dispatchers.IO) {

    @OptIn(kotlinx.coroutines.ExperimentalCoroutinesApi::class)
    private val fooChannel = ConflatedBroadcastChannel<Unit>()

    @OptIn(
        kotlinx.coroutines.FlowPreview::class,
        kotlinx.coroutines.ExperimentalCoroutinesApi::class
    )
    val fooFlow = fooChannel.asFlow().map { getFooFromRemoteSource() }.flowOn(dispatcher)

    private fun getFooFromRemoteSource(): String {
        val result = "result"
        return result
    }

    @OptIn(kotlinx.coroutines.ExperimentalCoroutinesApi::class)
    fun requestFoo() {
        fooChannel.offer(Unit)
    }
}