kotlin - 声明一个继承并初始化的对象

Question

我看过这个Kotlin代码:

sealed class BookingState {
    object ReasonOfTravel : BookingState()
}

在Kotlin文档中，我看到了声明对象的示例:

https://kotlinlang.org/docs/reference/object-declarations.html

但是我看不到没有花括号的声明。在上面的代码示例中，ReasonOfTravel继承自BookingState，但之后没有大括号。仅在您要通过BookingState()初始化对象时运行一些代码时才需要花括号吗？

Answer 1

就您而言，object仅用作标记，没有任何特定行为。稍后，您可以检查BookingState实例并执行以下操作:

sealed class BookingState {
    object ReasonOfTravel : BookingState()
    object ReasonOfTravel2 : BookingState()
    object ReasonOfTravel3 : BookingState()
}

fun test(bookingState: BookingState) = when(bookingState) {
    BookingState.ReasonOfTravel -> println("1")
    BookingState.ReasonOfTravel2 -> println("2")
    BookingState.ReasonOfTravel3 -> println("3")
}

您可以随时使用花括号添加特定行为:

sealed class BookingState {
    object ReasonOfTravel : BookingState() {
        fun printMe() = println("test")
    }

    object ReasonOfTravel2 : BookingState()
    object ReasonOfTravel3 : BookingState()
}

fun test(bookingState: BookingState) = when (bookingState) {
    is BookingState.ReasonOfTravel -> bookingState.printMe()
    BookingState.ReasonOfTravel2 -> println("2")
    BookingState.ReasonOfTravel3 -> println("3")
}

花括号中的代码无法在对象初始化时运行。您必须使用init块