kotlin - 为什么流程不执行，没有引发错误？

Question

GlobalScope或自定义的CoroutineScope实例都不起作用:

import kotlinx.coroutines.*
import kotlinx.coroutines.flow.*

fun makeFlow() = flow {
    println("sending first value")
    emit(1)
    println("first value collected, sending another value")
    emit(2)
    println("second value collected, sending a third value")
    emit(3)
    println("done")
}

@InternalCoroutinesApi
fun main() {
    val someScope = CoroutineScope(Dispatchers.Default)
    someScope.launch {
        makeFlow().collect { value ->
            println("value is $value")
        }
    }
    GlobalScope.launch {
        makeFlow().collect { value ->
            println("value is $value")
        }
    }
}

绝对没有输出或抛出错误

为什么？

甚至更陌生，当我添加一个runBlocking{}块时，一切都会执行:

val someScope = CoroutineScope(Dispatchers.Default)
    someScope.launch {
        makeFlow().collect { value ->
            println("someScope: value is $value")
        }
    }
    GlobalScope.launch {
        makeFlow().collect { value ->
            println("GlobalScope: value is $value")
        }
    }
    runBlocking {
        makeFlow().collect { value ->
            println("runBlocking: value is $value")
        }
    }

输出:

sending first value
sending first value
sending first value
runBlocking: value is 1
GlobalScope: value is 1
first value collected, sending another value
GlobalScope: value is 2
second value collected, sending a third value
GlobalScope: value is 3
done
someScope: value is 1
first value collected, sending another value
someScope: value is 2
second value collected, sending a third value
someScope: value is 3
done
first value collected, sending another value
runBlocking: value is 2
second value collected, sending a third value
runBlocking: value is 3
done

Answer 1

您启动的协程并没有完成，因为main函数在触发它们后立即返回而无需等待它们，从而完成了您的应用程序。没有发生任何错误，因为它是预期的行为。
runBlocking的lambda不会返回，直到其中的所有协程返回为止。在这种情况下，它恰好浪费了足够的时间来让其他两个协程有时间完成，可能是因为它是最后启动的，并且做的工作量差不多。

如果您对前两个延迟，则他们将没有机会完成:

fun main() {
    val someScope = CoroutineScope(Dispatchers.Default)
    someScope.launch {
        delay(100L)
        makeFlow().collect {
            makeFlow().collect { value ->
                println("value is $value")
            }
        }
    }
    GlobalScope.launch {
        delay(100L)
        makeFlow().collect { value ->
            println("value is $value")
        }
    }
    runBlocking {
        makeFlow().collect { value ->
            println("runBlocking: value is $value")
        }
    }
}

sending first value
runBlocking: value is 1
first value collected, sending another value
runBlocking: value is 2
second value collected, sending a third value
runBlocking: value is 3
done