Unresolved reference: async in Kotlin in 1.3

问题

I have multi module kotlin gradle project in github here.

One of my sub project introducing-coroutines with build file build.gradle.kts file is here

The contents of build.gradle.kts is -

    import org.jetbrains.kotlin.gradle.dsl.Coroutines
    import org.jetbrains.kotlin.gradle.tasks.KotlinCompile

    plugins {
        java
        kotlin("jvm") version "1.3.11"
    }

    group = "chapter2"
    version = "1.0-SNAPSHOT"

    repositories {
        mavenCentral()
    }

    dependencies {
        compile(kotlin("stdlib-jdk8"))
        compile("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.1.0")
        testCompile("junit", "junit", "4.12")
    }

    configure<JavaPluginConvention> {
        sourceCompatibility = JavaVersion.VERSION_1_8
        targetCompatibility = JavaVersion.VERSION_1_8
    }

    tasks.withType<KotlinCompile> {
        kotlinOptions.jvmTarget = "1.8"
    }

    kotlin {
        experimental {
            coroutines   = Coroutines.ENABLE
        }
    }

I'm trying to create my first coroutine program from this link.

import kotlinx.coroutines.*
import kotlinx.coroutines.async
import kotlin.system.*
import kotlin.system.measureTimeMillis

suspend  fun computecr(array: IntArray, low: Int, high: Int): Long {
    return if (high - low <= SEQUENTIAL_THRESHOLD) {
        (low until high)
                .map { array[it].toLong() }
                .sum()
    } else {
        val mid = low + (high - low) / 2
        val left = async { computecr(array, low, mid) }
        val right = compute(array, mid, high)
        return left.await() + right
    }
}

When I compile the program I get the below error -

e: /Users/rajkumar.natarajan/Documents/Coding/coroutines-demo/introducing-coroutines/src/main/kotlin/SumUsingCoroutines.kt: (15, 20): Unresolved reference: async
> Task :introducing-coroutines:compileKotlin FAILED

FAILURE: Build failed with an exception.

I can import import kotlinx.coroutines.async without any issue but not sure why I'm getting this error.

I have already verified similar issue here and added anko-commons dependency here

How can I resolve this error?

回答1:

First of all you have to remove from Gradle the part where you enable the experimental coroutine feature.

kotlin {
    experimental {
        coroutines   = Coroutines.ENABLE
    }
}

You cannot use async() function implicitly anymore. You have to call it explicitly for a global scope coroutine GlobalScope.async(){...} or you can call it from another coroutine scope using CoroutineScope(...).async{...} or from scoping functions coroutineScope {...}, withContext(...){...}.

I have written an example for personal use to understand myself how coroutines are working. I hope it is good and helpful.

回答2:

The problem is that async (the same as launch) is defined as an extension function on CoroutineScope. In the following example, it is invoked in the receiver CoroutineScope of withContext:

suspend  fun computecr(array: IntArray, low: Int, high: Int): Long {
    return if (high - low <= SEQUENTIAL_THRESHOLD) {
        (low until high)
            .map { array[it].toLong() }
            .sum()
    } else {
        withContext(Default) {
            val mid = low + (high - low) / 2
            val left = async { computecr(array, low, mid) }
            val right = computecr(array, mid, high)
            left.await() + right
        }
    }
}

来源：https://stackoverflow.com/questions/54283140/unresolved-reference-async-in-kotlin-in-1-3