[Coroutine&Flow] 14장 플로우 플래트

flatMapConcat

플로우에서는 3가지 유형의 flatmap을 자원하고 있다. flatMapConcat, flatMapMerge, flatMapLatest이다.

flatMapConcat 은 첫번째 요소에 대해서 플레트닝을 하고 두번째 요소를 합한다.

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

fun requestFlow(i: Int): Flow<String> = flow {
    emit("$i: First") 
    delay(500) // wait 500 ms
    emit("$i: Second")    
}

fun main() = runBlocking<Unit> { 
    val startTime = System.currentTimeMillis() // remember the start time 
    (1..3).asFlow().onEach { delay(100) } // a number every 100 ms 
        .flatMapConcat {
            requestFlow(it)
        }                                                                           
        .collect { value -> // collect and print 
            println("$value at ${System.currentTimeMillis() - startTime} ms from start") 
        } 
}

1: First at 135 ms from start
1: Second at 637 ms from start
2: First at 738 ms from start
2: Second at 1238 ms from start
3: First at 1339 ms from start
3: Second at 1839 ms from start

flatMapMerge

flatMapMerge는 첫 요소의 플레트닝을 시작하며 이어 다음 요소의 플레트닝을 시작한다.

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

fun requestFlow(i: Int): Flow<String> = flow {
    emit("$i: First") 
    delay(500) // wait 500 ms
    emit("$i: Second")    
}

fun main() = runBlocking<Unit> { 
    val startTime = System.currentTimeMillis() // remember the start time 
    (1..3).asFlow().onEach { delay(100) } // a number every 100 ms 
        .flatMapMerge {
             requestFlow(it) 
        }                                                                           
        .collect { value -> // collect and print 
            println("$value at ${System.currentTimeMillis() - startTime} ms from start") 
        } 
}

1: First at 170 ms from start
2: First at 266 ms from start
3: First at 367 ms from start
1: Second at 671 ms from start
2: Second at 766 ms from start
3: Second at 869 ms from start

flatMapLatest

flatMapLatest는 다음 요소의 플레트닝을 시작하며 이전에 진행 중이던 플레트닝을 취소한다.

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

fun requestFlow(i: Int): Flow<String> = flow {
    emit("$i: First") 
    delay(500) // wait 500 ms
    emit("$i: Second")    
}

fun main() = runBlocking<Unit> { 
    val startTime = System.currentTimeMillis() // remember the start time 
    (1..3).asFlow().onEach { delay(100) } // a number every 100 ms 
        .flatMapLatest {
            requestFlow(it) 
        }                                                                           
        .collect { value -> // collect and print 
            println("$value at ${System.currentTimeMillis() - startTime} ms from start") 
        } 
}

1: First at 145 ms from start
2: First at 281 ms from start
3: First at 382 ms from start
3: Second at 883 ms from start