最近一直在了解关于 Kotlin协程 的知识，那最好的学习资料自然是官方提供的学习文档了，看了看后我就萌生了翻译官方文档的想法。前后花了要接近一个月时间，一共九篇文章，在这里也分享出来，希望对读者有所帮助。个人知识所限，有些翻译得不是太顺畅，也希望读者能提出意见
协程官方文档：coroutines-guide
select 表达式可以同时等待多个挂起函数，并选择第一个可用的函数来执行
选择表达式是
kotlinx.coroutines的一个实验性的特性，这些 API 预计将在kotlinx.coroutines库的即将到来的更新中衍化，并可能会有突破性的变化
一、Selecting from channels
我们现在有两个字符串生产者：fizz 和 buzz 。其中 fizz 每 300 毫秒生成一个字符串“Fizz”：
fun CoroutineScope.fizz() = produce<String> {
while (true) { // sends "Fizz" every 300 ms
delay(300)
send("Fizz")
}
}
接着 buzz 每 500 毫秒生成一个字符串“Buzz!”：
fun CoroutineScope.buzz() = produce<String> {
while (true) { // sends "Buzz!" every 500 ms
delay(500)
send("Buzz!")
}
}
使用挂起函数 receive，我们可以从两个通道接收其中一个的数据。但是 select 表达式允许我们使用其 onReceive 子句同时从两者接收：
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'")
}
}
}
让我们运行代码 7 次：
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
}
运行结果：
fizz -> 'Fizz'
buzz -> 'Buzz!'
fizz -> 'Fizz'
fizz -> 'Fizz'
buzz -> 'Buzz!'
fizz -> 'Fizz'
buzz -> 'Buzz!'
二、Selecting on close
当通道关闭时，select 中的 onReceive 子句会失败并导致相应的 select 引发异常。我们可以使用 onReceiveOrNull 子句在通道关闭时执行特定操作。下面的示例还显示了 select 是一个返回其查询方法结果的表达式：
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'"
}
}
注意，onReceiveOrNull 是一个扩展函数，仅可用于具有不可为空元素的通道，这样就不会意外混淆通道是已关闭还是返回了空值这两种情况
让我们将其与生成四次“Hello”字符串的通道 a 和生成四次“World”字符串的通道 b 一起使用：
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
}
这段代码的结果非常有趣，所以我们将在细节中分析它：
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
从中可以观察到几点
首先，select 偏向于第一个子句。当同时可以选择多个子句时，将选择其中的第一个子句。在这里，两个通道都在不断地产生字符串，因此作为 select 中的第一个子句的通道获胜。但是，因为我们使用的是无缓冲通道，所以 a 在其发送调用时会不时地被挂起，从而给了 b 发送的机会
第二个观察结果是，当通道已经关闭时，onReceiveOrNull 将立即被选中
三、Selecting to send
select 表达式有 onSend 子句，可以与 selection 的偏向性质结合使用。
让我们写一个整数生产者的例子，当主通道上的消费者跟不上时，它会将其值发送到 side 通道：
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
}
}
}
消费者将会非常缓慢，每个数值处理需要 250 毫秒：
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
}
让我们看看会发生什么：
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
四、Selecting deferred values
延迟值可以使用 onAwait 子句来查询。让我们启动一个异步函数，它在随机的延迟后会延迟返回字符串：
fun CoroutineScope.asyncString(time: Int) = async {
delay(time.toLong())
"Waited for $time ms"
}
让我们随机启动十余个异步函数，每个都延迟随机的时间
fun CoroutineScope.asyncStringsList(): List<Deferred<String>> {
val random = Random(3)
return List(12) { asyncString(random.nextInt(1000)) }
}
现在，main 函数等待它们中的第一个完成，并统计仍处于活动状态的延迟值的数量。注意，我们在这里使用 select 表达式事实上是一种 Kotlin DSL，因此我们可以使用任意代码为它提供子句。在本例中，我们遍历一个延迟值列表，为每个延迟值提供 onAwait 子句。
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
}
输出结果：
Deferred 4 produced answer 'Waited for 128 ms'
11 coroutines are still active
五、Switch over a channel of deferred values
现在我们来编写一个通道生产者函数，它消费一个产生延迟字符串的通道，并等待每个接收的延迟值，但它只在下一个延迟值到达或者通道关闭之前处于运行状态。此示例将 onReceiveOrNull 和 onAwait 子句放在同一个 select 中：
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
}
}
}
为了测试它，我们将用一个简单的异步函数，它在特定的延迟后返回特定的字符串：
fun CoroutineScope.asyncString(str: String, time: Long) = async {
delay(time)
str
}
main 函数只是启动一个协程来打印 switchMapDeferreds 的结果并向它发送一些测试数据：
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
}
代码的执行结果：
BEGIN
Replace
END
Channel was closed
