kotlin - 扩展功能对于分派(dispatch)接收者来说是虚拟的？

Question

在 Kotlin 中 Declaraing extensions as members ，“此类函数的调度对于调度接收器类型而言是虚拟，但对于扩展接收器类型而言是静态是什么意思。”

这是否意味着“扩展函数不遵循其接收者的类类型。它遵循参数类型(？在此代码调用方法参数中)。”

请多多指教

open class Base { }

class Derived : Base() { }

open class BaseCaller {
    open fun Base.printFunctionInfo() {
        println("Base extension function in BaseCaller")
    }

    open fun Derived.printFunctionInfo() {
        println("Derived extension function in BaseCaller")
    }

    fun call(b: Base) {
        b.printFunctionInfo()   // call the extension function
    }
}

class DerivedCaller: BaseCaller() {
    override fun Base.printFunctionInfo() {
        println("Base extension function in DerivedCaller")
    }

    override fun Derived.printFunctionInfo() {
        println("Derived extension function in DerivedCaller")
    }
}

fun main() {
    BaseCaller().call(Base())   // "Base extension function in BaseCaller"
    DerivedCaller().call(Base())  // "Base extension function in DerivedCaller" - dispatch receiver is resolved virtually
    DerivedCaller().call(Derived())  // "Base extension function in DerivedCaller" - extension receiver is resolved statically
}

Answer 1

既然你已经链接了文档，我认为你已经阅读了以下部分

The instance of the class in which the extension is declared is called dispatch receiver, and the instance of the receiver type of the extension method is called extension receiver.

理解了上面的术语之后，你还需要理解以下几点

• 如果您不了解虚拟方法，请阅读 this

• Extensions are resolved statically .考虑以下代码块

  fun call(b: Base) {
       // This will always call extension function defined on the Base class
       // even if you pass an object of Derived class
       b.printFunctionInfo()   // call the extension function
  }
  
  // This calls the printFunctionInfo defined on the Base, even though we pass Derived
  DerivedCaller().call(Derived()) 
  

现在回答你的问题

the dispatch of such functions is virtual with regard to the dispatch receiver type, but static with regard to the extension receiver type.

有了扩展被静态解析这一点，我们已经确定无论您传递哪个对象(Base 或 Derived)，call 函数总是调用在 Base 类型上定义的扩展函数。

但是会调用哪个扩展函数？一个在基类中还是在派生类中？

这取决于调用 call 函数的对象类型，如果您使用 Base 对象调用调用，那么将调用基类中的扩展，如果您使用 Derived 对象然后将调用派生类中的扩展。