ตัวดำเนินการ Flow: map, filter, transform และ take

import kotlinx.coroutines.*
import kotlinx.coroutines.flow.*
fun numbers(): Flow<Int> = flow {
    repeat(5) { emit(it + 1) }
}
fun main() = runBlocking {
    numbers().collect { println(it) }
}

import kotlinx.coroutines.*
import kotlinx.coroutines.flow.*
fun main() = runBlocking {
    (1..5).asFlow()
        .map { it * it }
        .collect { println(it) } // 1 4 9 16 25
}

import kotlinx.coroutines.*
import kotlinx.coroutines.flow.*
fun main() = runBlocking {
    (1..10).asFlow()
        .filter { it % 2 == 0 }
        .collect { println(it) } // 2 4 6 8 10
}

import kotlinx.coroutines.*
import kotlinx.coroutines.flow.*
fun main() = runBlocking {
    (1..3).asFlow()
        .transform { n ->
            emit("Item $n")
            emit("Done $n")
        }
        .collect { println(it) }
}

import kotlinx.coroutines.*
import kotlinx.coroutines.flow.*
fun main() = runBlocking {
    flow {
        repeat(100) { emit(it) }
    }.take(3)
     .collect { println(it) } // 0 1 2
}

import kotlinx.coroutines.*
import kotlinx.coroutines.flow.*
fun main() = runBlocking {
    (1..10).asFlow()
        .takeWhile { it < 5 }
        .collect { println(it) } // 1 2 3 4

    (1..10).asFlow()
        .dropWhile { it < 5 }
        .collect { println(it) } // 5 6 7 8 9 10
}

import kotlinx.coroutines.*
import kotlinx.coroutines.flow.*
fun main() = runBlocking {
    (1..20).asFlow()
        .filter { it % 3 == 0 }
        .map { it * 2 }
        .take(4)
        .collect { println(it) } // 6 12 18 24
}

import kotlinx.coroutines.*
import kotlinx.coroutines.flow.*
fun main() = runBlocking {
    flowOf("1", "two", "3", "four", "5")
        .mapNotNull { it.toIntOrNull() }
        .collect { println(it) } // 1 3 5
}

import kotlinx.coroutines.*
import kotlinx.coroutines.flow.*
fun main() = runBlocking {
    (1..3).asFlow()
        .flatMapConcat { n -> flowOf("$n-a", "$n-b") }
        .collect { println(it) }
}

import kotlinx.coroutines.*
import kotlinx.coroutines.flow.*
fun main() = runBlocking {
    (1..5).asFlow()
        .onEach { println("Processing: $it") }
        .filter { it % 2 == 0 }
        .collect { println("Collected: $it") }
}

import kotlinx.coroutines.*
import kotlinx.coroutines.flow.*
fun main() = runBlocking {
    val flow = (1..3).asFlow().map { println("map $it"); it * 2 }
    println("Before collect")
    flow.collect { println("got $it") }
    println("Second collect:")
    flow.collect { println("got $it") }  // runs again
}