Testing coroutines

In this section, we will focus on how you can test your coroutines. You can create tests for classes such as ViewModel that launch a coroutine.

For coroutines launched using Dispatchers.Main, your unit tests will fail with the following error message:

java.lang.IllegalStateException: Module with the Main dispatcher had failed to initialize. For tests Dispatchers.setMain from kotlinx-coroutines-test module can be used

This exception happens because Dispatchers.Main uses Looper.getMainLooper(), the application’s main thread. This main looper is not available in Android for local unit tests. To make your tests work, you must use the Dispatchers.setMain extension function to change the Main dispatcher. For example, you can create a function in your test class that will run before your tests:

@Before

fun setUp() {

    Dispatchers.setMain(UnconfinedTestDispatcher())

}

The setUp function will run before the tests. It will...