Kotlin 协程执行顺序？

Question

Can someone enlighten me about what is going on in this code example:

import kotlinx.coroutines.*
import kotlin.time.*

fun subTask(scope: CoroutineScope): Job {
    val job = scope.launch(Dispatchers.IO){
        println("SubTask started.")
        delay(500L) 
        println("SubTask ending.")
    }
    job.invokeOnCompletion(object: CompletionHandler{
        override fun invoke(cause: Throwable?) {
            println("SubTask completed.")
        }
    })
    return job
}

fun main() {
    val duration = measureTime {
        runBlocking {
            val job = withContext(Dispatchers.IO) {
                subTask(this)
            }
            println("Waiting for SubTask") 
            job.join()
        }
    }
    println("Duration: $duration")
}

(Click here for Kotlin playground)

I was expecting to see

SubTask started.
Waiting for SubTask
SubTask ending.
SubTask completed.
Duration: 600ms

But I am getting

SubTask started.
SubTask ending.
SubTask completed.
Waiting for SubTask
Duration: 600ms

Why isn't the subTask run in parallel with main()?

Answer 1

withContext suspends until it is complete. Since you are passing the scope of the withContext to subTask() and subTask() uses it to launch its job, withContext will suspend until this other launched job completes. If subTask() used a different CoroutineScope , or if you had not wrapped your subTask() call in withContext , you would have had your expected behavior.