Deferred 值提供了一种在协程之间传递单个值的方便手段, 而 Channel 提供一种传递一系列值(即 Stream 流)的方法.
Channel 概念上与Java的 BlockingQueue 很相似, 但有一个重要的不同点, BlockingQueue 的 put 和 take 方法是阻塞的, 而Channel对应的 send 和 receive 方法是可挂起.
import kotlinx.coroutines.*
import kotlinx.coroutines.channels.*
fun main() = runBlocking {
val channel = Channel()
launch {
// this might be heavy CPU-consuming computation or async logic, we'll just send five squares
for (x in 1..5) channel.send(x * x)
}
// here we print five received integers:
repeat(5) { println(channel.receive()) }
println("Done!")
}
1
4
9
16
25
Done!
不像队列, 通道关闭意味着不会再有数据到来, 接收端可以使用 for 循环来接收通道的数据. 概念上, close 方法像是发送了一个关闭标记, 然后循环将停止.
import kotlinx.coroutines.*
import kotlinx.coroutines.channels.*
fun main() = runBlocking {
val channel = Channel()
launch {
for (x in 1..5) channel.send(x * x)
channel.close() // we're done sending
}
// here we print received values using `for` loop (until the channel is closed)
for (y in channel) println(y)
println("Done!")
}
生产者-消费者模式很常见, 使用 produce 协程构建器可以轻松地实现生产者. 消费端可以使用 consumeEach 扩展函数 , 它可以取代 for 循环迭代.
import kotlinx.coroutines.*
import kotlinx.coroutines.channels.*
fun CoroutineScope.produceSquares(): ReceiveChannel = produce { //注意返回的类型
for (x in 1..5) send(x * x)
}
// 注意: produceSquares() 是定义在 CoroutineScope 上的扩展方法
fun main() = runBlocking {
val squares = produceSquares()
squares.consumeEach { println(it) }
println("Done!")
}
管道是一种模式, 它有一个生产者协程, 可能生产出无限的一系列的值. 另外有一个或多个消费协程, 消费值, 然后也可能生产出另外一些值.
import kotlinx.coroutines.*
import kotlinx.coroutines.channels.*
fun main() = runBlocking {
val numbers = produceNumbers() // produces integers from 1 and on
val squares = square(numbers) // squares integers
for (i in 1..5) println(squares.receive()) // print first five
println("Done!") // we are done
coroutineContext.cancelChildren() // cancel children coroutines
}
//生产者
fun CoroutineScope.produceNumbers() = produce {
var x = 1
while (true) send(x++) // infinite stream of integers starting from 1
}
//消费者
fun CoroutineScope.square(numbers: ReceiveChannel): ReceiveChannel = produce {
for (x in numbers) send(x * x)
}
注意生产者和消费者都是定义在 CoroutineScope 上的扩展函数, 这样我们可以依赖结构化并发保证取消或结束时没有泄漏.
多个消费者协程可能消费同一个管道, 如下:
import kotlinx.coroutines.*
import kotlinx.coroutines.channels.*
fun main() = runBlocking {
val producer = produceNumbers()
repeat(5) { launchProcessor(it, producer) }
delay(950)
producer.cancel() // cancel producer coroutine and thus kill them all
}
fun CoroutineScope.produceNumbers() = produce {
var x = 1 // start from 1
while (true) {
send(x++) // produce next
delay(100) // wait 0.1s
}
}
fun CoroutineScope.launchProcessor(id: Int, channel: ReceiveChannel) = launch {
for (msg in channel) {
println("Processor #$id received $msg")
}
}
Processor #2 received 1
Processor #4 received 2
Processor #0 received 3
Processor #1 received 4
Processor #3 received 5
Processor #2 received 6
Processor #4 received 7
Processor #0 received 8
Processor #1 received 9
Processor #3 received 10
注意: 取消生产者协程会关闭管道, 最终终止在其上迭代的消费者协程. launchProcessor 方法内, 在 Channel 迭代的 for 循环与前面介绍的 consumeEach 不同, 即使在多协程模式下, 它也是完全安全的, 如果一个消费者协程失败了, 其他的仍然可以正确迭代Channel, 而使用 consumeEach 则会在异常结束时会取消 Channel, 这样会影响其他消费协程.
有时多个生产协程会往一个管道中生产数据, 而只有一个消费协程.
import kotlinx.coroutines.*
import kotlinx.coroutines.channels.*
fun main() = runBlocking {
val channel = Channel()
launch { sendString(channel, "foo", 200L) }
launch { sendString(channel, "BAR!", 500L) }
repeat(6) { // receive first six
println(channel.receive())
}
println("ddd")
coroutineContext.cancelChildren() // cancel all children to let main finish
}
suspend fun sendString(channel: SendChannel, s: String, time: Long) {
while (true) {
delay(time)
channel.send(s)
}
}
foo
foo
BAR!
foo
foo
foo
Channel 默认是不带缓存区的, 如果发送方先执行, 则需要挂起等待接收方消费后才会发送下一个数据, 反之亦然. Channel 的工厂方法以及 produce builder 方法有一个可选的 capacity 参数可以指定缓冲区的大小, 它使生产者在挂起等待前可以最多发送多少个数据, 与 BlockingQueue 的缓冲区大小参数类似, 如果 BlockingQueue 的缓冲区满了, 则发送操作会阻塞.
import kotlinx.coroutines.*
import kotlinx.coroutines.channels.*
fun main() = runBlocking {
val channel = Channel(4) // create buffered channel
val sender = launch { // launch sender coroutine
repeat(10) {
println("Sending $it") // print before sending each element
channel.send(it) // will suspend when buffer is full
}
}
// don't receive anything... just wait....
delay(1000)
sender.cancel() // cancel sender coroutine
}
Sending 0
Sending 1
Sending 2
Sending 3
Sending 4
前4个元素发送到管道了, 但第5个只是打印出日志, 并没有发送成功, 而是挂起了.
Channel 是公平的Channel 按调用的顺序来决定谁先发送或接收数据, 即 FIFO 算法.
data class Ball(var hits: Int)
fun main() = runBlocking {
val table = Channel() // a shared table
launch { player("ping", table) }
launch { player("pong", table) }
table.send(Ball(0)) // serve the ball
delay(1000) // delay 1 second
coroutineContext.cancelChildren() // game over, cancel them
}
suspend fun player(name: String, table: Channel) {
for (ball in table) { // receive the ball in a loop
ball.hits++
println("$name $ball")
delay(300) // wait a bit
// 这里发送数据, 但即使在for循环内马上接收数据, 但另一个协程已经在他处等待, 优先接收了.
table.send(ball)
}
}
ping Ball(hits=1)
pong Ball(hits=2)
ping Ball(hits=3)
pong Ball(hits=4)
虽然 Channel 是公平的, 但因为执行器(executor)的原因, 可能看出来不公平.
有时需要一个信号发生器, 每隔一定的周期或者延迟发送一个信号, 以便做一些同步动作或者重复性的工作. Ticker Channel 就是这种信号生产器通道. 它可以使用 ticker 工厂方法创建. Ticker Channel只产生Unit类型的数值, 并且有两种产生信号的时机模式, 固定周期(FIXED_PERIOD)或者固定延迟(FIXED_DELAY), 固定延迟以消费者接收到数据时开始计时.
import kotlinx.coroutines.*
import kotlinx.coroutines.channels.*
fun main() = runBlocking {
val tickerChannel = ticker(delayMillis = 100, initialDelayMillis = 0) // create ticker channel
var nextElement = withTimeoutOrNull(1) { tickerChannel.receive() }
println("Initial element is available immediately: $nextElement") // initial delay hasn't passed yet
nextElement = withTimeoutOrNull(50) { tickerChannel.receive() } // all subsequent elements has 100ms delay
println("Next element is not ready in 50 ms: $nextElement")
nextElement = withTimeoutOrNull(60) { tickerChannel.receive() }
println("Next element is ready in 100 ms: $nextElement")
// Emulate large consumption delays
println("Consumer pauses for 150ms")
delay(150)
// Next element is available immediately
nextElement = withTimeoutOrNull(1) { tickerChannel.receive() }
println("Next element is available immediately after large consumer delay: $nextElement")
// Note that the pause between `receive` calls is taken into account and next element arrives faster
nextElement = withTimeoutOrNull(60) { tickerChannel.receive() }
println("Next element is ready in 50ms after consumer pause in 150ms: $nextElement")
tickerChannel.cancel() // indicate that no more elements are needed
}
Initial element is available immediately: kotlin.Unit
Next element is not ready in 50 ms: null
Next element is ready in 100 ms: kotlin.Unit
Consumer pauses for 150ms
Next element is available immediately after large consumer delay: kotlin.Unit
Next element is ready in 50ms after consumer pause in 150ms: kotlin.Unit
Ticker 通道可以觉察可能暂停的消费者, 并且当消费者消费太慢时, 停止生产, 自我调整以维护一个固定的生产数据的频率.
