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Channels & Flows
in Practice
Mohit Sarveiya
/heyitsmohit
twitter.com
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Channels & Flows in Practice
• Channels
• Inside Channels
• Flows
• Implementing & Testing Polling
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Coroutine A
Problem
Coroutine B
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Coroutine A
Problem
Coroutine B
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Passing Data
• Deferred
• Channels
• Flows
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Channels
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What is a Channel?
Item
Channel
Buffer (optional)
Item
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What is a Channel?
interface Channel : SendChannel, ReceiveChannel
interface ReceiveChannel {
suspend fun receive(): E
}
interface SendChannel {
suspend fun send(element: E)
}
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Channel Types
• Rendezvous
• Buffered
• Unlimited
• Conflated
• Broadcast Channel
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Creating Channel
fun Channel(capacity: Int = RENDEZVOUS) =
when (capacity) {
RENDEZVOUS "-> RendezvousChannel()
UNLIMITED "-> LinkedListChannel()
CONFLATED "-> Con"flatedChannel()
BUFFERED "-> ArrayChannel(CHANNEL_DEFAULT_CAPACITY)
else "-> ArrayChannel(capacity)
}
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Using Channels
runBlocking {
val channel = Channel()
launch {
channel.send(1)
}
val num = channel.receive()
}
Consumer
Producer
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Rendezvous Channel
• No Buffer
• Waits for receiver to send data
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Rendezvous Channel
runBlocking {
val channel = Channel()
launch {
channel.send(1)
channel.send(2)
}
}
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Rendezvous Channel
runBlocking {
val channel = Channel()
launch {
channel.send(1)
channel.send(2)
}
}
SUSPEND
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Rendezvous Channel
runBlocking {
val channel = Channel()
launch {
channel.send(1)
channel.send(2)
}
val num = channel.receive()
}
Receiver
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runBlocking {
val channel = Channel()
launch {
channel.send(1)
channel.send(2)
}
val num = channel.receive()
}
Rendezvous Channel
RESUME
Receive value 1
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runBlocking {
val channel = Channel()
launch {
channel.send(1)
channel.send(2)
}
val num = channel.receive()
}
Rendezvous Channel
SUSPEND
Received value 1
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Buffered Channel
• Buffer with specific capacity.
• Caches sent items in Array
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runBlocking {
val channel = Channel(capacity = 3)
launch {
channel.send(1)
channel.send(2)
channel.send(3)
}
}
Buffered Channel
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Buffer
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runBlocking {
val channel = Channel(capacity = 3)
launch {
channel.send(1)
channel.send(2)
channel.send(3)
channel.send(4)
}
}
Buffered Channel
SUSPEND 1 2 3
Buffer
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Unlimited Channel
runBlocking {
val channel = Channel(Channel.UNLIMITED)
launch {
channel.send(1)
channel.send(2)
channel.send(3)
}
}
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Broadcast Channel
Coroutine A
Coroutine B
Coroutine C
Coroutine D
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Broadcast Channel
• Buffered
• Unlimited
• Conflated
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Broadcast Channel
runBlocking {
val channel = broadcast(capacity = 10, block = {
send(1)
})
val consumer1 = channel.openSubscription()
val consumer2 = channel.openSubscription()
launch {
println(consumer1.receive()) "// 1
println(consumer2.receive()) "// 1
}
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Creating Producers
runBlocking {
val channel = Channel()
launch {
for (i in 1"..5) {
channel.send(i)
}
channel.close()
}
}
Producer
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Producer Builder
• Creates Coroutine with Channel
• DSL for creating channels
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Producer Builder
fun CoroutineScope.produce(
context: CoroutineContext,
capacity: Int = 0,
block: suspend ProducerScope.() "-> Unit
): ReceiveChannel
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Creating Producers
suspend fun CoroutineScope.producer(): ReceiveChannel =
produce {
for (i in 1"..5) {
send(i)
}
close()
}
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Creating Producers
runBlocking {
val channel: ReceiveChannel = producer()
}
suspend fun CoroutineScope.producer(): ReceiveChannel =
produce {
for (i in 1"..5) { send(i) }
close()
}
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Consuming Producers
runBlocking {
val channel: ReceiveChannel = producer()
for (num in channel) {
println(num)
}
}
suspend fun CoroutineScope.producer(): ReceiveChannel =
produce {
for (i in 1"..5) { send(i) }
close()
}
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Inside Channels
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Inside Channels
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Inside Channels
val channel = Channel()
What happens under the hood?
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Inside Channels
• Queue (Doubly Linked List)
• Buffer (Array)
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Channel States
• Empty Queue
• Send Queued
• Send Buffered
• Closed
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Channel States
EmptyQueue
LockFreeLinkedListNode
SendBuffered SendQueued Closed
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Query Channel State
• Get most recent state
• Size of buffer
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class AbstractSendChannel {
fun toString() =
{$queueDebugStateString}$bufferDebugString”
}
Query Channel State
Current Channel State Buffer Size
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Rendezvous Channel
val channel = Channel()
EmptyQueue
Queue
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Rendezvous Channel
val channel = Channel()
channel.send(1)
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Rendezvous Channel
class SendElement(
val data: Any?,
val cont: CancellableContinuation
) : LockFreeLinkedListNode()
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Rendezvous Channel
val channel = Channel()
channel.send(1)
SendElement(1)
Queue
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Rendezvous Channel
val channel = Channel()
channel.send(1)
println(channel)
Output:
RendezvousChannel{SendQueued}
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Rendezvous Channel
val channel = Channel()
channel.send(1)
channel.receive()
EmptyQueue
Queue
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Unlimited Channel
runBlocking {
val channel = Channel(Channel.UNLIMITED)
launch {
channel.send(1)
channel.send(2)
channel.send(3)
}
}
SendBuffered(1) SendBuffered(2) SendBuffered(3)
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Consuming Values
runBlocking {
val channel: ReceiveChannel = producer()
for (num in channel) {
println(num)
}
}
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Coroutine A
Consuming Values
Coroutine B
1 2 3 x
Closed token
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Channel Iterator
interface ChannelIterator {
suspend fun hasNextSuspend(): E
fun next(): E
}
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Channel Iterator
fun hasNextSuspend() = suspendCancellable { cont "->
while (true) {
val result = channel.pollInternal()
this.result = result
if (result is Closed""<*>) {
cont.resume(false)
} else {
cont.resume(true)
}
…
}
}
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Coroutine A
Channel Iterator
Coroutine B
EmptyQueue
Channel Queue
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Coroutine A
Channel Iterator
Coroutine B
1
SendElement(1)
Channel Queue
hasNext()
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Coroutine A
Channel Iterator
Coroutine B
1
SendElement(1)
Channel Queue
next()
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Coroutine A
Channel Iterator
Coroutine B
1
SendElement(1)
Channel Queue
next()
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Coroutine A
Channel Iterator
Coroutine B
1
SendElement(3)
Channel Queue
next()
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Coroutine A
Channel Iterator
Coroutine B
1
Closed
Channel Queue
2 3 x
Closed token
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Coroutine A
Channel Iterator
Coroutine B
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Suspended
No Closed token
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Flows
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Coroutine A
Cold Stream
Coroutine B
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Coroutine A
Cold Stream
Coroutine B
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Coroutine A
Cold Stream
Coroutine B
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Creating Flows
• flowOf(…)
• flow { … }
• channelFlow { … }
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Creating Flows
runBlocking {
val "flow: "Flow = "flowOf(1, 2, 3)
}
fun "flowOf(vararg elements: T): "Flow
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Collecting Flows
runBlocking {
val "flow: "Flow = "flowOf(1, 2, 3)
"flow.collect {
println(it) "// 1 2 3
}
}
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Creating Flows
val "flow = "flow {
emit(1)
emit(2)
emit(3)
}
"flow.collect {
println(it) "// 1 2 3
}
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Creating Flows
val "flow = "flow {
emit(1)
emit(2)
emit(3)
}
"flow.collect { println(it) } "// 1 2 3
"flow.collect { println(it) } "// 1 2 3
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Terminal Operators
• Collect
• Single
• Reduce
• toList
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Single Operator
Collects only one value.
val "flow = "flowOf(1)
val num: Int = "flow.single()
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Single Operator
Collects only one value.
val "flow = "flowOf(1,2,3)
val num: Int = "flow.single()
java.lang.IllegalStateException: Expected only one element
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Flow Operators
• Map, Zip
• Buffer
• Retry
• Take
• delayEach
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DelayEach
runBlocking {
val "flow = "flowOf(1,2,3).delayEach(1000)
"flow.collect {
println(it)
}
}
Operators are not suspending functions
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DelayEach
runBlocking {
val "flow = get"Flow()
"flow.collect {
println(it)
}
}
fun get"Flow() = "flowOf(1,2,3).delayEach(1000)
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Flow Context
runBlocking {
val "flow = "flow {
emit(1)
emit(2)
emit(3)
}
"flow.collect {
println(it)
}
}
Which dispatcher does this run on?
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Flow Context
runBlocking {
val "flow = "flow {
emit(1)
emit(2)
emit(3)
}
"flow.collect {
println(it)
}
}
BlockingEventLoop
BlockingEventLoop
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Flow Context
runBlocking {
val "flow = "flow {
emit(1)
emit(2)
emit(3)
}
"flow
."flowOn(Dispatchers.IO)
.collect { println(it) }
}
Dispatchers.IO
BlockingEventLoop
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Flow Constraints
• Context Preservation
• Exception Handling
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Flow Constraints
Emit from the same coroutine
val "flow = "flow {
launch(Dispatchers.IO) {
emit(1)
}
}
Flow invariant is violated: emission from another coroutine is detected
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Channel Flow Builder
val "flow = channel"Flow {
launch(Dispatchers.IO) {
send(1)
}
launch(Dispatchers.IO) {
send(2)
}
}
Flow
Channel
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Channel Flow Builder
class Channel"FlowBuilder(
val block: suspend ProducerScope.() "-> Unit,
capacity: Int = BUFFERED
) : Channel"Flow(context, capacity)
Uses Buffered Channel
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Exception Handling
val "flow = "flow {
try {
makeRequest()
} catch (e: IOException) {
}
}
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Exception Handling
fun "Flow.catch(action:
suspend "FlowCollector.(cause: Throwable) "-> Unit
): "Flow
Emit exception in a wrapper
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Exception Handling
sealed class ApiResponse {
data class Success(val data: T): ApiResponse()
data class Error(val t: Throwable): ApiResponse()
}
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Exception Handling
val "flow = "flow> {
val data = makeRequest()
emit(ApiResponse.Success(data))
}.catch {
emit(ApiResponse.Error(it))
}
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Channel to Flow Bridge
runBlocking {
val bufferedChannel = Channel(capacity = 10)
val broadcastChannel = broadcast(capacity = 10) {
send(1)
}
val "flow1: "Flow = bufferedChannel.consumeAs"Flow()
val "flow2 = broadcastChannel.as"Flow()
}
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Use Case: Polling
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Polling
1 second
/feed
Data
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Polling
interface ApiService {
suspend fun getData(): Data
}
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Polling
interface Poller {
fun poll(delay: Long): "Flow
fun close()
}
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Polling
class CoroutinePoller(
val apiService: ApiService,
val dispatcher: CoroutineDispatcher
): Poller {
override fun poll(delay: Long): "Flow { }
override fun close() { }
}
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Polling
class CoroutinePoller(…): Poller {
override fun poll(delay: Long): "Flow {
return channel"Flow {
while (!isClosedForSend) {
val data = apiService.getData()
send(data)
delay(delay)
}
}."flowOn(dispatcher)
}
}
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Polling
class CoroutinePoller("..): Poller {
override fun cancel() {
dispatcher.cancel()
}
}
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Polling Usage
class MyViewModel(val poller: Poller): ViewModel {
fun startPolling() {
viewModelScope.launch {
val "flow: "Flow = poller.poll(1000)
"flow.collect { data !->
processData(data)
}
}
}
}
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Testing Strategy
Create coroutine for test
• Run blocking Coroutine
• Coroutine with test dispatcher and scope
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Testing
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Testing
• Dispatchers.setMain
• runBlockingTest
• TestCoroutineDispatcher
• advanceTimeBy, pauseDispatcher, resumeDispatcher
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Testing
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Testing Strategy
• Mock API Service
• Advance time every second
• Assert on Flow
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Testing
@Test
fun `should poll every second`() = runBlockingTest {
val data = mock()
val apiService = mock() {
onBlocking { getData() } doReturn data
}
val testDispatcher = TestCoroutineDispatcher()
val poller = CoroutinePoller(apiService, testDispatcher)
}
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Testing
@Test
fun `should poll every second`() = runBlockingTest {
val "flow = poller.poll(1_000)
launch {
val dataList = "flow.take(2).toList()
assertEquals(dataList.size, 2)
}
testDispatcher.advanceTimeBy(2_000)
poller.close()
}
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Testing
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Flow Assert
suspend fun "Flow.test(
validate: suspend "FlowAssert.() "-> Unit
) {
}
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Flow Assert
sealed class Event {
object Complete : Event()
data class Error(val t: Throwable) : Event()
data class Item(val item: T) : Event()
}
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Flow Assert
suspend fun "Flow.test(
validate: suspend "FlowAssert.() "-> Unit
) {
coroutineScope {
val events = Channel>(UNLIMITED)
launch {
try {
collect { item !->
events.send(Event.Item(item))
}
Event.Complete
}
}
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Flow Assert
suspend fun "Flow.test(
validate: suspend "FlowAssert.() "-> Unit
) {
coroutineScope {
val events = Channel>(UNLIMITED)
launch {
try {
…
} catch (t: Throwable) { Event.Error(t) }
if (terminalEvent "!= null) {
events.send(terminalEvent)
}
events.close()
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Flow Assert
class FlowAssert(val events: Channel>) {
suspend fun expectItem(): T
suspend fun expectComplete()
suspend fun expectError(): Throwable
…
}
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Flow Assert
@Test
fun `should poll every second`() = runBlockingTest {
val "flow = poller.poll(1_000)
launch {
"flow.test {
assertEquals(expectItem(), data)
assertEquals(expectItem(), data)
expectComplete()
}
}
testDispatcher.advanceTimeBy(2_000)
poller.close()
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Resources
• Kotlinx-coroutines-test
https://github.com/Kotlin/kotlinx.coroutines/tree/master/kotlinx-coroutines-test
• Coroutine JUnit Test Rule
https://craigrussell.io/2019/11/unit-testing-coroutine-suspend-functions-using-testcoroutinedispatcher/
• Corbind
https://github.com/LDRAlighieri/Corbind
• FlowAssert
https://github.com/cashapp/sqldelight/blob/master/extensions/coroutines-extensions/src
commonTest/kotlin/com/squareup/sqldelight/runtime/coroutines/FlowAssert.kt
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Thank you!
twitter.com/heyitsmohit