Flow

Learn the cold stream concept with Flow.

Flow: Cold stream

A Flow is a cold asynchronous stream in Kotlin that produces values over time. Cold means the flow does not produce anything until a consumer calls collect. This differs from hot channels, which produce whether or not anyone listens.

Flow lives in the kotlinx.coroutines.flow package. The simplest builder is flowOf(1, 2, 3), which creates a flow of fixed values. To consume a flow you call collect { ... }; the lambda runs for each value. collect is a suspend function, so it runs inside a coroutine context such as runBlocking.

The flowOf builder

flowOf is the simplest Flow builder: it creates a flow from given fixed values. To turn an Iterable or Sequence into a flow, use .asFlow().

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

fun numbers(): Flow<Int> = flowOf(1, 2, 3)

fun main() = runBlocking {
    numbers().collect { println(it)
    }
}

Output: 1, 2, 3 on separate lines. Because the flow is cold, calling numbers() alone produces nothing; collect triggers the values to be emitted in order.

Exercise Define a simple Flow and consume it

Define fun numbers(): Flow<Int> = flowOf(1, 2, 3). Mark main with runBlocking; inside it call numbers().collect { println(it) }. The output prints each value on its own line.

Show example solution

fun numbers(): Flow<Int> = flowOf(1, 2, 3)

fun main() = runBlocking {
    numbers().collect { println(it) }
}

Test yourself

What kind of stream is a Flow?
- A synchronous list
- A hot broadcast channel
- A cold asynchronous stream of values
- A function returning a single value
Show answer

A cold asynchronous stream of values — A Flow is a cold asynchronous stream; it produces when collect is called.

Complete the return type

fun numbers(): ____ = flowOf(1, 2, 3)

Show answer

fun numbers(): Flow<Int> = flowOf(1, 2, 3)

Match the Flow parts

flow

Stream
cold

Cold (collect triggers)
collect

Consumer
flowOf

Fixed-value builder

Next lesson: flow builder

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