Why do we need a Dispatcher as a rule to test coroutines

Description

• Dispatchers are not available for the test and androidTest packages so when testing the live data. There might be a possibility of failure.
• Dispatchers rely upon the Main looper and available only during the execution of production code.
• So we need to define a rule to handle the live data.

Code

@ExperimentalCoroutinesApi class MainCoroutineRule( private val dispatcher: CoroutineDispatcher = TestCoroutineDispatcher() ) : TestWatcher(), TestCoroutineScope by TestCoroutineScope(dispatcher) {

override fun starting(description: Description?) {
    super.starting(description)
    Dispatchers.setMain(dispatcher)
}

override fun finished(description: Description?) {
    super.finished(description)
    cleanupTestCoroutines()
    Dispatchers.resetMain()
}

}

• Here in the starting(..) method we replace our main dispatcher with the test dispatcher.
• In the finished(..) method before we complete we clean up the co-routines if any are running and replace the test dispatcher again with the main dispatcher.
• Usage of the rule

@ExperimentalCoroutinesApi
class ShoppingViewModelTest {

    @get:Rule
    var instantTaskExecutorRule = InstantTaskExecutorRule()

    @get:Rule
    var mainCoroutineRule = MainCoroutineRule()

    @Test
    fun `someTestCase`() {
      // Test case 
    }

}

• Rule will apply to all the test cases