FlowOn for changing dispatcher in Kotlin - Mini Project: Build & Apply

Using FlowOn to Change Dispatcher in Kotlin Flow

📖 Scenario: You are building a simple Kotlin program that uses Flow to emit numbers. You want to change the thread where the flow runs using flowOn. This helps you run the flow on a background thread instead of the main thread.

🎯 Goal: Create a Kotlin Flow that emits numbers 1 to 3. Use flowOn to make the flow run on the Dispatchers.Default thread. Finally, collect and print the numbers on the main thread.

📋 What You'll Learn

Create a Flow that emits numbers 1, 2, and 3

Use flowOn(Dispatchers.Default) to change the dispatcher

Collect the flow and print each number

Use runBlocking to run the coroutine in main

💡 Why This Matters

🌍 Real World

Changing the dispatcher with flowOn is useful when you want to run heavy or blocking tasks on background threads without freezing the main UI thread.

💼 Career

Understanding flowOn is important for Kotlin developers working with asynchronous data streams and concurrency in Android or backend applications.

1

Create a Flow that emits numbers 1 to 3

Create a flow called numberFlow that emits the numbers 1, 2, and 3 using emit() inside the flow { } builder.

Kotlin

# Create a flow called numberFlow that emits 1, 2, and 3
# Your code here

Need a hint?

Use val numberFlow = flow { emit(1); emit(2); emit(3) } to create the flow.

2

Add flowOn to change dispatcher to Dispatchers.Default

Add flowOn(Dispatchers.Default) to numberFlow to make it run on the Default dispatcher. Assign the result back to numberFlow.

Kotlin

import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.flow.flowOn

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

# Add flowOn(Dispatchers.Default) to numberFlow below
# Your code here

Need a hint?

Use numberFlow = numberFlow.flowOn(Dispatchers.Default) or chain .flowOn(Dispatchers.Default) after the flow builder.

3

Collect the flow and print each number

Inside a runBlocking block, collect numberFlow using collect. For each emitted number, print it using println.

Kotlin

import kotlinx.coroutines.runBlocking
import kotlinx.coroutines.flow.collect

runBlocking {
    # Collect numberFlow and print each number
    # Your code here
}

Need a hint?

Use runBlocking { numberFlow.collect { number -> println(number) } } to collect and print.

4

Print the collected numbers as output

Run the program and observe the printed numbers 1, 2, and 3 each on its own line.

Kotlin

import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.flow.flow
import kotlinx.coroutines.flow.flowOn
import kotlinx.coroutines.runBlocking
import kotlinx.coroutines.flow.collect

val numberFlow = flow {
    emit(1)
    emit(2)
    emit(3)
}.flowOn(Dispatchers.Default)

runBlocking {
    numberFlow.collect { number ->
        println(number)
    }
}

# Run the program to see the output

Need a hint?

The program should print:
1
2
3