Flow builder and collect in Kotlin - Step-by-Step Execution

Concept Flow - Flow builder and collect

Start Flow Builder

↓

Emit values one by one

↓

Flow created

↓

Call collect on Flow

↓

Collect each emitted value

↓

Process or print value

↓

End when all values emitted

The flow builder creates a flow that emits values one by one. When collect is called, it receives each value and processes it until all are emitted.

Execution Sample

Kotlin

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

fun main() = runBlocking {
    val flow = flow {
        emit(1)
        emit(2)
        emit(3)
    }
    flow.collect { value ->
        println(value)
    }
}

This code creates a flow that emits numbers 1, 2, and 3, then collects and prints each number.

Execution Table

Step	Action	Flow State	Collected Value	Output
1	Start flow builder	Flow created, no values emitted yet	None	None
2	Emit 1	Flow emits 1	1	Print 1
3	Emit 2	Flow emits 2	2	Print 2
4	Emit 3	Flow emits 3	3	Print 3
5	Flow completes	No more values	None	Collection ends

💡 All values emitted and collected, flow completes

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
flow	empty	emitted 1	emitted 2	emitted 3	completed
value	none	1	2	3	none

Key Moments - 2 Insights

Why does collect receive values one by one instead of all at once?

Because the flow emits values sequentially and collect processes each emitted value immediately, as shown in execution_table rows 2 to 4.

What happens if we don't call collect on the flow?

The flow builder creates the flow but no values are emitted or processed until collect is called, so nothing happens (see execution_table step 1 vs steps 2-5).

Visual Quiz - 3 Questions

Look at the execution table, what is the collected value at step 3?

A 1

B 2

C 3

D None

💡 Hint

Check the 'Collected Value' column at step 3 in the execution_table.

At which step does the flow complete and stop emitting values?

A Step 5

B Step 3

C Step 4

D Step 2

💡 Hint

Look for the step where 'Flow completes' is noted in the 'Action' column.

If we remove the collect call, what changes in the execution?

A Values are emitted but not collected

B Flow emits and collects automatically

C No values are emitted or collected

D Values are collected but not emitted

💡 Hint

Refer to key_moments about what happens if collect is not called.

Concept Snapshot

Flow builder creates a flow that emits values sequentially.
Use emit(value) inside flow { } to send values.
Call collect { value -> ... } to receive and process each emitted value.
Without collect, flow does not emit values.
Collect runs suspending and processes values one by one.

Full Transcript

In Kotlin, a flow builder creates a flow that emits values one at a time using emit(). When you call collect on the flow, it receives each emitted value and processes it, for example by printing. The flow completes after all values are emitted. If you don't call collect, the flow does not emit any values. This step-by-step trace shows how values 1, 2, and 3 are emitted and collected in order.