为什么流程不执行，没有抛出错误？答案

【问题标题】：Why flow doesn't execute, no error thrown?为什么流程不执行，没有抛出错误？
【发布时间】：2020-06-04 21:31:57
【问题描述】：

GlobalScope 或自定义 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")
        }
    }
}

绝对没有输出或错误抛出

为什么？

更奇怪的是，当我添加 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

【问题讨论】：

标签： kotlin kotlin-flow kotlin-coroutines

【解决方案1】：

您启动的协程没有完成，因为main 函数在触发它们后立即返回而无需等待它们，从而完成您的应用程序。没有发生错误，因为这是预期的行为。

runBlocking 的 lambda 将不会返回，直到其中的所有协程都返回。在这种情况下，它恰好浪费了足够的时间让其他两个协程有时间完成，可能是因为它是最后启动的，并且做的工作量大致相同。

如果你在前两个中引入延迟，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

【讨论】：