Solution: UserDownloader
This is the solution using a dispatcher limited to a single thread and a read-only list:
class UserDownloader(private val api: NetworkService) {
private var users = listOf<User>()
private val dispatcher = Dispatchers.IO
.limitedParallelism(1)
fun downloaded(): List<User> = users
suspend fun getUser(id: Int) = withContext(dispatcher) {
val newUser = api.getUser(id)
users += newUser
}
}
This is the solution using a dispatcher limited to a single thread and a mutable list:
class UserDownloader(private val api: NetworkService) {
private val users = mutableListOf<User>()
private val dispatcher = Dispatchers.IO
.limitedParallelism(1)
suspend fun downloaded(): List<User>=withContext(dispatcher){
users.toList()
}
suspend fun getUser(id: Int) = withContext(dispatcher) {
val newUser = api.getUser(id)
users += newUser
}
}
This is the solution using a synchronized block and a mutable list:
class UserDownloader(private val api: NetworkService) {
private val users = mutableListOf<User>()
private val lock = Any()
suspend fun downloaded(): List<User> = synchronized(lock) {
users.toList()
}
suspend fun getUser(id: Int) {
val newUser = api.getUser(id)
synchronized(lock) {
users += newUser
}
}
}
This is the solution using a concurrent set:
class UserDownloader(private val api: NetworkService) {
private val users = ConcurrentHashMap.newKeySet<User>()
fun downloaded(): List<User> = users.toList()
suspend fun getUser(id: Int) {
val newUser = api.getUser(id)
users += newUser
}
}
Example solution in playground
import kotlinx.coroutines.*
import org.junit.Test
import kotlin.test.assertEquals
data class User(val name: String)
interface NetworkService {
suspend fun getUser(id: Int): User
}
class FakeNetworkService : NetworkService {
override suspend fun getUser(id: Int): User {
delay(2)
return User("User$id")
}
}
class UserDownloader(private val api: NetworkService) {
private var users = listOf<User>()
private val dispatcher = Dispatchers.IO
.limitedParallelism(1)
fun downloaded(): List<User> = users
suspend fun getUser(id: Int) = withContext(dispatcher) {
val newUser = api.getUser(id)
users += newUser
}
}
suspend fun main(): Unit = coroutineScope {
val downloader = UserDownloader(FakeNetworkService())
coroutineScope {
repeat(1_000_000) {
launch {
downloader.getUser(it)
}
}
}
print(downloader.downloaded().size) // ~714725
}
class UserDownloaderTest {
@Test
fun test() = runBlocking {
val downloader = UserDownloader(FakeNetworkService())
coroutineScope {
repeat(1_000_000) {
launch(Dispatchers.Default) {
downloader.getUser(it)
}
}
}
assertEquals(1_000_000, downloader.downloaded().size)
}
}
