How does the use of functions runBlockingTest, runTest, and runBlocking differ in the context of testing asynchronous code in Kotlin?

In Kotlin, various functions are used for testing asynchronous code that help manage coroutines in tests:

• runBlocking — launches a coroutine and blocks the current thread until it completes. Used for running coroutines in regular code and tests, but not optimal for testing as it blocks the thread.

• runBlockingTest — an outdated function from the kotlinx-coroutines-test library, which runs a coroutine in a test context with control over virtual time (timers). It allowed testing delays and timeouts without real waiting. It has now been replaced by runTest.

• runTest — the modern way to test coroutines, providing a test context with virtual time. It allows running asynchronous code, controlling delays and timeouts, speeding up tests, and making them deterministic.

Example of using runTest:

import kotlinx.coroutines.test.runTest
import kotlinx.coroutines.delay
import kotlin.test.Test
import kotlin.test.assertEquals

class ExampleTest {
    @Test
    fun testDelay() = runTest {
        var result = 0
        val job = launch {
            delay(1000)
            result = 42
        }
        advanceTimeBy(1000) // advance virtual time
        job.join()
        assertEquals(42, result)
    }
}

Thus, for modern asynchronous code testing, it is recommended to use runTest, as it is more flexible and efficient.