Exercise: Callback function wrappers
In the FetchTasksUseCase class, implement the following functions that should each fetch tasks using fetchTasks method from callbackUseCase, but return it using different types:
fetchTasks, which should return tasks or throw an exception in the case of an error.fetchTasksResult, which should returnResultrepresenting either a success or a failure.fetchTasksOrNull, which should return tasks ornullin the case of an error.
class FetchTasksUseCase(
private val callbackUseCase: FetchTasksCallbackUseCase
) {
@Throws(ApiException::class)
suspend fun fetchTasks(): List<Task> =
TODO()
suspend fun fetchTasksResult(): Result<List<Task>> =
TODO()
suspend fun fetchTasksOrNull(): List<Task>? =
TODO()
}
interface FetchTasksCallbackUseCase {
fun fetchTasks(
onSuccess: (List<Task>) -> Unit,
onError: (Throwable) -> Unit
): Cancellable
}
This problem can either be solved in the below playground or you can clone kotlin-exercises project and solve it locally. In the project, you can find code template for this exercise in coroutines/suspension/FetchTasksUseCase.kt. You can find there starting code and unit tests.
Once you are done with the exercise, you can check your solution here.
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> =
TODO()
suspend fun fetchTasksResult(): Result<List<Task>> =
TODO()
suspend fun fetchTasksOrNull(): List<Task>? =
TODO()
}
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() }
}
}
}
