Exercise: mapAsync
Practice shows that many projects require asynchronous mapping of elements in a collection. To avoid repeating this pattern, implement an mapAsync function, which should map all elements in a collection asynchronously.
suspend fun <T, R> Iterable<T>.mapAsync(
transformation: suspend (T) -> R
): List<R> = TODO()
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/recipes/MapAsync.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.*
import kotlinx.coroutines.test.currentTime
import kotlinx.coroutines.test.runTest
import org.junit.Test
import kotlin.coroutines.CoroutineContext
import kotlin.test.assertEquals
import kotlin.test.assertTrue
suspend fun <T, R> Iterable<T>.mapAsync(
transformation: suspend (T) -> R
): List<R> = TODO()
class MapAsyncTest {
@Test
fun should_behave_like_a_regular_map_for_a_list_and_a_set() = runTest {
val list = ('a'..'z').toList()
assertEquals(list.map { c -> c.inc() }, list.mapAsync { c -> c.inc() })
assertEquals(list.map { c -> c.code }, list.mapAsync { c -> c.code })
assertEquals(list.map { c -> c.uppercaseChar() }, list.mapAsync { c -> c.uppercaseChar() })
val set = (1..10).toSet()
assertEquals(set.map { i -> i * i }, set.mapAsync { i -> i * i })
assertEquals(set.map { i -> "A$i" }, set.mapAsync { i -> "A$i" })
assertEquals(set.map { i -> i.toChar() }, set.mapAsync { i -> i.toChar() })
}
@Test
fun should_map_async_and_keep_elements_order() = runTest {
val transforms: List<suspend () -> String> = listOf(
{ delay(3000); "A" },
{ delay(2000); "B" },
{ delay(4000); "C" },
{ delay(1000); "D" },
)
val res = transforms.mapAsync { it() }
assertEquals(listOf("A", "B", "C", "D"), res)
assertEquals(4000, currentTime)
}
@Test
fun should_support_context_propagation() = runTest {
var ctx: CoroutineContext? = null
val name1 = CoroutineName("Name 1")
withContext(name1) {
listOf("A").mapAsync {
ctx = currentCoroutineContext()
it
}
}
assertEquals(name1, ctx?.get(CoroutineName))
val name2 = CoroutineName("Some name 2")
withContext(name2) {
listOf("B").mapAsync {
ctx = currentCoroutineContext()
it
}
}
assertEquals(name2, ctx?.get(CoroutineName))
}
@Test
fun should_propagate_exceptions_from_transformation_and_cancel_other_transformations() = runTest {
// given
val e = object : Throwable() {}
val bodies = listOf(
suspend { "A" },
suspend { delay(1000); "B" },
suspend { delay(500); throw e },
suspend { "C" }
)
val jobs = mutableListOf<CoroutineContext>()
// when
val result = runCatching {
bodies.mapAsync {
jobs += currentCoroutineContext()
it()
}
}
// then should propagate exception
assertTrue(result.isFailure)
assertEquals(e, result.exceptionOrNull())
without waiting for slower transformations
assertEquals(500, currentTime)
// and cancel slower transformations
assert(jobs.all { it.job.isCompleted })
}
}
