[코루틴] CoroutineContext

CoroutineContext란?

public fun CoroutineScope.launch(
    context: CoroutineContext = EmptyCoroutineContext,
    start: CoroutineStart = CoroutineStart.DEFAULT,
    block: suspend CoroutineScope.() -> Unit
): Job

public fun <T> CoroutineScope.async(
    context: CoroutineContext = EmptyCoroutineContext,
    start: CoroutineStart = CoroutineStart.DEFAULT,
    block: suspend CoroutineScope.() -> T
): Deferred<T>

 

코루틴 빌더의 첫 번째 변수로 넣어주던 객체이다.

 

CoroutineContext

- 코루틴을 실행하는 실행 환경을 설정하고 관리하는 Interface이다.

- Element들의 집합이다. 각 Element는 고유한 key를 가진다.

@SinceKotlin("1.3")
public interface CoroutineContext {
    // key에 따른 context의 Element 반환
    public operator fun <E : Element> get(key: Key<E>): E?
    // 초기값을 시작으로 주어진 람다 함수를 이용하여 Context Element들을 병합한 후 결과 반환
    public fun <R> fold(initial: R, operation: (R, Element) -> R): R
    // 현재 Context와 주어진 다른 Context가 가지는 Element들을 모두 포함하는 CoroutineContext 반환
    public operator fun plus(context: CoroutineContext): CoroutineContext =
        if (context === EmptyCoroutineContext) this else // fast path -- avoid lambda creation
            context.fold(this) { acc, element ->
                ...
            }
    // Context에서 해당 key를 갖는 Element들을 제외한 새로운 context 반환
    public fun minusKey(key: Key<*>): CoroutineContext
    public interface Key<E : Element>
    
    public interface Element : CoroutineContext {
        public val key: Key<*>
        public override operator fun <E : Element> get(key: Key<E>): E? =
            @Suppress("UNCHECKED_CAST")
            if (this.key == key) this as E else null
        public override fun <R> fold(initial: R, operation: (R, Element) -> R): R =
            operation(initial, this)
        public override fun minusKey(key: Key<*>): CoroutineContext =
            if (this.key == key) EmptyCoroutineContext else this
    }
}

 

 

CoroutineContext 구성 요소

크게 Job, CoroutineDispatcher, CoroutineName, CoroutineExceptionHandler로 이루어져 있다.

- Job : 코루틴의 추상체로 코루틴을 조작

- CoroutineDispatcher : 코루틴을 스레드에 할당

- CoroutineName : 코루틴의 이름

- CoroutineExceptionHandler : 코루틴에서 발생한 예외 처리

 

 

CoroutineContext 구성하기

Key-value로 구성 요소 관리

각 Element는 고유한 key를 가지며 중복된 값은 허용되지 않는다.

키	값
CoroutineName key	CoroutineName 객체
CoroutineDispatcher key	CoroutineDispatcher 객체
Job key	Job 객체
CoroutineExceptionHandler key	CoroutineExceptionHandler 객체

 

 

Element 추가

키에 값을 직접 대입하지 않고 + 연산자를 사용해 CoroutineContext 객체 구성

val coroutineContext: CoroutineContext = newSingleThreadContext("MyThread") + CoroutineName("MyCoroutine")

// 사용
fun main() = runBlocking<Unit> {
    val coroutineContext: CoroutineContext = newSingleThreadContext("MyThread") + CoroutineName("MyCoroutine")
    launch(context = coroutineContext) {
        println("[${Thread.currentThread().name}, ${currentCoroutineContext()[CoroutineName]} 실행]")
    }
}
// 결과
[MyThread, CoroutineName(MyCoroutine) 실행]

키	값
CoroutineName key	CoroutineName("MyCoroutine")
CoroutineDispatcher key	newSingleThreadContext("My Thread")
Job key	X
CoroutineExceptionHandler key	X

 

구성 요소가 없는 CoroutineContext는 EmptyCoroutineContext 사용

 

 

Element 덮어씌우기

같은 Element가 둘 이상 더해지면 나중에 추가된 Element가 이전의 값을 덮어씌운다.

fun main() = runBlocking<Unit> {
    val coroutineContext: CoroutineContext = newSingleThreadContext("MyThread") + CoroutineName("MyCoroutine")
    val newCoroutineContext: CoroutineContext = coroutineContext + CoroutineName("NewCoroutine")
    launch(context = newCoroutineContext) {
        println("[${Thread.currentThread().name}, ${currentCoroutineContext()[CoroutineName]} 실행]")
    }
}
// 결과
[MyThread, CoroutineName(NewCoroutine) 실행]

 

 

여러 Element로 이루어진 CoroutineContext 합치기

동일한 key를 가진 Element가 있다면 나중에 들어온 값이 선택된다.

fun main() = runBlocking<Unit> {
    val coroutineContext1: CoroutineContext = CoroutineName("MyCoroutine1") + newSingleThreadContext("MyThread1")
    val coroutineContext2: CoroutineContext = CoroutineName("MyCoroutine2") + newSingleThreadContext("MyThread2")
    val combinedContext = coroutineContext1 + coroutineContext2
    launch(context = combinedContext) {
        println("[${Thread.currentThread().name}, ${currentCoroutineContext()[CoroutineName]} 실행]")
    }
}
// 결과
[MyThread2, CoroutineName(MyCoroutine2) 실행]

 

 

CoroutineContext에 Job 생성해 추가하기

val myJob = Job()
val coroutineContext: CoroutineContext = Dispatchers.IO + myJob

키	값
CoroutineName key	X
CoroutineDispatcher key	Dispatchers.IO
Job key	myJob
CoroutineExceptionHandler key	X

 

하지만, Job 객체를 추가하면 코루틴의 구조화가 깨지기 때문에 주의가 필요하다.

 

 

CoroutineContext Element에 접근하는 방법

구성 요소에 접근하기 위해서는 key가 필요하다.

 

CoroutineContext Element 키

일반적으로 Element는 내부에 키를 싱글톤 객체로 구현한다.

public data class CoroutineName(
    val name: String
) : AbstractCoroutineContextElement(CoroutineName) {
    public companion object Key : CoroutineContext.Key<CoroutineName>
    
    override fun toString(): String = "CoroutineName($name)"
}

public interface CoroutineExceptionHandler : CoroutineContext.Element {
    public companion object Key : CoroutineContext.Key<CoroutineExceptionHandler>
    public fun handleException(context: CoroutineContext, exception: Throwable)
}

public abstract class CoroutineDispatcher :
    AbstractCoroutineContextElement(ContinuationInterceptor), ContinuationInterceptor {

    // 아직 실험중인 API
    @ExperimentalStdlibApi
    public companion object Key : AbstractCoroutineContextKey<ContinuationInterceptor, CoroutineDispatcher>(
        ContinuationInterceptor,
        { it as? CoroutineDispatcher })
    ...
}

public interface Job : CoroutineContext.Element {
    public companion object Key : CoroutineContext.Key<Job>
}

 

 

키를 사용해 CoroutineContext Element에 접근

// Singleton Key 사용
fun main() = runBlocking<Unit> {
    val coroutineContext = CoroutineName("MyCoroutine") + Dispatchers.IO
    val nameFromContext = coroutineContext[CoroutineName.Key]
    println(nameFromContext)
}
// 결과
CoroutineName(MyCoroutine)

// Element 자체를 Key로 사용. 자동으로 CoroutineName.Key를 사용해 연산 처리
fun main() = runBlocking<Unit> {
    val coroutineContext = CoroutineName("MyCoroutine") + Dispatchers.IO
    val nameFromContext = coroutineContext[CoroutineName]
    println(nameFromContext)
}
// 결과
CoroutineName(MyCoroutine)

// Element의 key 프로퍼티를 사용해 접근
fun main() = runBlocking<Unit> {
    val coroutineName = CoroutineName("MyCoroutine")
    val dispatcher: CoroutineDispatcher = Dispatchers.IO
    val coroutineContext = coroutineName + dispatcher
    println(coroutineContext[coroutineName.key])
    println(coroutineContext[dispatcher.key])
}
// 결과
CoroutineName(MyCoroutine)
Dispatchers.IO

 

각 Element는 같은 Key 객체를 공유한다.

fun main() = runBlocking<Unit> {
    val coroutineName = CoroutineName("MyCoroutine")
    
    if (coroutineName.key === CoroutineName.Key) {
        println("coroutineName.key와 CoroutineName.Key는 동일하다")
    }
}
// 결과
coroutineName.key와 CoroutineName.Key는 동일하다

 

 

CoroutineContext Element 제거

minusKey 함수 사용

@OptIn(ExperimentalStdlibApi::class)
fun main() = runBlocking<Unit> {
    val coroutineName = CoroutineName("MyCoroutine")
    val dispatcher = Dispatchers.IO
    val myJob = Job()
    val coroutineContext: CoroutineContext = coroutineName + dispatcher + myJob
    println("${coroutineContext[CoroutineName]}, ${coroutineContext[CoroutineDispatcher]}, ${coroutineContext[Job]}")
    
    val deletedCoroutineContext = coroutineContext.minusKey(CoroutineName)
    println("${deletedCoroutineContext[CoroutineName]}, ${deletedCoroutineContext[CoroutineDispatcher]}, ${deletedCoroutineContext[Job]}")
}
// 결과
CoroutineName(MyCoroutine), Dispatchers.IO, JobImpl{Active}@a7e666
null, Dispatchers.IO, JobImpl{Active}@a7e666

 

주의할 점

minusKey 함수는 새로운 CoroutineContext 반환.

@OptIn(ExperimentalStdlibApi::class)
fun main() = runBlocking<Unit> {
    val coroutineName = CoroutineName("MyCoroutine")
    val dispatcher = Dispatchers.IO
    val myJob = Job()
    val coroutineContext: CoroutineContext = coroutineName + dispatcher + myJob
    val deletedCoroutineContext = coroutineContext.minusKey(CoroutineName)
    println("${coroutineContext[CoroutineName]}, ${coroutineContext[CoroutineDispatcher]}, ${coroutineContext[Job]}")
}
// 결과
CoroutineName(MyCoroutine), Dispatchers.IO, JobImpl{Active}@a7e666