Testing coroutines

Learn coroutine testing with runTest.

Testing coroutines with runTest

runTest is a test builder in the kotlinx-coroutines-test library. It opens a TestScope to test suspend functions and uses virtual time instead of waiting in real time.

Example: @Test fun testFetch() = runTest { val result = fetchUser(1); assertEquals("Alex", result) }. Inside the runTest block, suspend calls are written like normal functions; delay is skipped instantly. This makes coroutine tests fast and deterministic.

TestScope and delay skipping

runTest returns a TestScope. Inside this scope, suspend functions are called like normal functions. delay calls are skipped automatically; there is no real waiting.

The kotlinx-coroutines-test dependency is required. Import it with import kotlinx.coroutines.test.runTest. Use assertEquals to check expected values. This pattern keeps coroutine tests fast and predictable.

Exercise Test a suspend function with runTest

Define a suspend function fetchUser(id: Int): String that returns "Alex" when id is 1. In main open a runTest block and print the result of fetchUser(1) with println.

Show example solution

import kotlinx.coroutines.test.runTest

suspend fun fetchUser(id: Int): String {
    return "Alex"
}

fun main() = runTest {
    println(fetchUser(1))
}

Test yourself

Which of these can be done directly inside a runTest block?
- Only synchronous function calls
- suspend function calls
- Only UI rendering
- Only file reads
Show answer

suspend function calls — runTest provides a TestScope; suspend calls run directly.

Complete the runTest block

@Test
fun testFetch() = ____ {
    val result = fetchUser(1)
    assertEquals("Alex", result)
}

Show answer

@Test
fun testFetch() = runTest {
    val result = fetchUser(1)
    assertEquals("Alex", result)
}

Match the runTest parts

runTest { }

Test builder
TestScope

Provided scope
suspend fetch()

Called function
delay

Skipped wait

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