子类的 kotlin 工厂

Question

我有一个类需要一个工厂，当提供一个子类的样本值时，它将创建一个具有子类类的新对象。例如

open class Base(val i:Int){
    fun <T:Base>factory(sample:T, n:Int) = Base(i) // need sample." invoke constructor" (i) as T
}

关于如何在使用 Base 的任何子类作为参数调用而不需要添加样板文件或反射超出 KotlinJS 可用的反射以覆盖每个未来子类的工厂时如何使这项工作工作？

Answer 1

open class Base(val i: Int) {

    fun <T : Base> factory(sample: T, @Suppress("UNUSED_PARAMETER") n: Int): Base {
        @Suppress("UNUSED_VARIABLE") val constructor = sample::class.js

        return js("new constructor(n)") as Base
    }

    override fun toString() = "Base($i)"
}

class Derived(i: Int) : Base(i) {

    override fun toString() = "Derived($i)"
}


fun main() {
    println(Base(10).factory(Derived(20), n = 5))  // "Derived(5)"
}

Answer 2

首先，你最好使用reified类型参数，这样你的工厂就不需要sample参数。 其次，如果您的工厂将返回一个泛型类型的对象，那么您将不需要在子类中重写它 - 它会返回所需类型的实例。

但是如果没有reflection，我看不出有什么办法：

open class Base (val i: Int) {
    inline fun <reified T : Base> factory(n: Int): T {
        val kClass = T::class
        val constructor = kClass.constructors.find { it.parameters.size == 1 && it.parameters[0].type == Int::class }
            ?: throw RuntimeException("Constructor with single Int parameter was not found for ${kClass.qualifiedName} class")
        return constructor.call(n)
    }
}

用法：

open class Derived(i: Int) : Base(i)
open class Derived2(i: Int) : Derived(i)

//any subclass of Base or Base itself when called on instance of Base
val base = Base(1)
val a: Base = base.factory(1) //or val a = base.factory<Base>(1)
val b: Derived = base.factory(1)
val c: Derived2 = base.factory(1)

//any subclass of Base or Base itself when called on any instance of Base subclass
val derived2 = Derived2(1)
val a1: Base = derived2.factory(1)
val b1: Derived = derived2.factory(1)
val c1: Derived2 = derived2.factory(1)