Kotlin select使用方法介绍

2022-11-28 12:07 开发作者：且听真言

一、select是什么

select——>用于选择更快的结果。

基于场景理解

比如客户端要查询一个商品的详情。两个服务：缓存服务，速度快但信息可能是旧的；网络服务，速度慢但信息一定是最新的。

如何实现上述逻辑：

   runblocking {
        suspend fun getCacheInfo(productId: String): Product {
            delay(100L)
            return Product(productId, 8.9)
        }
        suspend fun getNetworkInfo(productId: String): Product? {
            delay(200L)
            return Product(productId, 8.8)
        }
        fun updateUI(product: Product) {
            println("${product.productId} : ${product.price}")
        }
        val startTime = System.currentTimeMillis()
        val productId = "001"
        val cacheInfo = getCacheInfo(productId)
        if (cacheInfo != null) {
            updateUI(cacheInfo)
            println("Time cost: ${System.currentTimeMillis() - startTime}")
        }
        val latestInfo = getNetworkInfo(productId)
        if (latestInfo != null) {
            updateUI(latestInfo)
            println("Time cost: ${System.currentTimeMillis() - startTime}")
        }
    }

001 : 8.9
Time cost: 113
001 : 8.8
Time cost: 324

上述程序分为四步：第一步：查询缓存信息；第二步：缓存服务返回信息，更新 UI；第三步：查询网络服务；第四步：网络服务返回信息，更新 UI。

用户可以第一时间看到商品的信息，虽然它暂时会展示旧的信息，但由于我们同时查询了网络服务，旧缓存信息也马上会被替代成新的信息。但是可能存在一些问题：如果程序卡在了缓存服务，获取网络服务就会无法执行。是因为 getCacheInfo() 它是一个挂起函数，只有这个程序执行成功以后，才可以继续执行后面的任务。能否做到：两个挂起函数同时执行，谁返回的速度更快，就选择哪个结果。答案是使用select。

runBlocking {
        suspend fun getCacheInfo(productId: String): Product {
            delay(100L)
            return Product(productId, 8.9)
        }
        suspend fun getNetworkInfo(productId: String): Product {
            delay(200L)
            return Product(productId, 8.8)
        }
        fun updateUI(product: Product) {
            println("${product.productId} : ${product.price}")
        }
        val startTime = System.currentTimeMillis()
        val productId = "001"
        val product = select<Product?> {
            async {
                getCacheInfo(productId)
            }.onAwait {
                it
            }
            async {
                getNetworkInfo(productId)
            }.onAwait {
                it
            }
        }
        if (product != null) {
            updateUI(product)
            println("Time cost: ${System.currentTimeMillis() - startTime}")
        }
    }

001 : 8.9
Time cost: 134

Process finished with exit code 0

由于缓存的服务更快，所以，select 确实帮我们选择了更快的那个结果。我们的 select 可以在缓存服务出现问题的时候，灵活选择网络服务的结果。从而避免用户等待太长的时间，得到糟糕的体验。

在上述代码中，用户大概率是会展示旧的缓存信息。但实际场景下，我们是需要进一步更新最新信息的。

runBlocking {
        suspend fun getCacheInfo(productId: String): Product {
            delay(100L)
            return Product(productId, 8.9)
        }
        suspend fun getNetworkInfo(productId: String): Product {
            delay(200L)
            return Product(productId, 8.8)
        }
        fun updateUI(product: Product) {
            println("${product.productId} : ${product.price}")
        }
        val startTime = System.currentTimeMillis()
        val productId = "001"
        val cacheDeferred = async {
            getCacheInfo(productId)
        }
        val latestDeferred = async {
            getNetworkInfo(productId)
        }
        val product = select<Product?> {
 
            cacheDeferred.onAwait {
                it.copy(isCache = true)
            }
            latestDeferred.onAwait {
                it.copy(isCache = false)
            }
        }
        if (product != null) {
            updateUI(product)
            println("Time cost: ${System.currentTimeMillis() - startTime}")
        }
        if (product != null && product.isCache) {
            val latest = latestDeferred.await() ?: return@runBlocking
            updateUI(latest)
          android  println("Time cost: ${System.currentTimeMillis() - startTime}")
        }
    }

001 : 8.9
Time cost: 124
001 : 8.8
Time cost: 228

Process finished with exit code 0

如果是多个服务的缓存场景呢？

 runBlocking {
        val startTime = System.currentTimeMillis()
        val prodphpuctId = "001"
        suspend fun getCacheInfo(productId: String): Product {
            delay(100L)
            return Product(productId, 8.9)
        }
        suspend fun getCacheInfo2(productId: String): Product {
            delay(50L)
            return Product(productId, 8.85)
        }
        suspend fun getNetworkInfo(productId: String): Product {
            delay(200L)
            return Product(productId, 8.8)
        }
        fun updateUI(product: Product) {
            println("${product.productId} : ${product.price}")
        }
        val cacheDeferred = async {
            getCacheInfo(productId)
        }
        val cacheDeferred2 = async {
            getCacheInfo2(productId)
        }
        val latestDeferred = async {
            getNetworkInfo(productId)
        }
        val product = select<Product?> {
            cacheDeferred.onAwait {
                it.copy(isCache = true)
            }
            cacheDeferred2.onAwait {
                it.copy(isCache = true)
            }
            latestDeferred.onAwait {
                it.copy(isCache = true)
            }
        }
        if (product != null) {
            updateUI(product)
            println("Time cost: ${System.currentTimeMillis() - startTime}")
        }
        if (product != null && product.isCache) {
            val latest = latestDeferred.await()
            updateUI(latest)
            println("Time cost: ${System.currentTimeMillis() - startTime}")
        }
    }

Log

001 : 8.85
Time cost: 79
001 : 8.8
Time cost: 229

Process finished with exit code 0

select 代码模式，可以提升程序的整体响应速度。

二、select和Channel

runBlocking {
        val startTime = System.currentTimeMillis()
        val channel1 = produce {
            send(1)
            delay(200L)
            send(2)
            delay(200L)
            send(3)
        }
        val channel2 = produce {
            delay(100L)
            send("a")
            delay(200L)
            send("b")
            delay(200L)
            send("c")
        }
        channel1.consumeEach {
            println(it)
        }
        channel2.consumeEach {
            println(it)
        }
        println("Time cost: ${System.currentTimeMillis() - startTime}")
    }

Log

1
2
3
a
b
c
Time cost: 853

Process finished with exit code 0

上述代码串行执行，可以使用select进行优化。

  runBlocking {
        val startTime = System.currentTimeMillis()
        val channel1 = produce {
            send(1)
            delay(200L)
            send(2)
            delay(200L)
            send(3)
        }
        val channel2 = produce {
            delay(100L)
            send("a")
            delay(200L)
            send("b")
            delay(200L)
            send("c")
        }
        suspend fun selectChannel(
            channel1: ReceiveChannel<Int>,
            channel2: ReceiveChannel<String>
        ): Any {
            return select<Any> {
                if (!channel1.isClosedForReceive) {
                    channel1.onReceive {
                        it.also {
                            println(it)
                        }
                    }
                }
                if (!channel2.isClosedForReceive) {
                    channel2.onReceive {
                        it.also {
                            println(it)
                        }
                    }
                }
            }
        }
        repeat(6) {
            selectChannel(channel1, channel2)
        }
        println("Time cost: ${System.currentTimeMillis() - startTime}")
    }

Log
1
a
2
b
3
c
Time cost: 574

Process finished with exit code 0

从代码执行结果可以发现程序的执行耗时有效减少。onReceive{} 是 Channel 在 select 当中的语法，当 Channel 当中有数据以后，它就会被回调，通过这个 Lambda，将结果传出去。执行了 6 次 select，目的是要把两个管道中的所有数据都消耗掉。

如果Channel1不生产数据了，程序会如何执行？

runBlocking {
        val startTime = System.currentTimeMillis()
        val channel1 = produce<String> {
            delay(5000L)
        }
        val channel2 = produce<String> {
            delay(100L)
            send("a")
            delay(200L)
            send("b")
            delay(200L)
            send("c")
        }
        suspend fun selectChannel(
            channel1: ReceiveChannel<String>,
            channel2: ReceiveChannel<String>
        ): String = select<String> {
            channel1.onReceive {
                it.also {
                    println(it)
                }
            }
            channel2.onReceive {
                it.also {
                    println(it)
                }
            }
        }
        repeat(3) {
            selectChannel(channel1, channel2)
        }
        println("Time cost: ${System.currentTimeMillis() - startTime}")
    }

Log
a
b
c
Time cost: 570

Process finished with exit code 0

如果不知道Channel的个数，如何避免ClosedReceiveChannelException?

使用：onReceiveCatching{}

runBlocking {
        val startTime = System.currentTimeMillis()
        val channel1 = produce<String> {
            dela编程y(5000L)
        }
        val channel2 = produce<String> {
            delay(100L)
            send("a")
            delay(200L)
            send("b")
            delay(200L)
            send("c")
        }
        suspend fun selectChannel(
            channel1: ReceiveChannel<String>,
            channel2: ReceiveChannel<String>
        ): String = select<String> {
            channel1.onReceiveCatching {
                it.getOrNull() ?: "channel1 is closed!"
            }
            channel2.onReceiveCatching {
                it.getOrNull() ?: "channel2 is closed!"
            }
  cYagfRlJo      }
        repeat(6) {
            val result = selectChannel(channel1, channel2)
            println(result)
        }
        println("Time cost: ${System.currentTimeMillis() - startTime}")
    }

Log
a
b
c
channel2 is closed!
channel2 is closed!
channel2 is closed!
Time cost: 584

Process finished with exit code 0

得到所有结果以后，程序不会立即退出，因为 channel1 一直在 delay()。

所以我们需要在6次repeat之android后将channel关闭。

runBlocking {
        val startTime = System.currentTimeMillis()
        val channel1 = produce<String> {
            delay(15000L)
        }
        val channel2 = produce<String> {
            delay(100L)
            send("a")
            delay(200L)
            send("b")
            delay(200L)
            send("c")
        }
        suspend fun selectChannel(
            channel1: ReceiveChannel<String>,
            channel2: ReceiveChannel<String>
        ): String = select<String> {
            channel1.onReceiveCatching {
                it.getOrNull() ?: "channel1 is closed!"
            }
            channel2.onReceiveCatching {
                it.getOrNull() ?: "channel2 is closed!"
            }
        }
        repeat(6) {
            val result = selectChannel(channel1, channel2)
            println(result)
        }
        channel1.cancel()
        channel2.cancel()
        println("Time cost: ${System.currentTimeMillis() - startTime}")
    }

Log
a
b
c
channel2 is closed!
channel2 is closed!
channel2 is closed!
Time cost: 612

Process finished with exit code 0

Deferred、Channel 的 API:

public interface Deferred : CoroutineContext.Element {
    public suspend fun join()
    public suspend fun await(): T
    public val onJoin: Sele开发者_C培训ctClause0
    public val onAwait: SelectClause1<T>
}
public interface SendChannel<in E> 
    public suspend fun send(element: E)
    public val onSend: SelectClause2<E, SendChannel<E>>
}
public interface ReceiveChannel<out E> {
    public suspend fun receive(): E
    public suspend fun receiveCatching(): ChannelResult<E>
    public val onReceive: SelectClause1<E>
    public val onReceiveCatching: SelectClause1<ChannelResult<E>>
}

当 select 与 Deferred 结合使用的时候，当并行的 Deferred 比较多的时候，你往往需要在得到一个最快的结果以后，去取消其他的 Deferred。

通过 async 并发执行协程，也可以借助 select 得到最快的结果。

 runBlocking {
        suspend fun <T> fastest(vararg deferreds: Deferred<T>): T = select {
            fun cancelAll() = deferreds.forEach {
                it.cancel()
            }
            for (deferred in deferreds) {
                deferred.onAwait {
                    cancelAll()
                    it
                }
            }
        }
        val deferred1 = async {
            delay(100L)
            println("done1")
            "result1"
        }
        val deferred2 = async {
            delay(200L)
            println("done2")
            "result2"
        }
        val deferred3 = async {
            delay(300L)
            println("done3")
            "result3"
        }
        val deferred4 = async {
            delay(400L)
            println("done4")
            "result4"
        }
        val deferred5 = async {
            delay(5000L)
            println("done5")
            "result5"
        }
        val fastest = fastest(deferred1, deferred2, deferred3, deferred4, deferred5)
        println(fastest)
    }

Log

done1
result1

Process finished with exit code 0

到此这篇关于Kotlin select使用方法介绍的文章就介绍到这了,更多相关Kotlin select内容请搜索我们以前的文章或继续浏览下面的相关文章希望大家以后多多支持我们！

继续阅读：Kotlin select Kotlin select使用方法

Kotlin select使用方法介绍

目录

一、select是什么

二、select和Channel

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