Cahining coroutines by using extension functions in Kotlin - kotlin

Question

I want to chain 3 coroutines by using Kotlin's extension functions. I know how to do it with regular ones, but can't manage it with extension functions. In fact, in the 2nd coroutine I can receive only one data sent from the 1st coroutine, but that's all. The program works but all I get on the console is Doc: 1st Document. What I'm doing wrong?
fun main(args: Array<String>) = runBlocking {
produceDocs().docLength().report().consumeEach {
println(it)
}
}
private fun CoroutineScope.produceDocs() = produce {
fun getDocs(): List<String> {
return listOf("1st Document", "2nd Newer Document")
}
while (this.isActive) {
val docs = getDocs()
for (doc in docs) {
send(doc)
}
delay(TimeUnit.SECONDS.toMillis(2))
}
}
private suspend fun ReceiveChannel<String>.docLength(): ReceiveChannel<Int> = coroutineScope {
val docsChannel: ReceiveChannel<String> = this#docLength
produce {
for (doc in docsChannel) {
println("Doc: $doc") // OK. This works.
send(doc.count()) // ??? Not sure where this sends data to?
}
}
}
private suspend fun ReceiveChannel<Int>.report(): ReceiveChannel<String> = coroutineScope {
val docLengthChannel: ReceiveChannel<Int> = this#report
produce {
for (len in docLengthChannel) {
println("Length: $len") // !!! Nothing arrived.
send("Report. Document contains $len characters.")
}
}
}

Answer 1

You have to consume each channel independently in order to make emissions go through the chain, otherwise the first emission will never be consumed:
private fun CoroutineScope.produceDocs() = produce {
fun getDocs(): List<String> {
return listOf("1st Document", "2nd Newer Document")
}
while (this.isActive) {
val docs = getDocs()
for (doc in docs) {
send(doc)
}
delay(TimeUnit.SECONDS.toMillis(2))
}
}
private suspend fun ReceiveChannel<String>.docLength() : ReceiveChannel<Int> = CoroutineScope(coroutineContext).produce {
for (doc in this#docLength) {
println("Doc: $doc") // OK. This works.
send(doc.count()) // ??? Not sure where this sends data to?
}
}
private suspend fun ReceiveChannel<Int>.report(): ReceiveChannel<String> = CoroutineScope(coroutineContext).produce {
for (len in this#report) {
println("Length: $len") // !!! Nothing arrived.
send("Report. Document contains $len characters.")
}
}
I suggest you a better approach to do the exact same thing using Flow:
private fun produceDocs(): Flow<String> = flow {
fun getDocs(): List<String> {
return listOf("1st Document", "2nd Newer Document")
}
while (true) {
val docs = getDocs()
for (doc in docs) {
emit(doc)
}
delay(TimeUnit.SECONDS.toMillis(2))
}
}
private fun Flow<String>.docLength(): Flow<Int> = flow {
collect { doc ->
println("Doc: $doc")
emit(doc.count())
}
}
private fun Flow<Int>.report(): Flow<String> = flow {
collect { len ->
println("Length: $len")
emit("Report. Document contains $len characters.")
}
}
Or better like this:
private fun produceDocs(): Flow<String> = flow {
fun getDocs(): List<String> {
return listOf("1st Document", "2nd Newer Document")
}
while (true) {
val docs = getDocs()
for (doc in docs) {
emit(doc)
}
delay(TimeUnit.SECONDS.toMillis(2))
}
}
private fun Flow<String>.docLength(): Flow<Int> = transform { doc ->
println("Doc: $doc")
emit(doc.count())
}
private fun Flow<Int>.report(): Flow<String> = transform { len ->
println("Length: $len")
emit("Report. Document contains $len characters.")
}
And collect it like this:
produceDocs().docLength().report().collect {
println(it)
}
Or even better like this:
produceDocs()
.map { doc ->
println("Doc: $doc")
doc.count()
}
.map { len ->
println("Length: $len")
"Report. Document contains $len characters."
}
.collect {
println(it)
}