57. Flows

A Flow represents an asynchronous stream of values produced over time. Unlike a channel, a flow is cold: the code inside flow { } does not run until you collect it, and it runs again for each collector. Note: requires the kotlinx-coroutines-core dependency.

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

fun numbers(): Flow<Int> = flow {
    for (i in 1..5) {
        delay(50)               // emit values over time, suspending in between
        emit(i)                 // hand a value to the collector
    }
}

fun main() = runBlocking {
    numbers()
        .filter { it % 2 == 1 } // keep odd numbers
        .map { it * it }        // square them
        .collect { println(it) }  // terminal operator: starts the flow
}

Intermediate operators like map and filter build a new flow lazily and declare what to do; nothing happens until a terminal operator such as collect runs. Each emitted value passes through the operator chain in order, so 1, 3, 5 are squared into 1, 9, 25.

Running it:

$ kotlin run
1
9
25

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