onEach and flowOn

OperatorFlowOnTest.kt

package fr.x

import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.delay
import kotlinx.coroutines.flow.flow
import kotlinx.coroutines.flow.flowOn
import kotlinx.coroutines.flow.onEach
import kotlinx.coroutines.runBlocking
import org.junit.Test

/**
 * Unit tests of [MaasPaymentMethodsViewModel].
 */
class MaasPaymentMethodsViewModelTest   {
 

    @Test
    fun main() {
        println(
            """ 
                *Start: running in : ${Thread.currentThread()}
                    """.trimIndent(),
        )
        runBlocking {
            flow {
                for (i in 1..5) {
                    emit(i)
                    delay(200)
                    println(
                        """ 
                        *emitting: running in : ${Thread.currentThread()}
                    """.trimIndent(),
                    )
                }
            }
                .onEach {
                    delay(100)
                    println(
                        """ 
                        *delaying: running in : ${Thread.currentThread()}
                    """.trimIndent(),
                    )
                }
                .flowOn(Dispatchers.IO)
                .collect { println(it) }

        }
    }
}

*Start: running in : Thread[Test worker,5,main]
*delaying: running in : Thread[DefaultDispatcher-worker-1 @coroutine#2,5,main]
1
*emitting: running in : Thread[DefaultDispatcher-worker-1 @coroutine#2,5,main]
*delaying: running in : Thread[DefaultDispatcher-worker-1 @coroutine#2,5,main]
2
*emitting: running in : Thread[DefaultDispatcher-worker-1 @coroutine#2,5,main]
*delaying: running in : Thread[DefaultDispatcher-worker-1 @coroutine#2,5,main]
3
*emitting: running in : Thread[DefaultDispatcher-worker-1 @coroutine#2,5,main]
*delaying: running in : Thread[DefaultDispatcher-worker-1 @coroutine#2,5,main]
4
*emitting: running in : Thread[DefaultDispatcher-worker-1 @coroutine#2,5,main]
*delaying: running in : Thread[DefaultDispatcher-worker-1 @coroutine#2,5,main]
5
*emitting: running in : Thread[DefaultDispatcher-worker-1 @coroutine#2,5,main]

changing the code to this:

.flowOn(Dispatchers.IO)
 .onEach {
  delay(100)
println(
 """ 
 *delaying: running in : ${Thread.currentThread()}
""".trimIndent(),
}
.collect { println(it) }

will give this output:
*Start: running in : Thread[Test worker,5,main]
*delaying: running in : Thread[Test worker @coroutine#1,5,main]
1
*emitting: running in : Thread[DefaultDispatcher-worker-1 @coroutine#2,5,main]
*delaying: running in : Thread[Test worker @coroutine#1,5,main]
2
*emitting: running in : Thread[DefaultDispatcher-worker-1 @coroutine#2,5,main]
*delaying: running in : Thread[Test worker @coroutine#1,5,main]
3
*emitting: running in : Thread[DefaultDispatcher-worker-1 @coroutine#2,5,main]
*delaying: running in : Thread[Test worker @coroutine#1,5,main]
4
*emitting: running in : Thread[DefaultDispatcher-worker-1 @coroutine#2,5,main]
*delaying: running in : Thread[Test worker @coroutine#1,5,main]
5
*emitting: running in : Thread[DefaultDispatcher-worker-1 @coroutine#2,5,main]