Coroutine testing

The runBlocking coroutine builder can be used for testing. This creates a coroutine that uses a current thread. The test within the JUnit framework may look as follows:

class ExampleUnitTest {

    @Test
    fun comicLoading() = runBlocking {
        val image = async { loadImage() }.await()
        assertNotNull(image)
    }
}

This snippet loads an image using the async coroutine builder, and checks that the image is not null. The source code of the runBlocking function looks as follows:

@Throws(InterruptedException::class)
public fun <T> runBlocking(context: CoroutineContext = EmptyCoroutineContext, block: suspend CoroutineScope.() -> T): T {
    val currentThread = Thread.currentThread()
    val contextInterceptor = context[ContinuationInterceptor]
    val privateEventLoop = contextInterceptor == null // create private event loop if no dispatcher is specified
    val eventLoop = if (privateEventLoop) BlockingEventLoop(currentThread) else contextInterceptor as? EventLoop
    val newContext = GlobalScope.newCoroutineContext(
        if (privateEventLoop) context + (eventLoop as ContinuationInterceptor) else context
    )
    val coroutine = BlockingCoroutine<T>(newContext, currentThread, eventLoop, privateEventLoop)
    coroutine.start(CoroutineStart.DEFAULT, coroutine, block)
    return coroutine.joinBlocking()
}

As you can see, the runBlocking coroutine builder uses the currentThread function to obtain an instance of the Thread class. When you run this test, you will see the following window:

This window shows that the test has passed successfully.