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Kotlin协程（三）async和await
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 {await和async使用方法
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

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