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📖 17.1 플로우 연산자로 플로우 조작

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📖 17.2 중간 연산자는 업스트림 플로우에 적용되고 다운스트림 플로우를 반환한다

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🔖 17.2.1 업스트림 원소별로 임의의 값을 배출: transform 함수

\n\n fun main() {\n val names = flow {\n emit("Jo")\n emit("May")\n emit("Sue")\n }\n val uppercasedNames = names.map {\n it.uppercase()\n }\n runBlocking {\n uppercasedNames.collect { println("$it ") }\n }\n}\n \n\n\n fun main() {\n val names = flow {\n emit("Jo")\n emit("May")\n emit("Sue")\n }\n val upperAndLowercasedNames = names.transform {\n emit(it.uppercase())\n emit(it.lowercase())\n }\n runBlocking {\n upperAndLowercasedNames.collect { println("$it ") }\n }\n}\n \n\n

🔖 17.2.2 take나 관련 연산자는 플로우를 취소할 수 있다

\n\n\n fun main() {\n val temps = getTemperatures()\n temps\n .take(5)\n .collect {\n log(it)\n }\n}\n \n\n

🔖 17.2.3 플로우의 각 단계 후킹: onStart, onEach, onCompletion, onEmpty

\n\n fun main() {\n val temps = getTemperatures()\n temps\n .take(5)\n .onCompletion { cause ->\n if (cause != null) {\n println("An error occurred! $cause")\n } else {\n println("Completed!")\n }\n }\n .collect {\n println(it)\n }\n}\n \n\n\n fun main() {\n flow\n .onEmpty {\n println("Nothing - emitting default value!")\n emit(0)\n }\n .onStart {\n println("Starting")\n }\n .onEach {\n println("On $it")\n }\n .onCompletion {\n println("Done!")\n }\n .collect()\n}\n \n\n

🔖 17.2.4 다운스트림 연산자와 수집자를 위한 원소 버퍼링: buffer 연산자

\n\n fun getAllUserIds(): Flow<Int> {\n return flow {\n repeat(3) {\n delay(200.milliseconds)\n log("Emitting!")\n emit(it)\n }\n }\n}\n\nsuspend fun getProfileFromNetwork(id: Int): String {\n delay(2.seconds)\n return "Profile[$id]"\n}\n\nfun main() {\n val ids = getAllUserIds()\n runBlocking {\n ids\n .map { getProfileFromNetwork(it) }\n .collect { log("Got $it") }\n }\n}\n \n\n\n 0 [main @coroutine#1] Emitting!\n2028 [main @coroutine#1] Got Profile[0]\n2234 [main @coroutine#1] Emitting!\n4240 [main @coroutine#1] Got Profile[1]\n4447 [main @coroutine#1] Emitting!\n6453 [main @coroutine#1] Got Profile[2]\n \n\n\n fun main() {\n val ids = getAllUserIds()\n runBlocking {\n ids\n .buffer(3)\n .map { getProfileFromNetwork(it) }\n .collect { log("Got $it") }\n }\n}\n \n\n 0 [main @coroutine#2] Emitting!\n213 [main @coroutine#2] Emitting!\n414 [main @coroutine#2] Emitting!\n2030 [main @coroutine#1] Got Profile[0]\n4040 [main @coroutine#1] Got Profile[1]\n6046 [main @coroutine#1] Got Profile[2]\n \n\n

🔖 17.2.5 중간값을 버리는 연산자: conflate 연산자

\n\n fun main() {\n runBlocking {\n val temps = getTemperatures()\n temps\n .onEach {\n log("Read $it from sensor")\n }\n .conflate()\n .collect { \n log("Collected $it")\n delay(1.seconds)\n }\n }\n}\n \n\n 142709 [main @coroutine#1] Collected 6\n142864 [main @coroutine#2] Read 28 from sensor\n143370 [main @coroutine#2] Read 24 from sensor\n143711 [main @coroutine#1] Collected 24\n143876 [main @coroutine#2] Read -2 from sensor\n144382 [main @coroutine#2] Read 1 from sensor\n144715 [main @coroutine#1] Collected 1\n144888 [main @coroutine#2] Read 1 from sensor\n145391 [main @coroutine#2] Read 29 from sensor\n145719 [main @coroutine#1] Collected 29\n145893 [main @coroutine#2] Read 9 from sensor\n146396 [main @coroutine#2] Read 1 from sensor\n146725 [main @coroutine#1] Collected 1\n \n\n

🔖 17.2.6 일정 시간 동안 값을 필터링하는 연산자: debounce 연산자

\n\n val searchQuery = flow {\n emit("K")\n delay(100.milliseconds)\n emit("Ko")\n delay(200.milliseconds)\n emit("Kotl")\n delay(500.milliseconds)\n emit("Kotlin")\n}\n \n\n\n fun main() = runBlocking {\n searchQuery\n .debounce(250.milliseconds)\n .collect {\n log("Searching for $it")\n }\n}\n \n\n 0 [main @coroutine#1] Searching for Kotl\n247 [main @coroutine#1] Searching for Kotlin\n \n\n

🔖 17.2.7 플로우가 실행되는 코루틴 콘텍스트를 바꾸기: flowOn 연산자

\n\n fun main() {\n runBlocking {\n flowOf(1)\n .onEach { log("A") }\n .flowOn(Dispatchers.Default)\n .onEach { log("B") }\n .flowOn(Dispatchers.IO)\n .onEach { log("C") }\n .collect()\n }\n}\n \n\n 0 [DefaultDispatcher-worker-3 @coroutine#3] A\n24 [DefaultDispatcher-worker-1 @coroutine#2] B\n26 [main @coroutine#1] C\n \n\n

📖 17.3 커스텀 중간 연산자 만들기

\n\n\n fun Flow<Double>.averageOfLast(n: Int): Flow<Double> =\n flow {\n val numbers = mutableListOf<Double>()\n collect { \n if (numbers.size > n) {\n numbers.removeFirst()\n }\n numbers.add(it)\n emit(numbers.average())\n }\n }\n\nfun main() = runBlocking {\n flowOf(1.0, 2.0, 30.0, 121.0)\n .averageOfLast(3)\n .collect {\n print("$it ")\n }\n}\n \n\n

📖 17.4 최종 연산자는 업스트림 플로우를 실행하고 값을 계산한다

\n\n\n fun main() = runBlocking {\n getTemperatures()\n .onEach { \n log(it)\n }\n .collect()\n}\n \n\n\n fun main() = runBlocking {\n getTemperatures()\n .first()\n}\n \n\n

🔖 17.4.1 프레임워크는 커스텀 연산자를 제공한다

\n\n @Composable fun TemperatureDisplay(temps: Flow<Int>) {\n val temperature = temps.collectAsState(null)\n Box {\n temperature.value?.let {\n Text("The current temperature is $it!")\n }\n }\n}\n \n","excerpt":"📖 17.1 플로우 연산자로 플로우 조작 시퀀스와 마찬가지로 플로우도 중간 연산자와 최종 연산자를 구분 중간 연산자는 코드를 실행하지 않고 변경된 플로우를 반환하며, 최종 연산자는 컬렉션, 개별 원소, 계산된 값을 반환하거나 아무 값도 반환하지 않으면서 플로우를 수집하고 실제 코드를 실행한다. 📖 17.…","fields":{"slug":"/backend/kotlin-in-action/17장-플로우_연산자/"},"frontmatter":{"title":"Kotlin in Action - 17장 플로우 연산자","thumbnail":{"childImageSharp":{"fluid":{"src":"/static/f7f7ddfa31d1614405dc5af1487b9ec4/9c94d/kotlin-in-action.png"}}},"draft":false,"category":"Back-End","tags":["Kotlin"],"date":"June 29, 2025"}},"previous":null,"node":{"id":"674cdded-b7f4-544a-8575-a58074fcfdbe","html":"

📖 18.1 코루틴 내부에서 던져진 오류 처리

\n\n\n fun main(): Unit = runBlocking {\n try {\n launch {\n throw UnsupportedOperationException("Ouch!")\n }\n } catch (u: UnsupportedOperationException) {\n println("Handled $u")\n }\n}\n \n\n\n fun main(): Unit = runBlocking {\n launch {\n try {\n throw UnsupportedOperationException("Ouch!")\n } catch (u: UnsupportedOperationException) {\n println("Handled $u")\n }\n }\n}\n \n\n\n fun main(): Unit = runBlocking {\n val myDeferredInt: Deferred<Int> = async {\n throw UnsupportedOperationException("Ouch!")\n }\n try {\n val i: Int = myDeferredInt.await()\n println(i)\n } catch (u: UnsupportedOperationException) {\n println("Handled: $u")\n }\n}\n \n\n

📖 18.2 코틀린 코루틴에서의 오류 전파

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🔖 18.2.1 자식이 실패하면 모든 자식을 취소하는 코루틴

\n\n fun main(): Unit = runBlocking {\n launch {\n try {\n while (true) {\n println("Heartbeat!")\n delay(500.milliseconds)\n }\n } catch (e: Exception) {\n println("Heartbeat terminated: $e")\n throw e\n }\n }\n launch {\n delay(1.seconds)\n throw UnsupportedOperationException("Ow!")\n }\n}\n \n\n Heartbeat!\nHeartbeat!\nHeartbeat terminated: kotlinx.coroutines.JobCancellationException: Parent job is Cancelling; job="coroutine#1":BlockingCoroutine{Cancelling}@130f889\nException in thread "main" java.lang.UnsupportedOperationException: Ow!\n \n\n

🔖 18.2.2 구조적 동시성은 코루틴 스코프를 넘는 예외에만 영향을 미친다

\n\n fun main(): Unit = runBlocking {\n launch {\n try {\n while (true) {\n println("Heartbeat!")\n delay(500.milliseconds)\n }\n } catch (e: Exception) {\n println("Heartbeat terminated: $e")\n throw e\n }\n }\n launch {\n try {\n delay(1.seconds)\n throw UnsupportedOperationException("Ow!")\n } catch (u: UnsupportedOperationException) {\n println("Caught $u")\n }\n }\n}\n \n\n

🔖 18.2.3 슈퍼바이저는 부모와 형제가 취소되지 않게 한다

\n\n fun main(): Unit = runBlocking {\n supervisorScope {\n launch {\n try {\n while (true) {\n println("Heartbeat!")\n delay(500.milliseconds)\n }\n } catch (e: Exception) {\n println("Heartbeat terminated: $e")\n throw e\n }\n }\n launch {\n delay(1.seconds)\n throw UnsupportedOperationException("Ow!")\n }\n }\n}\n \n\n

📖 18.3 CoroutineExceptionHandler: 예외 처리를 위한 마지막 수단

\n\n\n fun main(): Unit = runBlocking {\n val supervisor = ComponentWithScope()\n supervisor.action()\n delay(1.seconds)\n}\n\nclass ComponentWithScope(dispatcher: CoroutineDispatcher = Dispatchers.Default) {\n private val exceptionHandler = CoroutineExceptionHandler { _, e ->\n println("[ERROR] ${e.message}")\n }\n private val scope = CoroutineScope(\n SupervisorJob() + dispatcher + exceptionHandler\n )\n\n fun action() = scope.launch {\n throw UnsupportedOperationException("Ouch!")\n }\n}\n \n\n\n private val topLevelHandler = CoroutineExceptionHandler { _, e ->\n println("[TOP] ${e.message}")\n}\n\nprivate val intermediateHandler = CoroutineExceptionHandler { _, e ->\n println("[INTERMEDIATE] ${e.message}")\n}\n\n@OptIn(DelicateCoroutinesApi::class)\nfun main() {\n GlobalScope.launch(topLevelHandler) {\n launch(intermediateHandler) {\n throw UnsupportedOperationException("Ouch!")\n }\n }\n Thread.sleep(1000)\n}\n \n\n

🔖 18.3.1 CoroutineExceptionHandler를 launch와 async에 적용할 때의 차이점

\n\n class ComponentWithScope(dispatcher: CoroutineDispatcher = Dispatchers.Default) {\n private val exceptionHandler = CoroutineExceptionHandler { _, e ->\n println("[ERROR] ${e.message}")\n }\n private val scope = CoroutineScope(\n SupervisorJob() + dispatcher + exceptionHandler\n )\n\n fun action() = scope.launch {\n async {\n throw UnsupportedOperationException("Ouch!")\n }\n }\n}\n\nfun main() {\n val supervisor = ComponentWithScope()\n supervisor.action()\n delay(1.seconds)\n}\n \n\n

📖 18.4 플로우에서 예외 처리

\n\n class UnhappyFlowException : Exception()\n\nval exceptionalFlow = flow {\n repeat(5) { number ->\n emit(number)\n }\n throw UnhappyFlowException()\n}\n \n\n

🔖 18.4.1 catch 연산자로 업스트림 예외 처리

\n\n fun main() = runBlocking {\n exceptionalFlow\n .catch { cause ->\n println("\\nHandled: $cause")\n emit(-1)\n }\n .collect {\n println("$it ")\n }\n}\n \n\n\n fun main() = runBlocking {\n exceptionalFlow\n .map { \n it + 1\n }\n .catch { cause ->\n println("\\nHandled: $cause")\n }\n .onEach { \n throw UnhappyFlowException()\n }\n .collect()\n}\n \n\n

🔖 18.4.2 술어가 참일 때 플로우의 수집 재시도: retry 연산자

\n\n val unreliableFlow = flow {\n println("Starting the flow!")\n repeat(10) { number ->\n if (Random.nextDouble() < 0.1) throw CommunicationException()\n emit(number)\n }\n}\n\nfun main() = runBlocking {\n unreliableFlow\n .retry(5) { cause ->\n println("\\nHandled: $cause")\n cause is CommunicationException\n }\n .collect { number ->\n println("$number")\n }\n}\n \n\n

📖 18.5 코루틴과 플로우 테스트

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🔖 18.5.1 코루틴을 사용하는 테스트를 빠르게 만들기: 가상 시간과 테스트 디스패처

\n\n class PlaygroundTest {\n\n @Test\n fun testDelay() = runTest {\n val startTime = System.currentTimeMillis()\n delay(20.seconds)\n println(System.currentTimeMillis() - startTime)\n }\n}\n \n\n\n @OptIn(ExperimentalCoroutinesApi::class)\n@Test\nfun testDelay() = runTest {\n var x = 0\n launch {\n delay(500.milliseconds)\n x++\n }\n launch {\n delay(1.seconds)\n x++\n }\n println(currentTime)\n\n delay(600.milliseconds)\n assertEquals(1, x)\n println(currentTime)\n\n delay(500.milliseconds)\n assertEquals(2, x)\n println(currentTime)\n}\n \n\n\n @OptIn(ExperimentalCoroutinesApi::class)\n@Test\nfun testDelay() = runTest {\n var x = 0\n launch { \n x++\n launch { \n x++\n }\n }\n launch { \n delay(200.milliseconds)\n x++\n }\n runCurrent()\n assertEquals(2, x)\n advanceUntilIdle()\n assertEquals(3, x)\n}\n \n\n

🔖 18.5.2 터빈으로 플로우 테스트

\n\n val myFlow = flow { \n emit(1)\n emit(2)\n emit(3)\n}\n\n@Test\nfun doTest() = runTest { \n val results = myFlow.toList()\n assertEquals(3, results.size)\n}\n \n\n\n @Test\nfun doTest() = runTest { \n val results = myFlow.test {\n assertEquals(1, awaitItem())\n assertEquals(2, awaitItem())\n assertEquals(3, awaitItem())\n awaitComplete()\n }\n}\n \n","excerpt":"📖 18.1 코루틴 내부에서 던져진 오류 처리 일시 중단 함수나 코루틴 빌더 안에 작성한 코드도 예외를 발생시킬 수 있다. 코루틴 빌더는 실행할 새로운 코루틴을 생성하는데, 이 새로운 코루틴에서 발생한 예외는 catch 블록에 의해 잡히지 않는다 위 코드는 예외가 잡히지 않는다. launch에 전달되는 람다 블록안에서는 잡힌다. await를 감싼 try-catch에서 예외를 잡지만 동시에 예외를 출력한다. 자식 코루틴은 잡히지 않은 예외를 항상 부모 코루틴에 전파한다. 📖 18.…","fields":{"slug":"/backend/kotlin-in-action/18장-오류_처리와_테스트/"},"frontmatter":{"title":"Kotlin in Action - 18장 오류 처리와 테스트","thumbnail":{"childImageSharp":{"fluid":{"src":"/static/f7f7ddfa31d1614405dc5af1487b9ec4/9c94d/kotlin-in-action.png"}}},"draft":false,"category":"Back-End","tags":["Kotlin"],"date":"July 06, 2025"}}}},"staticQueryHashes":["2374173507","2996537568","3691437124"]}