Solution: Continuation storage
var continuation: Continuation<String>? = null
suspend fun continuationSteal(console: Console) {
console.println("Before")
val result = suspendCancellableCoroutine {
continuation = it
}
console.println(result)
console.println("After")
}
Example solution in playground
import kotlinx.coroutines.delay
import kotlinx.coroutines.launch
import kotlinx.coroutines.runBlocking
import kotlinx.coroutines.suspendCancellableCoroutine
import kotlinx.coroutines.test.UnconfinedTestDispatcher
import kotlinx.coroutines.test.runTest
import org.junit.Test
import kotlin.coroutines.Continuation
import kotlin.coroutines.resume
import kotlin.test.assertEquals
import kotlin.test.assertNotNull
var continuation: Continuation<String>? = null
suspend fun continuationSteal(console: Console) {
console.println("Before")
val result = suspendCancellableCoroutine {
continuation = it
}
console.println(result)
console.println("After")
}
interface Console {
fun println(text: Any?)
}
fun main(): Unit = runBlocking<Unit> {
launch {
continuationSteal(object : Console {
override fun println(text: Any?) {
kotlin.io.println(text)
}
})
}
delay(1000)
continuation?.resume("This is some text")
}
// Before
// (1 sec)
// This is some text
// After
@Suppress("FunctionName")
class ContinuationStealTests {
private val fakeText = "This is some text"
class FakeConsole : Console {
val printed = mutableListOf<Any?>()
override fun println(text: Any?) {
printed += text
}
}
@Test
fun `At the beginning function says Before`() = runTest(UnconfinedTestDispatcher()) {
val fakeConsole = FakeConsole()
val job = launch {
continuationSteal(fakeConsole)
}
delay(100)
assertEquals("Before", fakeConsole.printed.first())
job.cancel()
}
@Test
fun `At the end function says After`() = runTest(UnconfinedTestDispatcher()) {
val fakeConsole = FakeConsole()
val job = launch {
continuationSteal(fakeConsole)
}
continuation?.resume(fakeText)
assertEquals("After", fakeConsole.printed.last())
}
@Test
fun `In the middle, we suspend function`() = runTest(UnconfinedTestDispatcher()) {
val fakeConsole = FakeConsole()
val job = launch {
continuationSteal(fakeConsole)
}
assertEquals(mutableListOf<Any?>("Before"), fakeConsole.printed)
job.cancel()
}
@Test
fun `Function should return continuation`() = runTest(UnconfinedTestDispatcher()) {
val fakeConsole = FakeConsole()
launch {
continuationSteal(fakeConsole)
}
assertNotNull(continuation).resume(fakeText)
assertEquals("After", fakeConsole.printed.last())
}
@Test
fun `Only Before is printed before resume`() = runTest(UnconfinedTestDispatcher()) {
val fakeConsole = FakeConsole()
val job = launch {
continuationSteal(fakeConsole)
}
assertEquals("Before", fakeConsole.printed.first())
job.cancel()
}
@Test
fun `After resume function should print text to resume`() = runTest(UnconfinedTestDispatcher()) {
val fakeConsole = FakeConsole()
launch {
continuationSteal(fakeConsole)
}
continuation?.resume(fakeText)
assertEquals(3, fakeConsole.printed.size)
assertEquals(fakeText, fakeConsole.printed[1])
}
}
