flow builder
- Kotlin course
- Intermediate
- Lesson 89/116
- 5 min
Learn to create Flows with flow { }.
The flow { } builder
The flow { ... } builder lets you build a flow imperatively. You write normal Kotlin code in the body; emit(value) sends each value to the consumer.
fun counter(): Flow<Int> = flow {
for (i in 1..3) {
emit(i)
}
}Here the loop calls emit(i) on each turn, passing the value to collect. emit is a suspend function; it waits until the consumer is ready. This makes the flow sequential: the next emit runs only after the consumer finishes.
emit and sequential execution
A flow body runs sequentially with its consumer. On each emit call the flow pauses; the collect lambda runs, and when it finishes the flow continues. So emit and collect handshake.
flow {
emit("a")
println("body")
}.collect { println(it) }Output order: a, body. First emit("a") goes to collect, the lambda prints, then emit returns and the body continues. This is a consequence of the cold, sequential nature of flows.
Practice Build a number flow with the flow builder
Define fun counter(): Flow<Int> = flow { ... }. In the body use a for (i in 1..3) loop to emit(i) each value. Mark main with runBlocking; inside it call counter().collect { println(it) }.
Starter code
import kotlinx.coroutines.flow.*
import kotlinx.coroutines.runBlocking
// counter function: inside flow { } emit 1, 2, 3
// main: inside runBlocking, collect the counter flow and print
fun main() = runBlocking {
// write here
}Sample solution
fun counter(): Flow<Int> = flow {
for (i in 1..3) {
emit(i)
}
}
fun main() = runBlocking {
counter().collect { println(it) }
}Expected output
1 2 3
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Send a value
fun counter(): Flow<Int> = flow {
for (i in 1..3) {
____(i)
}
}fun counter(): Flow<Int> = flow {
for (i in 1..3) {
emit(i)
}
}Explanation: emit sends a value inside a flow body.
2 Match the flow builder parts
| Term | Meaning |
|---|---|
flow { } |
Flow builder |
emit(value) |
Sends a value |
for (i in 1..3) |
Loop |
Flow<Int> |
Return type |
3 Which function sends a value to the consumer inside a flow { } body?
- return
- yield
- send
- emit Correct answer
Explanation: emit is the suspend function that sends a value inside a flow body.