Kotlin 协程(三) async和await

Posted 2023-02-16

参考技术A Kotlin 协程 (一)

Kotlin 协程(二) -协程取消与超时

如果顺序执行,调起两个挂起函数,执行这两个挂起函数的总是时间等于分别执行这个两个函数的总和,如示例(1)

示例(1):

fun main() = runBlocking<Unit>

    val time = measureTimeMillis

        val one = doSomethingUsefulOne()

        val two = doSomethingUsefulTwo()

        println("The result is $one + two")

    

    println("Completed in $time ms")



suspend fun doSomethingUsefulOne(): Int

    delay(1000L) 

    return 10



suspend fun doSomethingUsefulTwo(): Int

    delay(1000L) 

    return 33



示例(1)执行结果:

The result is 43

Completed in 2046 ms

async函数类似于 launch函数.它启动了一个单独的协程,这是一个轻量级的线程并与其它所有的协程一起并发的工作.不同之处在于 launch 返回一个 Job 并且不附带任何结果值,而 async 返回一个 Deferred —— 一个轻量级的非阻塞 future,这代表了一个将会在稍后提供结果的 promise.你可以使用 .await() 在一个延期的值上得到它的最终结果， 但是 Deferred 也是一个 Job，所以如果需要的话，你可以取消它.如示例(2)

示例(2):

fun main() = runBlocking<Unit>

    val time = measureTimeMillis

        val one = async doSomethingUsefulOne()

        val two = async doSomethingUsefulTwo()

        val result1 = one.await()

        val result2 = two.await()

        println("The result is $result1 + result2")

    

    println("Completed in $time ms")



suspend fun doSomethingUsefulOne(): Int

    delay(1000L) 

    return 10



suspend fun doSomethingUsefulTwo(): Int

    delay(1000L)

    return 33



示例(2)的执行结果:

The result is 43

Completed in 1032 ms

通过示例(1)和示例(2)的执行结果,我们可以看出,示例(2)的两个挂起函数是并发执行的,即:async函数是新起一个协程,并发执行的

async 可以通过将 start 参数设置为 CoroutineStart.LAZY 而变为惰性的.在这种情况下,只有结果通过 await 获取的时候协程才会启动,或者在 Job 的 start 函数调用的时候,如示例(3)

示例(3):

fun main() = runBlocking<Unit>

    val time = measureTimeMillis

        val one = async(start = CoroutineStart.LAZY) doSomethingUsefulOne()

        val two = async(start = CoroutineStart.LAZY) doSomethingUsefulTwo()

        one.start()

        two.start()

        val result1 = one.await()

        val result2 = two.await()

        println("The result is $result1 + result2")

    

    println("Completed in $time ms")



suspend fun doSomethingUsefulOne(): Int

    delay(1000L) 

    return 9



suspend fun doSomethingUsefulTwo(): Int

    delay(1000L) 

    return 33



示例(3)的执行结果:

The result is 42

Completed in 1026 ms

在示例(3)中,如果没有执行start,默认执行await方法,则one 和 two是顺序执行的,one执行完之后才执行two,如示例(4)

示例(4):

fun main() = runBlocking<Unit>

    val time = measureTimeMillis

        val one = async(start = CoroutineStart.LAZY) doSomethingUsefulOne()

        val two = async(start = CoroutineStart.LAZY) doSomethingUsefulTwo()

        val result1 = one.await()

        val result2 = two.await()

        println("The result is $result1 + result2")

    

    println("Completed in $time ms")



suspend fun doSomethingUsefulOne(): Int

    delay(1000L) 

    return 9



suspend fun doSomethingUsefulTwo(): Int

    delay(1000L) 

    return 33



示例(4)的执行结果:

The result is 42

Completed in 2026 ms