kotlin - 为什么委托(delegate)类方法getValue 和setValue 需要用operator 关键字标记？

Question

这是来自 Delegated properties 的示例文档。

import kotlin.reflect.KProperty

class Delegate {
    operator fun getValue(thisRef: Any?, property: KProperty<*>): String {
        return "$thisRef, thank you for delegating '${property.name}' to me!"
    }

    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: String) {
        println("$value has been assigned to '${property.name}' in $thisRef.")
    }
}

Answer 1

这是因为委托(delegate)属性是 defined by convention .这意味着:

[its] semantics are defined through syntactic expansion of one syntax form into another syntax form.

可以看到委托(delegate)属性的扩展further down the documentation page :

class C {
    var prop: Type by MyDelegate()
}

// this code is generated by the compiler instead:
class C {
    private val prop$delegate = MyDelegate()
    var prop: Type
        get() = prop$delegate.getValue(this, this::prop)
        set(value: Type) = prop$delegate.setValue(this, this::prop, value)
}

按照惯例定义的语法的一个特征是(来自第一个链接):

All call expressions that are produced by expansion are only allowed to use operator functions.

还有:

This expansion of a particular syntax form to a different piece of code is usually defined in the terms of operator functions.

只是为了给您提供更多示例，第一个链接显示了更多按约定定义的语法示例。下面是与它们相关的相应算子函数:

<表类="s-表"><头>约定的语法相关算子函数<正文>算术和比较运算符 plus, compareTo 等 调用约定 调用运算符形式赋值 plusAssign, minusAssign 等For 循环语句 迭代器, hasNext, next委托(delegate)属性 设置值, 获取值解构声明 component1, component2 等

请注意，您需要在所有这些函数上加上单词 operator 才能使相应的语法起作用。换句话说，“operator”表示该函数可以在约定定义的语法中使用。