withContextってスレッド切り替え以外にも使えるって知ってた？

KotlinではCoroutineを用いることで、非同期処理を手続き的に記述できます。
Coroutineの利用例として「ネットワークアクセスをwithContext(Dispatchers.IO)でラップすることでIOスレッドで行う」という例が紹介されることが多いです。
そのため、「withContextはスレッド切り替え時に使う」という理解をされている方は多いと思います。
しかし、実際にはwithContextはCoroutineContextを切り替えるために利用するものであり、スレッド切り替え以外にもさまざまな活用法があります。
本セッションでは、CoroutineContextについて触れたうえで、筆者のチームがwithContextをどのような場面で活用しているかを紹介したいと思います。

Transcript

1. withContextͬͯ εϨου੾Γସ͑Ҏ֎ʹ΋ ࢖͑Δͬͯ஌ͬͯͨʁ 2024/06/22 (Sat) Kotlin Fest 2024 T45K @getupmax
2. ࢿྉ͸ͪ͜Β https://x.com/getupmax
3. Kotlin Coroutine ࢖ͬͯ·͔͢ʁ 🙋
4. Coroutine͓͞Β͍ ~ AndroidΛ୊ࡐͱͯ͠ ~ https://developer.android.com/kotlin/coroutines
5. UIεϨου্Ͱ ͕͔͔࣌ؒΔॲཧΛͦͷ··ॻ͘ Coroutine͓͞Β͍ https://developer.android.com/kotlin/coroutines#executing-in-a-background-thread
6. UIεϨου্Ͱ ͕͔͔࣌ؒΔॲཧΛͦͷ··ॻ͘ ↓ ϑϦʔζ͢Δ Coroutine͓͞Β͍ https://developer.android.com/kotlin/coroutines#executing-in-a-background-thread
7. class LoginRepository(private val responseParser: LoginResponseParser) { private const val loginUrl = "https://example.com/login" // ݱࡏͷεϨουΛϒϩοΫ͢ΔɺωοτϫʔΫϦΫΤετΛߦ͏ؔ਺ // ͭ·ΓɺϩάΠϯ͕׬ྃ͢Δ·ͰΞϓϦ͕ϑϦʔζ͢Δ fun makeLoginRequest(jsonBody: String): Result<LoginResponse> { val url = URL(loginUrl) (url.openConnection() as? HttpURLConnection)?.run { ... } return ... } } https://developer.android.com/kotlin/coroutines#executing-in-a-background-thread
8. APIݺͼग़͠ΛIOεϨουͰ΍Γ͍ͨ Coroutine͓͞Β͍
9. APIݺͼग़͠ΛIOεϨουͰ΍Γ͍ͨ ↓ withContext(Dispatchers.IO)ͰғΉ Coroutine͓͞Β͍
10. class LoginRepository(...) { ... suspend fun makeLoginRequest(jsonBody: String): Result<LoginResponse> { // APIݺͼग़͠ΛIO DispatcherͰ࣮ߦ͢Δ return withContext(Dispatchers.IO) { val url = URL(loginUrl) (url.openConnection() as? HttpURLConnection)?.run { ... } return ... } } } https://developer.android.com/kotlin/coroutines#use-coroutines-for-main-safety
11. class LoginViewModel(private val loginRepository: LoginRepository) : ViewModel() { fun login(username: String, token: String) { viewModelScope.launch { val jsonBody = "{ username: \"$username\", token: \"$token\"}" // makeLoginRequestͷ࣮ߦ͸IOεϨουͰߦΘΕΔ // ͦͷؒɺUIεϨου͸ϒϩοΫ͞Εͳ͍ͷͰɺϢʔβૢ࡞Λड͚෇͚ΒΕΔ val result = loginRepository.makeLoginRequest(jsonBody) // ݁ՌΛUIεϨουͰॲཧ͢Δ when (result) { is Result.Success<LoginResponse> -> ... else -> ... } } } }
12. ϩάΠϯதʹΞϓϦ͕ ϑϦʔζ͠ͳ͘ͳͬͨ 🎉 Coroutine͓͞Β͍
13. withContext(Dispatchers.XXX) ? Coroutine͓͞Β͍ ωοτϫʔΫॲཧ ܭࢉྔଟ͍ॲཧ UIपΓͷॲཧ Dispatchers.IO Dispatchers.Default Dispatchers.Main → → →
14. withContext͸ εϨου੾Γସ͑༻ʁ🤔
15. 🙅
16. withContext͸ CoroutineContextΛ ੾Γସ͑Δ
17. https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.coroutines/-coroutine-context/
18. CoroutineContext.KeyΛΩʔ CoroutineContext.ElementΛόϦϡʔ ͱͨࣙ͠ॻ CoroutineContext
19. https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.coroutines/-coroutine-context/
20. // StringܕΛอ࣋͢ΔίϯςΩετ class CoroutineContextA(val value: String) : CoroutineContext.Element { companion object Key : CoroutineContext.Key<CoroutineContextA> override val key: CoroutineContext.Key<*> = Key } // IntܕΛอ࣋͢ΔίϯςΩετ class CoroutineContextB(val value: Int) : CoroutineContext.Element { companion object Key : CoroutineContext.Key<CoroutineContextB> override val key: CoroutineContext.Key<*> = Key }
21. val contextA = CoroutineContextA("test") val contextB = CoroutineContextB(1) contextA[CoroutineContextA.Key]?.value == "test" contextB[CoroutineContextA.Key] == null val contextAB = contextA + contextB contextAB[CoroutineContextA.Key]?.value == "test" contextAB[CoroutineContextB.Key]?.value == 1 val contextABMinusA = contextAB.minusKey(CoroutineContextA.Key) contextABMinusA[CoroutineContextA.Key] == null
22. val contextA = CoroutineContextA("test") val contextB = CoroutineContextB(1) contextA[CoroutineContextA.Key]?.value == "test" contextB[CoroutineContextA.Key] == null val contextAB = contextA + contextB contextAB[CoroutineContextA.Key]?.value == "test" contextAB[CoroutineContextB.Key]?.value == 1 val contextABMinusA = contextAB.minusKey(CoroutineContextA.Key) contextABMinusA[CoroutineContextA.Key] == null
23. val contextA = CoroutineContextA("test") val contextB = CoroutineContextB(1) contextA[CoroutineContextA.Key]?.value == "test" contextB[CoroutineContextA.Key] == null val contextAB = contextA + contextB contextAB[CoroutineContextA.Key]?.value == "test" contextAB[CoroutineContextB.Key]?.value == 1 val contextABMinusA = contextAB.minusKey(CoroutineContextA.Key) contextABMinusA[CoroutineContextA.Key] == null
24. val contextA = CoroutineContextA("test") val contextB = CoroutineContextB(1) contextA[CoroutineContextA.Key]?.value == "test" contextB[CoroutineContextA.Key] == null val contextAB = contextA + contextB contextAB[CoroutineContextA.Key]?.value == "test" contextAB[CoroutineContextB.Key]?.value == 1 val contextABMinusA = contextAB.minusKey(CoroutineContextA.Key) contextABMinusA[CoroutineContextA.Key] == null
25. 1. Ҿ਺Ͱ༩͑ΒΕͨCoroutineContextΛ 2. ݱࡏͷCoroutineContextʹՃ͑ͯ 3. ϒϩοΫΛ࣮ߦ͢Δ withContext
26. val contextA = CoroutineContextA("test") val contextB = CoroutineContextB(1) withContext(a) { withContext(b) { currentCoroutineContext()[CoroutineContextA.Key]?.value == "test" currentCoroutineContext()[CoroutineContextB.Key]?.value == 1 } currentCoroutineContext()[CoroutineContextA.Key]?.value == "test" currentCoroutineContext()[CoroutineContextB.Key] == null }
27. Կʹ࢖͑Δʁ
28. CoroutineContextʹ஋ΛೖΕͯ withContextʹ౉͢͜ͱͰ ίϯςΩετม਺ͱͯ͠ѻ͑Δ
29. ࠓ·ͰThreadLocalͱ͔Ͱ ؅ཧ͍ͯͨ͠ΞϨίϨΛ CoroutineͰ΋࢖͑ͦ͏
30. ར༻ྫ1 τϥϯβΫγϣϯ؅ཧ
31. ఻౷తͳWeb FrameworkͰ͸ ThreadLocalͰτϥϯβΫγϣϯΛ؅ཧ τϥϯβΫγϣϯ؅ཧ
32. ྫ) Spring Framework
33. Coroutine͸࣮ߦεϨουΛ੾Γସ͑Δ →ThreadLocal͸࢖͑ͳ͍ τϥϯβΫγϣϯ؅ཧ
34. Coroutine͸࣮ߦεϨουΛ੾Γସ͑Δ →ThreadLocal͸࢖͑ͳ͍ ͷͰɺࣗલ؅ཧ τϥϯβΫγϣϯ؅ཧ
35. εϨουΛ؅ཧ͢ΔΦϒδΣΫτΛ CoroutineContextʹೖΕͯ؅ཧ͢Δ τϥϯβΫγϣϯ؅ཧ
36. jOOQͰͷτϥϯβΫγϣϯ؅ཧ val dsl: DSLContext = xxx dsl.transactionCoroutine { config -> config.dsl().execute("insert into table1 ...") config.dsl().execute("insert into table2 ...") }
37. class UseCase(private val dsl: DSLContext, private val service: Service) { suspend fun execute() { dsl.transactionCoroutine { config -> service.execute(config.dsl()) } } } class Service(private val repo: Repository) { suspend fun execute(ctx: DSLContext) { repo.save(ctx) } } class Repository { suspend fun save(ctx: DSLContext) { ctx.execute("insert into table1 ...") } }
38. DSLContextΦϒδΣΫτΛ CoroutineContextʹೖΕͯ؅ཧ͢Δ jOOQͷτϥϯβΫγϣϯ؅ཧ
39. // DSLContextΛอ࣋͢ΔElement class TransactionElement(val ctx: DSLContext) : AbstractCoroutineContextElement(TransactionElement) { companion object Key : CoroutineContext.Key<TransactionElement> } class TransactionManager(private val ctx: DSLContext) { suspend fun <T> execute(block: suspend () -> T): T = ctx.transactionCoroutine { config -> withContext(TransactionElement(config.dsl())) { block() } } }
40. class Repository(private val dsl: DSLContext) { suspend fun save() { (coroutineContext[TransactionElement]?.ctx ?: this.ctx) .execute("insert into table1 ...") } } class UseCase( private val manager: TransactionManager, private val service: Service ) { suspend fun execute() { manager.execute { service.execute(config.dsl()) } } }
41. ར༻ྫ2 Ϣʔβ৘ใͷ؅ཧ
42. ϦΫΤετॲཧͷ࠷ॳ͔Β࠷ޙ·Ͱ ৘ใΛอ͍࣋ͨ͠ e.g., Tenant IDͳͲ Ϣʔβ৘ใͷ؅ཧ
43. class Controller(private val service: Service) { suspend fun get(session: Session) { val tenantId = session.getTenantId() service.execute(tenantId) } } class Service(private val repo: Repository) { suspend fun execute(tenantId: Long) { repo.select(tenantId) } } class Repository(private val ctx: DSLContext) { suspend fun select(tenantId: Long) { ctx.execute("select * table where tenant_id = ?", tenantId) } }
44. ϦΫΤετॲཧͷ࠷ॳ͔Β࠷ޙ·Ͱ ৘ใΛอ͍࣋ͨ͠ e.g., Tenant IDͳͲ Ϣʔβ৘ใͷ؅ཧ
45. ϦΫΤετॲཧͷ࠷ॳ͔Β࠷ޙ·Ͱ ৘ใΛอ͍࣋ͨ͠ e.g., Tenant IDͳͲ → CoroutineContextʹೖΕΔ Ϣʔβ৘ใͷ؅ཧ
46. class UserInfoElement(val tenantId: Long) : AbstractCoroutineContextElement(UserInfoElement) { companion object Key : CoroutineContext.Key<UserInfoElement> } suspend fun getTenantId(): Long = coroutineContext[UserInfoElement]!!.tenantId class Controller(private val service: Service) { suspend fun get(session: Session) = withContext(UserInfoElement(session.getTenantId())){ service.execute() } } class Service(private val repo: Repository) { suspend fun execute() { repo.select() } } class Repository(private val ctx: DSLContext) { suspend fun select() { ctx.execute("select * table where tenant_id = ?", getTenantId()) } }
47. ͱ͜ΖͰ
48. ͜Εͬͯ Context Parameterͷํ͕ྑ͍ʁ
49. with಺ͷม਺Λݺͼग़͠ઌͰ΋ࢀরͰ͖Δػೳ ʢ΋ͱ΋ͱ͸Context Receiverʣ Kotlin 2.1ΑΓಋೖ༧ఆ ʢexperimentalͱͯ͠ར༻Մೳʣ Context Parameter https://github.com/Kotlin/KEEP/blob/context-parameters/proposals/context-receivers.md
50. data class UserInfo(val tenantId: String) class ItemDao { context(UserInfo) fun selectById(itemId: Int): Item? = execute(""" select * from item where id = $itemId and tenant_id = $tenantId """) } get("/") { with(UserInfo("123456")) { call.respond(...) } }
51. ͸CoroutineContextͱൺֱͯ͠... 👍 ίϯύΠϧ࣌ʹίϯςΩετͷݕ͕ࠪ͞ΕΔ 👍 Coroutine͡Όͳͯ͘΋࢖͑Δ 👎 Experimental 👎 ϥΠϒϥϦ/ϑϨʔϜϫʔΫͷαϙʔτ͕ෆे෼ Context Parameter
52. • Ϣʔβ৘ใ͸Context Receiver͕ྑͦ͞͏ ◦ શͯͷϝιουʹ͓͍ͯඞਢ ◦ ଘࡏ͢Δ͜ͱΛίϯύΠϧ࣌ʹอূ͍ͨ͠ • τϥϯβΫγϣϯ͸CoroutineContext͕ྑͦ͞͏ ◦ ଘࡏ͢Δ͜ͱΛίϯύΠϧ࣌ʹอূ͢Δඞཁ͕ͳ͍ ◦ ʢSpring AOP͕Context ReceiverʹରԠ͍ͯ͠ͳ͍ʣ ࢖͍෼͚ https://github.com/Kotlin/KEEP/blob/context-parameters/proposals/context-parameters.md
53. ·ͱΊ • withContext͸CoroutineContextΛ੾Γସ͑Δ • ಠࣗͷCoroutineContextΛఆٛ͢Δ͜ͱͰ
 ίϯςΩετม਺Λѻ͑Δ • Context Parameterͱ͸੗Έ෼͚Ͱ͖ͦ͏ • Kotlin 2.1ָ͠Έ✌
55. ͨ·͝εϙϯαʔ΍ͬͯ·͢ ϊϕϧςΟ഑෍தͰ͢ KotlinΤϯδχΞืूͯ͠·͢ Android/αʔόʔαΠυ໰Θͣ એ఻