Mock main thread handler for use in Android unit tests (requires Mockito and PowerMock)

TestHandler.kt

package <Whatever Package Ya Want Really>

import android.os.Handler
import io.mockk.every
import io.mockk.mockk
import java.util.concurrent.CountDownLatch

/**
 * TestHandler provides a way to execute tests that require a [Handler].
 *
 * The intended use of this class is to create a new instance of a [TestHandler] within the test
 * class. From there anywhere a [Handler] is needed, it should be provided through the [handler]
 * property. When running the test, it should be executed through the [runBlocking] function.
 *
 * @property handler provides the test handler which ensures that the any [Runnable] is executed
 * without any delay.
 */
class TestHandler {
    private val latch = CountDownLatch(2)

    val handler: Handler = mockk(relaxed = true) {
        every { post(any()) } answers {
            try {
                val runnable = invocation.args[0] as Runnable?
                runnable?.run()
            } finally {
                latch.countDown()
            }
            true
        }
    }

    fun runBlocking(block: () -> Unit) {
        try {
            block()
        } finally {
            latch.countDown()
        }
        latch.await()
    }
}