Flow builder and collect in Kotlin

Introduction

Flow builder helps create a stream of data that you can watch and react to. Collect lets you get each piece of data from that stream.

When you want to handle a series of values over time, like user clicks or sensor data.

When you need to process data that comes in chunks, like downloading a file in parts.

When you want to update your app UI as new data arrives without blocking the main thread.

Syntax

Kotlin

val flow = flow {
    emit(value) // send a value to the flow
}

flow.collect { value ->
    // handle each value here
}

flow {} is used to create a flow that emits values.

collect is a suspending function that receives each emitted value.

Examples

This flow emits three numbers one by one, and collect prints each.

Kotlin

val flow = flow {
    emit(1)
    emit(2)
    emit(3)
}

flow.collect { value ->
    println(value)
}

This flow emits multiples of 10 from 10 to 50, and collect prints each with a message.

Kotlin

val flow = flow {
    for (i in 1..5) {
        emit(i * 10)
    }
}

flow.collect { value ->
    println("Received: $value")
}

Sample Program

This program creates a flow that emits numbers 1 to 3. Then it collects and prints each number.

Kotlin

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

fun main() = runBlocking {
    val numbersFlow = flow {
        for (i in 1..3) {
            emit(i)
        }
    }

    numbersFlow.collect { number ->
        println("Number: $number")
    }
}

Important Notes

You must call collect inside a coroutine or suspend function.

Flow is cold, meaning it does not start emitting until you collect it.

Summary

Use flow {} to build a stream of data.

Use collect to receive and handle each emitted value.

Flows help manage data that arrives over time without blocking your app.

Practice

1. What is the primary purpose of the flow {} builder in Kotlin?

easy

A. To create a stream of values that can be emitted over time

B. To immediately execute a block of code and return a single value

C. To collect values from a list synchronously

D. To define a suspend function without any emissions

Solution

1. Step 1: Understand the role of flow {}

   The flow {} builder creates a stream that can emit multiple values asynchronously over time.

2. Step 2: Compare with other options

   Options B, C, and D describe synchronous or non-stream behaviors, which do not match flow {} usage.

3. Final Answer:

   To create a stream of values that can be emitted over time -> Option A

4. Quick Check:

   Flow builder = stream of values [OK]

Hint: Remember: flow builds streams, not single immediate values [OK]

Common Mistakes:

• Confusing flow with suspend functions returning single values
• Thinking flow collects values instead of emitting them
• Assuming flow is synchronous

2. Which of the following is the correct syntax to collect values from a flow named myFlow inside a suspend function?

easy

A. myFlow.collect(value -> println(value))

B. myFlow.collect { println(value) }

C. myFlow.collect { value -> println(value) }

D. myFlow.collect { value => println(value) }

Solution

1. Step 1: Identify correct lambda syntax in Kotlin

   Kotlin uses curly braces with parameter and arrow: { param -> action }.

2. Step 2: Match syntax with options

   myFlow.collect { value -> println(value) } correctly uses { value -> println(value) }. Options A lacks braces, B misses parameter declaration (value undefined), D uses wrong arrow symbol (=>).

3. Final Answer:

   myFlow.collect { value -> println(value) } -> Option C

4. Quick Check:

   Correct lambda syntax = myFlow.collect { value -> println(value) } [OK]

Hint: Use { param -> action } syntax for Kotlin lambdas [OK]

Common Mistakes:

• Omitting lambda parameter name
• Using Java-style arrows (->) incorrectly
• Using parentheses instead of curly braces

3. What will be the output of the following Kotlin code?

import kotlinx.coroutines.flow.*
suspend fun main() {
    val numbers = flow {
        emit(1)
        emit(2)
        emit(3)
    }
    numbers.collect { println(it * 2) }
}

medium

A. 1
2
3

B. 2
4
6

C. 3
6
9

D. Compilation error

Solution

1. Step 1: Analyze emitted values

   The flow emits 1, 2, and 3 sequentially.

2. Step 2: Understand the collect block

   Each emitted value is multiplied by 2 and printed, so outputs are 2, 4, and 6.

3. Final Answer:

   2 4 6 -> Option B

4. Quick Check:

   Emitted values * 2 = 2,4,6 [OK]

Hint: Multiply emitted values inside collect to get output [OK]

Common Mistakes:

• Printing original values without multiplication
• Expecting a list output instead of separate prints
• Assuming compilation error due to missing coroutine context

4. Identify the error in this Kotlin flow code snippet:

val flow = flow {
    emit(1)
    emit(2)
}
flow.collect { println(it) }

medium

A. collect should be called with parentheses

B. Incorrect flow builder syntax

C. emit cannot be called inside flow builder

D. Missing suspend modifier on the function calling collect

Solution

1. Step 1: Check flow and collect usage

   The flow builder and emit calls are correct.

2. Step 2: Understand collect context

   Collect is a suspend function and must be called from a suspend function or coroutine scope. Calling it directly without suspend context causes error.

3. Final Answer:

   Missing suspend modifier on the function calling collect -> Option D

4. Quick Check:

   collect requires suspend context [OK]

Hint: Remember: collect is suspend, needs suspend function or coroutine [OK]

Common Mistakes:

• Trying to call collect outside suspend or coroutine
• Confusing emit usage inside flow
• Thinking collect syntax is wrong

5. You want to create a flow that emits only even numbers from 1 to 5 and then collect and print them. Which code snippet correctly achieves this?

hard

A. val flow = flow {
for (i in 1..5) {
if (i % 2 == 0) emit(i)
}
}
suspend fun main() {
flow.collect { println(it) }
}

B. val flow = flowOf(1, 2, 3, 4, 5).filter { it % 2 == 0 }
fun main() {
flow.collect { println(it) }
}

C. val flow = flow {
emit(2)
emit(4)
}
flow.collect { println(it) }

D. val flow = flow {
for (i in 1..5) emit(i)
}
flow.collect { if (it % 2 == 0) println(it) }

Solution

1. Step 1: Check flow emission of even numbers

   val flow = flow { for (i in 1..5) { if (i % 2 == 0) emit(i) } } suspend fun main() { flow.collect { println(it) } } emits only even numbers inside the flow builder using a for loop and condition.

2. Step 2: Verify collection and printing

   val flow = flow { for (i in 1..5) { if (i % 2 == 0) emit(i) } } suspend fun main() { flow.collect { println(it) } } collects the flow inside a suspend function and prints each emitted value correctly.

3. Final Answer:

   val flow = flow { for (i in 1..5) { if (i % 2 == 0) emit(i) } } suspend fun main() { flow.collect { println(it) } } -> Option A

4. Quick Check:

   Emit filtered values inside flow, collect in suspend [OK]

Hint: Filter inside flow builder to emit only needed values [OK]

Common Mistakes:

• Calling collect outside suspend function
• Filtering after emitting all values instead of inside flow
• Not using suspend function for collect