CoroutineUseCaseTest.kt

import android.util.Log
import com.novonordisk.shared.MainCoroutineRule
import io.mockk.every
import io.mockk.mockkStatic
import io.mockk.unmockkAll
import kotlinx.coroutines.CoroutineDispatcher
import kotlinx.coroutines.test.runTest
import org.junit.After
import org.junit.Before
import org.junit.Rule
import org.junit.Test

class CoroutineUseCaseTest {

    // Overrides Dispatchers.Main used in Coroutines
    @get:Rule
    var coroutineRule = MainCoroutineRule()

    private val testDispatcher = coroutineRule.testDispatcher

    @Before
    fun setUp() {
        mockkStatic(Log::class)
        every { Log.d(any(), any()) } returns 0
    }

    @After
    fun teardown() {
        unmockkAll()
    }

    @Test
    fun `UseCase exception returns failure`() = runTest {
        val useCase = ExceptionUseCase(testDispatcher)
        val result = useCase(Unit)
        assert(result.isFailure)
    }

    private class ExceptionUseCase(dispatcher: CoroutineDispatcher) : UseCase<Unit, Unit>(dispatcher) {
        override suspend fun execute(parameters: Unit) {
            throw Exception("Test exception")
        }
    }

}

UseCase.kt

import android.util.Log
import kotlinx.coroutines.CoroutineDispatcher
import kotlinx.coroutines.withContext


/**
 * Executes business logic synchronously or asynchronously using Coroutines.
 */
abstract class UseCase<in P, R>(private val coroutineDispatcher: CoroutineDispatcher) {

    internal open val TAG: String = "UseCase"

    /** Executes the use case asynchronously and returns a [Result].
     *
     * @return a [Result].
     *
     * @param params the input parameters to run the use case with
     */
    suspend operator fun invoke(params: P): Result<R> {
        return try {
            // Moving all use case's executions to the injected dispatcher
            // In production code, this is usually the Default dispatcher (background thread)
            // In tests, this becomes a TestCoroutineDispatcher
            withContext(coroutineDispatcher) {
                execute(params).let {
                    Result.success(it)
                }
            }
        } catch (e: Exception) {
            Log.d(TAG, "invoke: ", e)
            Result.failure(e)
        }
    }

    /**
     * Override this to set the code to be executed.
     */
    @Throws(RuntimeException::class)
    protected abstract suspend fun execute(parameters: P): R
}