07. Testing Error Handling

In this step you'll test your application reacts appropriately when there's an error (such as data being unavailable from the network).

Step 1: Add an error flag to test double

1. Open test > data > source > FakeTestRepository.
2. Add a boolean flag called shouldReturnError and set it initially to false, which means that by default an error is not returned.

FakeTestRepository.kt

private var shouldReturnError = false

3. Create a setReturnError method that changes whether or not the repository should return errors:

FakeTestRepository.kt

fun setReturnError(value: Boolean) {
    shouldReturnError = value
}

4. Wrap getTask and getTasks in if statements so that if shouldReturnError is true, the method returns Result.Error:

FakeTestRepository.kt

override suspend fun getTask(taskId: String, forceUpdate: Boolean): Result<Task> {
    if (shouldReturnError) {
        return Error(Exception("Test exception"))
    }
    tasksServiceData[taskId]?.let {
        return Success(it)
    }
    return Error(Exception("Could not find task"))
}

override suspend fun getTasks(forceUpdate: Boolean): Result<List<Task>> {
    if (shouldReturnError) {
        return Error(Exception("Test exception"))
    }
    return Success(tasksServiceData.values.toList())
}

Step 2: Write a test for returned Error

In StatisticsViewModel, there are two LiveData booleans (error and empty):

StatisticsViewModel.kt

class StatisticsViewModel(
    private val tasksRepository: TasksRepository
) : ViewModel() {

    private val tasks: LiveData<Result<List<Task>>> = tasksRepository.observeTasks()

    // Other variables...    

    val error: LiveData<Boolean> = tasks.map { it is Error }
    val empty: LiveData<Boolean> = tasks.map { (it as? Success)?.data.isNullOrEmpty() }

    // Rest of the code...    
}

These represent whether or not tasks loaded properly. If there is an error, error and empty should both be true.

1. Open StatisticsViewModelTest.
2. Create a new test called loadStatisticsWhenTasksAreUnavailable_callErrorToDisplay
3. Call setReturnErrors() on tasksRepository, setting it to true.
4. Call statisticsViewModel.refresh() to trigger a data load.
5. Check that statisticsViewModel.empty and statisticsViewModel.error are both true.

The full code for this test is below:

StatisticsViewModelTest.kt

@Test
fun loadStatisticsWhenTasksAreUnavailable_callErrorToDisplay() {
    // Make the repository return errors.
    tasksRepository.setReturnError(true)
    statisticsViewModel.refresh()

    // Then empty and error are true (which triggers an error message to be shown).
    assertThat(statisticsViewModel.empty.getOrAwaitValue(), `is`(true))
    assertThat(statisticsViewModel.error.getOrAwaitValue(), `is`(true))
}

5. Run your test and observe it passing!