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Select expression (experimental)

Select expression makes it possible to await multiple suspending functions simultaneously and select the first one that becomes available.

Selecting from channels

Let us have two producers of strings: fizz and buzz. The fizz produces "Fizz" string every 300 ms:

fun CoroutineScope.fizz() = produce<String> { while (true) { // sends "Fizz" every 300 ms delay(300) send("Fizz") } }

And the buzz produces "Buzz!" string every 500 ms:

fun CoroutineScope.buzz() = produce<String> { while (true) { // sends "Buzz!" every 500 ms delay(500) send("Buzz!") } }

Using receive suspending function we can receive either from one channel or the other. But select expression allows us to receive from both simultaneously using its onReceive clauses:

suspend fun selectFizzBuzz(fizz: ReceiveChannel<String>, buzz: ReceiveChannel<String>) { select<Unit> { // <Unit> means that this select expression does not produce any result fizz.onReceive { value -> // this is the first select clause println("fizz -> '$value'") } buzz.onReceive { value -> // this is the second select clause println("buzz -> '$value'") } } }

Let us run it all seven times:

import kotlinx.coroutines.* import kotlinx.coroutines.channels.* import kotlinx.coroutines.selects.* fun CoroutineScope.fizz() = produce<String> { while (true) { // sends "Fizz" every 300 ms delay(300) send("Fizz") } } fun CoroutineScope.buzz() = produce<String> { while (true) { // sends "Buzz!" every 500 ms delay(500) send("Buzz!") } } suspend fun selectFizzBuzz(fizz: ReceiveChannel<String>, buzz: ReceiveChannel<String>) { select<Unit> { // <Unit> means that this select expression does not produce any result fizz.onReceive { value -> // this is the first select clause println("fizz -> '$value'") } buzz.onReceive { value -> // this is the second select clause println("buzz -> '$value'") } } } fun main() = runBlocking<Unit> { //sampleStart val fizz = fizz() val buzz = buzz() repeat(7) { selectFizzBuzz(fizz, buzz) } coroutineContext.cancelChildren() // cancel fizz & buzz coroutines //sampleEnd }

The result of this code is:

fizz -> 'Fizz' buzz -> 'Buzz!' fizz -> 'Fizz' fizz -> 'Fizz' buzz -> 'Buzz!' fizz -> 'Fizz' buzz -> 'Buzz!'

Selecting on close

The onReceive clause in select fails when the channel is closed causing the corresponding select to throw an exception. We can use onReceiveOrNull clause to perform a specific action when the channel is closed. The following example also shows that select is an expression that returns the result of its selected clause:

suspend fun selectAorB(a: ReceiveChannel<String>, b: ReceiveChannel<String>): String = select<String> { a.onReceiveOrNull { value -> if (value == null) "Channel 'a' is closed" else "a -> '$value'" } b.onReceiveOrNull { value -> if (value == null) "Channel 'b' is closed" else "b -> '$value'" } }

Note that onReceiveOrNull is an extension function defined only for channels with non-nullable elements so that there is no accidental confusion between a closed channel and a null value.

Let's use it with channel a that produces "Hello" string four times and channel b that produces "World" four times:

import kotlinx.coroutines.* import kotlinx.coroutines.channels.* import kotlinx.coroutines.selects.* suspend fun selectAorB(a: ReceiveChannel<String>, b: ReceiveChannel<String>): String = select<String> { a.onReceiveOrNull { value -> if (value == null) "Channel 'a' is closed" else "a -> '$value'" } b.onReceiveOrNull { value -> if (value == null) "Channel 'b' is closed" else "b -> '$value'" } } fun main() = runBlocking<Unit> { //sampleStart val a = produce<String> { repeat(4) { send("Hello $it") } } val b = produce<String> { repeat(4) { send("World $it") } } repeat(8) { // print first eight results println(selectAorB(a, b)) } coroutineContext.cancelChildren() //sampleEnd }

The result of this code is quite interesting, so we'll analyze it in more detail:

a -> 'Hello 0' a -> 'Hello 1' b -> 'World 0' a -> 'Hello 2' a -> 'Hello 3' b -> 'World 1' Channel 'a' is closed Channel 'a' is closed

There are couple of observations to make out of it.

First of all, select is biased to the first clause. When several clauses are selectable at the same time, the first one among them gets selected. Here, both channels are constantly producing strings, so a channel, being the first clause in select, wins. However, because we are using unbuffered channel, the a gets suspended from time to time on its send invocation and gives a chance for b to send, too.

The second observation, is that onReceiveOrNull gets immediately selected when the channel is already closed.

Selecting to send

Select expression has onSend clause that can be used for a great good in combination with a biased nature of selection.

Let us write an example of producer of integers that sends its values to a side channel when the consumers on its primary channel cannot keep up with it:

fun CoroutineScope.produceNumbers(side: SendChannel<Int>) = produce<Int> { for (num in 1..10) { // produce 10 numbers from 1 to 10 delay(100) // every 100 ms select<Unit> { onSend(num) {} // Send to the primary channel side.onSend(num) {} // or to the side channel } } }

Consumer is going to be quite slow, taking 250 ms to process each number:

import kotlinx.coroutines.* import kotlinx.coroutines.channels.* import kotlinx.coroutines.selects.* fun CoroutineScope.produceNumbers(side: SendChannel<Int>) = produce<Int> { for (num in 1..10) { // produce 10 numbers from 1 to 10 delay(100) // every 100 ms select<Unit> { onSend(num) {} // Send to the primary channel side.onSend(num) {} // or to the side channel } } } fun main() = runBlocking<Unit> { //sampleStart val side = Channel<Int>() // allocate side channel launch { // this is a very fast consumer for the side channel side.consumeEach { println("Side channel has $it") } } produceNumbers(side).consumeEach { println("Consuming $it") delay(250) // let us digest the consumed number properly, do not hurry } println("Done consuming") coroutineContext.cancelChildren() //sampleEnd }

So let us see what happens:

Consuming 1 Side channel has 2 Side channel has 3 Consuming 4 Side channel has 5 Side channel has 6 Consuming 7 Side channel has 8 Side channel has 9 Consuming 10 Done consuming

Selecting deferred values

Deferred values can be selected using onAwait clause. Let us start with an async function that returns a deferred string value after a random delay:

fun CoroutineScope.asyncString(time: Int) = async { delay(time.toLong()) "Waited for $time ms" }

Let us start a dozen of them with a random delay.

fun CoroutineScope.asyncStringsList(): List<Deferred<String>> { val random = Random(3) return List(12) { asyncString(random.nextInt(1000)) } }

Now the main function awaits for the first of them to complete and counts the number of deferred values that are still active. Note that we've used here the fact that select expression is a Kotlin DSL, so we can provide clauses for it using an arbitrary code. In this case we iterate over a list of deferred values to provide onAwait clause for each deferred value.

import kotlinx.coroutines.* import kotlinx.coroutines.selects.* import java.util.* fun CoroutineScope.asyncString(time: Int) = async { delay(time.toLong()) "Waited for $time ms" } fun CoroutineScope.asyncStringsList(): List<Deferred<String>> { val random = Random(3) return List(12) { asyncString(random.nextInt(1000)) } } fun main() = runBlocking<Unit> { //sampleStart val list = asyncStringsList() val result = select<String> { list.withIndex().forEach { (index, deferred) -> deferred.onAwait { answer -> "Deferred $index produced answer '$answer'" } } } println(result) val countActive = list.count { it.isActive } println("$countActive coroutines are still active") //sampleEnd }

The output is:

Deferred 4 produced answer 'Waited for 128 ms' 11 coroutines are still active

Switch over a channel of deferred values

Let us write a channel producer function that consumes a channel of deferred string values, waits for each received deferred value, but only until the next deferred value comes over or the channel is closed. This example puts together onReceiveOrNull and onAwait clauses in the same select:

fun CoroutineScope.switchMapDeferreds(input: ReceiveChannel<Deferred<String>>) = produce<String> { var current = input.receive() // start with first received deferred value while (isActive) { // loop while not cancelled/closed val next = select<Deferred<String>?> { // return next deferred value from this select or null input.onReceiveOrNull { update -> update // replaces next value to wait } current.onAwait { value -> send(value) // send value that current deferred has produced input.receiveOrNull() // and use the next deferred from the input channel } } if (next == null) { println("Channel was closed") break // out of loop } else { current = next } } }

To test it, we'll use a simple async function that resolves to a specified string after a specified time:

fun CoroutineScope.asyncString(str: String, time: Long) = async { delay(time) str }

The main function just launches a coroutine to print results of switchMapDeferreds and sends some test data to it:

import kotlinx.coroutines.* import kotlinx.coroutines.channels.* import kotlinx.coroutines.selects.* fun CoroutineScope.switchMapDeferreds(input: ReceiveChannel<Deferred<String>>) = produce<String> { var current = input.receive() // start with first received deferred value while (isActive) { // loop while not cancelled/closed val next = select<Deferred<String>?> { // return next deferred value from this select or null input.onReceiveOrNull { update -> update // replaces next value to wait } current.onAwait { value -> send(value) // send value that current deferred has produced input.receiveOrNull() // and use the next deferred from the input channel } } if (next == null) { println("Channel was closed") break // out of loop } else { current = next } } } fun CoroutineScope.asyncString(str: String, time: Long) = async { delay(time) str } fun main() = runBlocking<Unit> { //sampleStart val chan = Channel<Deferred<String>>() // the channel for test launch { // launch printing coroutine for (s in switchMapDeferreds(chan)) println(s) // print each received string } chan.send(asyncString("BEGIN", 100)) delay(200) // enough time for "BEGIN" to be produced chan.send(asyncString("Slow", 500)) delay(100) // not enough time to produce slow chan.send(asyncString("Replace", 100)) delay(500) // give it time before the last one chan.send(asyncString("END", 500)) delay(1000) // give it time to process chan.close() // close the channel ... delay(500) // and wait some time to let it finish //sampleEnd }

The result of this code:

BEGIN Replace END Channel was closed
Last modified: 16 April 2021