봉인 계층 구조와 함께 사용하는 완전한 when

모든 경우를 처리하는 when

개방형 타입에서 모든 경우를 처리하지 않는 when

open class Shape
class Circle : Shape()
class Square : Shape()
fun describe(s: Shape) = when (s) {
    is Circle -> "circle"
    is Square -> "square"
    else      -> "unknown" // required
}
fun main() { println(describe(Circle())) }

봉인 타입에서 모든 경우를 처리하는 when

sealed class Result
class Success(val value: Int) : Result()
class Failure(val msg: String) : Result()
fun describe(r: Result) = when (r) {
    is Success -> "ok: ${r.value}"
    is Failure -> "err: ${r.msg}"
}
fun main() {
    println(describe(Success(42)))
    println(describe(Failure("nope")))
}

봉인 인터페이스

sealed interface Animal
class Dog : Animal
class Cat : Animal
fun speak(a: Animal) = when (a) {
    is Dog -> "woof"
    is Cat -> "meow"
}
fun main() {
    println(speak(Dog()))
    println(speak(Cat()))
}

분기에서의 스마트 캐스트

sealed class Event
data class Click(val x: Int, val y: Int) : Event()
data class Key(val char: Char) : Event()
fun handle(e: Event) = when (e) {
    is Click -> "clicked at (${e.x},${e.y})" // smart-cast
    is Key   -> "key=${e.char}"
}
fun main() {
    println(handle(Click(10, 20)))
    println(handle(Key('A')))
}

경우 하나를 빠뜨리는 문제

sealed class Status
object Idle : Status()
object Running : Status()
// If we add object Done : Status(), every exhaustive when must update.
fun text(s: Status): String = when (s) {
    is Idle -> "idle"
    is Running -> "running"
}
fun main() { println(text(Idle)) }

데이터 클래스를 사용하는 봉인 타입

sealed class UiState {
    object Loading : UiState()
    data class Success(val data: List<String>) : UiState()
    data class Error(val message: String) : UiState()
}
fun render(state: UiState) = when (state) {
    UiState.Loading -> "loading..."
    is UiState.Success -> "got ${state.data.size} items"
    is UiState.Error -> "error: ${state.message}"
}
fun main() {
    println(render(UiState.Loading))
    println(render(UiState.Success(listOf("a", "b"))))
    println(render(UiState.Error("404")))
}

문으로 사용하는 when

sealed class Cmd
object Start : Cmd()
object Stop : Cmd()
fun execute(c: Cmd) {
    // Statement form: no error if incomplete (avoid this style)
    when (c) {
        Start -> println("started")
        Stop -> println("stopped")
    }
}
fun main() { execute(Start); execute(Stop) }

문에서도 모든 경우를 검사하도록 강제하기

sealed class Cmd
object Start : Cmd()
object Stop : Cmd()
fun execute(c: Cmd) {
    val handled: Unit = when (c) { // expression context now
        Start -> println("started")
        Stop -> println("stopped")
    }
}
fun main() { execute(Start) }

여러 경우를 묶는 분기

sealed class Event
class Click : Event()
class Tap : Event()
class Drag : Event()
fun isPointer(e: Event) = when (e) {
    is Click, is Tap, is Drag -> true
}
fun main() { println(isPointer(Tap())) }

중첩된 봉인 타입

sealed class Network {
    sealed class Wifi : Network() {
        object Open : Wifi()
        data class Secured(val ssid: String) : Wifi()
    }
    object Cellular : Network()
}
fun describe(n: Network) = when (n) {
    Network.Wifi.Open -> "open wifi"
    is Network.Wifi.Secured -> "secured: ${n.ssid}"
    Network.Cellular -> "cellular"
}
fun main() {
    println(describe(Network.Wifi.Secured("home")))
    println(describe(Network.Cellular))
}

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