Solution: Callback function wrappers
class FetchTasksUseCase(
private val callbackUseCase: FetchTasksCallbackUseCase
) {
@Throws(ApiException::class)
suspend fun fetchTasks(): List<Task> =
suspendCancellableCoroutine { cont ->
val cancellable = callbackUseCase.fetchTasks(
onSuccess = { cont.resume(it) },
onError = { cont.resumeWithException(it) }
)
cont.invokeOnCancellation { cancellable.cancel() }
}
suspend fun fetchTasksResult(): Result<List<Task>> =
suspendCancellableCoroutine { cont ->
val cancellable = callbackUseCase.fetchTasks(
onSuccess = { cont.resume(Result.success(it)) },
onError = { cont.resume(Result.failure(it)) }
)
cont.invokeOnCancellation { cancellable.cancel() }
}
suspend fun fetchTasksOrNull(): List<Task>? =
suspendCancellableCoroutine { cont ->
val cancellable = callbackUseCase.fetchTasks(
onSuccess = { cont.resume(it) },
onError = { cont.resume(null) }
)
cont.invokeOnCancellation { cancellable.cancel() }
}
}
Example solution in playground
import kotlinx.coroutines.launch
import kotlinx.coroutines.suspendCancellableCoroutine
import kotlinx.coroutines.test.runCurrent
import kotlinx.coroutines.test.runTest
import org.junit.Test
import kotlin.coroutines.resume
import kotlin.coroutines.resumeWithException
import kotlin.test.assertEquals
import kotlin.test.assertNotNull
import kotlin.test.assertTrue
class FetchTasksUseCase(
private val callbackUseCase: FetchTasksCallbackUseCase
) {
@Throws(ApiException::class)
suspend fun fetchTasks(): List<Task> =
suspendCancellableCoroutine { cont ->
val cancellable = callbackUseCase.fetchTasks(
onSuccess = { cont.resume(it) },
onError = { cont.resumeWithException(it) }
)
cont.invokeOnCancellation { cancellable.cancel() }
}
suspend fun fetchTasksResult(): Result<List<Task>> =
suspendCancellableCoroutine { cont ->
val cancellable = callbackUseCase.fetchTasks(
onSuccess = { cont.resume(Result.success(it)) },
onError = { cont.resume(Result.failure(it)) }
)
cont.invokeOnCancellation { cancellable.cancel() }
}
suspend fun fetchTasksOrNull(): List<Task>? =
suspendCancellableCoroutine { cont ->
val cancellable = callbackUseCase.fetchTasks(
onSuccess = { cont.resume(it) },
onError = { cont.resume(null) }
)
cont.invokeOnCancellation { cancellable.cancel() }
}
}
interface FetchTasksCallbackUseCase {
fun fetchTasks(
onSuccess: (List<Task>) -> Unit,
onError: (Throwable) -> Unit
): Cancellable
}
fun interface Cancellable {
fun cancel()
}
data class Task(val name: String, val priority: Int)
class ApiException(val code: Int, message: String): Throwable(message)
class FetchTasksTests {
val someTasks = listOf(Task("1", 123), Task("2", 456))
val someException = ApiException(500, "Some exception")
@Test
fun `fetchTasks should resume with result`() = runTest {
// given
val fakeFetchTaskCallback = FakeFetchTasksCallbackUseCase()
val useCase = FetchTasksUseCase(fakeFetchTaskCallback)
var result: List<Task>? = null
// when
launch {
result = useCase.fetchTasks()
}
// then
runCurrent()
assertEquals(null, result)
fakeFetchTaskCallback.onSuccess?.invoke(someTasks)
runCurrent()
assertEquals(someTasks, result)
}
@Test
fun `fetchTasks should resume with exception`() = runTest {
// given
val fakeFetchTaskCallback = FakeFetchTasksCallbackUseCase()
val useCase = FetchTasksUseCase(fakeFetchTaskCallback)
var exception: Throwable? = null
// when
launch {
try {
useCase.fetchTasks()
} catch (e: Throwable) {
exception = e
}
}
// then
runCurrent()
assertEquals(null, exception)
fakeFetchTaskCallback.onError?.invoke(someException)
runCurrent()
assertEquals(someException, exception)
}
@Test
fun `fetchTasks should support cancellation`() = runTest {
// given
val fakeFetchTaskCallback = FakeFetchTasksCallbackUseCase()
val useCase = FetchTasksUseCase(fakeFetchTaskCallback)
var cancelled = false
fakeFetchTaskCallback.onCancelled = { cancelled = true }
// when
val job = launch {
useCase.fetchTasks()
}
// then
runCurrent()
assertEquals(false, cancelled)
job.cancel()
assertEquals(true, cancelled)
}
@Test
fun `fetchTasksResult should resume with result`() = runTest {
// given
val fakeFetchTaskCallback = FakeFetchTasksCallbackUseCase()
val useCase = FetchTasksUseCase(fakeFetchTaskCallback)
var result: Result<List<Task>>? = null
// when
launch {
result = useCase.fetchTasksResult()
}
// then
runCurrent()
assertEquals(null, result)
fakeFetchTaskCallback.onSuccess?.invoke(someTasks)
runCurrent()
assertNotNull(result)
assertTrue(result!!.isSuccess)
assertEquals(someTasks, result!!.getOrNull())
}
@Test
fun `fetchTasksResult should resume with failure`() = runTest {
// given
val fakeFetchTaskCallback = FakeFetchTasksCallbackUseCase()
val useCase = FetchTasksUseCase(fakeFetchTaskCallback)
var result: Result<List<Task>>? = null
// when
launch {
result = useCase.fetchTasksResult()
}
// then
runCurrent()
assertEquals(null, result)
fakeFetchTaskCallback.onError?.invoke(someException)
runCurrent()
assertNotNull(result)
assertTrue(result!!.isFailure)
assertEquals(someException, result!!.exceptionOrNull())
}
@Test
fun `fetchTasksResult should support cancellation`() = runTest {
// given
val fakeFetchTaskCallback = FakeFetchTasksCallbackUseCase()
val useCase = FetchTasksUseCase(fakeFetchTaskCallback)
var cancelled = false
fakeFetchTaskCallback.onCancelled = { cancelled = true }
// when
val job = launch {
useCase.fetchTasksResult()
}
// then
runCurrent()
assertEquals(false, cancelled)
job.cancel()
assertEquals(true, cancelled)
}
@Test
fun `fetchTasksOrNull should resume with result`() = runTest {
// given
val fakeFetchTaskCallback = FakeFetchTasksCallbackUseCase()
val useCase = FetchTasksUseCase(fakeFetchTaskCallback)
val NO_VALUE = Any()
var result: Any? = NO_VALUE
// when
launch {
result = useCase.fetchTasksOrNull()
}
// then
runCurrent()
assertEquals(NO_VALUE, result)
fakeFetchTaskCallback.onSuccess?.invoke(someTasks)
runCurrent()
assertEquals(someTasks, result)
}
@Test
fun `fetchTasksOrNull should resume with failure`() = runTest {
// given
val fakeFetchTaskCallback = FakeFetchTasksCallbackUseCase()
val useCase = FetchTasksUseCase(fakeFetchTaskCallback)
val NO_VALUE = Any()
var result: Any? = NO_VALUE
// when
launch {
result = useCase.fetchTasksOrNull()
}
// then
runCurrent()
assertEquals(NO_VALUE, result)
fakeFetchTaskCallback.onError?.invoke(someException)
runCurrent()
assertEquals(null, result)
}
@Test
fun `fetchTasksOrNull should support cancellation`() = runTest {
// given
val fakeFetchTaskCallback = FakeFetchTasksCallbackUseCase()
val useCase = FetchTasksUseCase(fakeFetchTaskCallback)
var cancelled = false
fakeFetchTaskCallback.onCancelled = { cancelled = true }
// when
val job = launch {
useCase.fetchTasksOrNull()
}
// then
runCurrent()
assertEquals(false, cancelled)
job.cancel()
assertEquals(true, cancelled)
}
class FakeFetchTasksCallbackUseCase: FetchTasksCallbackUseCase {
var onSuccess: ((List<Task>) -> Unit)? = null
var onError: ((Throwable) -> Unit)? = null
var onCancelled: (()->Unit)? = null
override fun fetchTasks(onSuccess: (List<Task>) -> Unit, onError: (Throwable) -> Unit): Cancellable {
this.onSuccess = onSuccess
this.onError = onError
return Cancellable { onCancelled?.invoke() }
}
}
}
