Kotlin Coroutine Launch 在另一个启动中被顺序调用

Question

我有 2 个通过启动调用的挂起方法，这两个方法都通过启动创建了更多的协程。 我看到结果是连续的。 如果我删除第二级启动，那么我会看到这些操作是并行执行的。 有人可以解释为什么这不是以异步/并行方式运行吗？

private suspend fun coroutineA() = coroutineScope {
    println("[${printThreadInfo()}] Starting coroutineA()")
    (1..10).forEach {
        launch {
            delay(1000)
            println("[${printThreadInfo()}] A$it")
        }
    }
}

private suspend fun coroutineB() = coroutineScope {
    println("[${printThreadInfo()}] Starting coroutineB()")
    (1..10).forEach {
        launch {
            delay(1000)
            println("[${printThreadInfo()}] B$it")
        }
    }
}


// -Dkotlinx.coroutines.debug
fun main() {
    runBlocking {
        launch {
            println("[${printThreadInfo()}] Launching coroutineA")
            coroutineA()
        }
        launch {
            println("[${printThreadInfo()}] Launching coroutineB")
            coroutineB()
        }
    }
}

我看到的结果是：

[2020-11-20T07:53:17.749 Thread=main] Start of main()
[2020-11-20T07:53:17.859 Thread=main @coroutine#1] Launching launchMultipleCoroutines
[2020-11-20T07:53:17.866 Thread=main @coroutine#2] Launching coroutineA
[2020-11-20T07:53:17.868 Thread=main @coroutine#2] Starting coroutineA()
[2020-11-20T07:53:17.871 Thread=main @coroutine#3] Launching coroutineB
[2020-11-20T07:53:17.872 Thread=main @coroutine#3] Starting coroutineB()
[2020-11-20T07:53:18.878 Thread=main @coroutine#4] A1
[2020-11-20T07:53:18.878 Thread=main @coroutine#5] A2
[2020-11-20T07:53:18.878 Thread=main @coroutine#6] A3
[2020-11-20T07:53:18.879 Thread=main @coroutine#7] A4
[2020-11-20T07:53:18.879 Thread=main @coroutine#8] A5
[2020-11-20T07:53:18.879 Thread=main @coroutine#9] A6
[2020-11-20T07:53:18.879 Thread=main @coroutine#10] A7
[2020-11-20T07:53:18.879 Thread=main @coroutine#11] A8
[2020-11-20T07:53:18.880 Thread=main @coroutine#12] A9
[2020-11-20T07:53:18.880 Thread=main @coroutine#13] A10
[2020-11-20T07:53:18.880 Thread=main @coroutine#14] B1
[2020-11-20T07:53:18.880 Thread=main @coroutine#15] B2
[2020-11-20T07:53:18.880 Thread=main @coroutine#16] B3
[2020-11-20T07:53:18.880 Thread=main @coroutine#17] B4
[2020-11-20T07:53:18.881 Thread=main @coroutine#18] B5
[2020-11-20T07:53:18.881 Thread=main @coroutine#19] B6
[2020-11-20T07:53:18.881 Thread=main @coroutine#20] B7
[2020-11-20T07:53:18.881 Thread=main @coroutine#21] B8
[2020-11-20T07:53:18.881 Thread=main @coroutine#22] B9
[2020-11-20T07:53:18.881 Thread=main @coroutine#23] B10

而如果我删除 coroutineA 和 coroutineB 中的 launch()，我会看到结果看起来像是并行执行的：

[2020-11-20T08:02:49.356 Thread=main @coroutine#2] Launching coroutineA
[2020-11-20T08:02:49.407 Thread=main @coroutine#2] Starting coroutineA()
[2020-11-20T08:02:49.415 Thread=main @coroutine#3] Launching coroutineB
[2020-11-20T08:02:49.415 Thread=main @coroutine#3] Starting coroutineB()
[2020-11-20T08:02:49.512 Thread=main @coroutine#2] A1
[2020-11-20T08:02:49.516 Thread=main @coroutine#3] B1
[2020-11-20T08:02:49.612 Thread=main @coroutine#2] A2
[2020-11-20T08:02:49.617 Thread=main @coroutine#3] B2
[2020-11-20T08:02:49.716 Thread=main @coroutine#2] A3
[2020-11-20T08:02:49.718 Thread=main @coroutine#3] B3
[2020-11-20T08:02:49.817 Thread=main @coroutine#2] A4
[2020-11-20T08:02:49.819 Thread=main @coroutine#3] B4
[2020-11-20T08:02:49.921 Thread=main @coroutine#2] A5
...

Answer 1

发现我们需要使用launch(Dispatchers.Default)来实现协程的并行运行。 显示上下文有助于缩小范围。

[2020-11-20T08:47:47.858 Thread=main @coroutine#2] [[CoroutineId(2), "coroutine#2":StandaloneCoroutine{Active}@3d04a311, BlockingEventLoop@7a46a697]] Launching coroutineA
[2020-11-20T08:47:47.912 Thread=main @coroutine#2] [[CoroutineId(2), "coroutine#2":ScopeCoroutine{Active}@57fa26b7, BlockingEventLoop@7a46a697]] Starting coroutineA()
[2020-11-20T08:47:47.921 Thread=main @coroutine#3] Launching coroutineB
[2020-11-20T08:47:47.922 Thread=main @coroutine#3] [[CoroutineId(3), "coroutine#3":ScopeCoroutine{Active}@44e81672, BlockingEventLoop@7a46a697]] Starting coroutineB()
[2020-11-20T08:47:48.929 Thread=DefaultDispatcher-worker-6 @coroutine#17] [[CoroutineId(17), "coroutine#17":StandaloneCoroutine{Active}@7e782624, Dispatchers.Default]] B4
[2020-11-20T08:47:48.929 Thread=DefaultDispatcher-worker-5 @coroutine#4] [[CoroutineId(4), "coroutine#4":StandaloneCoroutine{Active}@28a58b8f, Dispatchers.Default]] A1

添加调度程序类型从 blocksEvenLoop 更改为独立协程。