• • Selecting from channels
  • Selecting to send
  • Switch over a channel of deferred values

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

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

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

You can get the full code here.

The result of this code is:

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

The 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 -> '$value'"
        }
        b.onReceiveOrNull { value -> 
            if (value == null) 
            else    
                "b -> '$value'"
        }
    }

Let’s use it with channel a that produces “Hello” string four times and channel that produces “World” four times:

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 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.

Select expression has 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

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.

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 $index produced answer '$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

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 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: