Testing coroutines with runTest in Kotlin - Mini Project: Build & Apply

Testing coroutines with runTest

📖 Scenario: You are writing a simple Kotlin function that uses coroutines to simulate a delayed greeting. You want to test this function properly using runTest from the Kotlin coroutines testing library.

🎯 Goal: Build a coroutine function that returns a greeting after a delay, then write a test using runTest to verify it returns the expected greeting.

📋 What You'll Learn

Create a suspend function called delayedGreeting that returns a String after a 100ms delay

Use delay(100) inside the function to simulate work

Write a test function called testDelayedGreeting using runTest

Inside the test, call delayedGreeting and assert the returned value is exactly "Hello, Coroutine!"

Print the test result string to the console

💡 Why This Matters

🌍 Real World

Testing coroutine functions is important in Android apps and backend services where asynchronous code is common.

💼 Career

Knowing how to test coroutines with <code>runTest</code> is a valuable skill for Kotlin developers working on reliable, maintainable asynchronous code.

1

Create the suspend function with delay

Write a suspend function called delayedGreeting that returns a String. Inside it, use delay(100) to pause for 100 milliseconds, then return the string "Hello, Coroutine!".

Kotlin

# Your code here

Need a hint?

Use suspend fun to define a coroutine function. Use delay(100) to pause inside the coroutine.

2

Set up the test function with runTest

Add a test function called testDelayedGreeting. Inside it, use runTest from kotlinx.coroutines.test to run coroutine code. For now, just call runTest { } with an empty block.

Kotlin

import kotlinx.coroutines.test.runTest

// Your code here

Need a hint?

Import runTest and create a function that calls it with a coroutine block.

3

Call delayedGreeting and check the result

Inside the runTest block in testDelayedGreeting, call delayedGreeting() and store the result in a variable called result. Then check if result equals "Hello, Coroutine!" using an if statement.

Kotlin

fun testDelayedGreeting() = runTest {
    // Call delayedGreeting and check result
    // Your code here
}

Need a hint?

Store the call result in result. Use if to compare and print the test outcome.

4

Run the test and print the output

Call the testDelayedGreeting() function in the main program to run the test and print the result to the console.

Kotlin

fun main() {
    // Call the test function here
    // Your code here
}

Need a hint?

Call testDelayedGreeting() inside main() to run the test and print the result.