Kotlin调用成员其他类的扩展功能 [英] Kotlin call member Extension function from other class

问题描述

我想在多个类中使用此功能:

I would like to use this function in multiple classes:

fun <T> T?.ifNull(function: (T?, s:String) -> Unit) {
}

我该怎么做?

这就是我想使用它的方式:

This is how I would like to use it:

class A{
fun <T> T?.ifNull(function: (T?, s:String) -> Unit) {
    }
}

class B{
constructor(){
    val a = A()
    //I want to use the function here
}}

推荐答案

如果将扩展功能定义为类A的成员，则该扩展功能仅在A的上下文中可用.也就是说，您当然可以直接在A内部使用它.但是，从另一个类B来看，它不是直接可见的. Kotlin具有所谓的范围函数，例如with，可用于将您的类带入A的范围.以下内容演示了如何在B内部调用扩展函数:

If you define an extension function as a member of a class A, that extension function is only usable in the context of A. That means, you can use it inside A directly, of course. From another class B though, it's not directly visible. Kotlin has so called scope functions like with, which may be used for bringing your class into the scope of A. The following demonstrates how the extension function is called inside B:

class B {
    init {
        with(A()) {
            "anything".ifNull { it, s -> }
        }
    }
}

作为一种替代方法(通常是推荐的方法)，您可以在顶级(即直接在文件中)定义扩展功能:

As an alternative, and this is mostly the recommended approach, you would define extension functions top-level, i.e. in a file directly:

fun <T> T?.ifNull(function: (T?, s: String) -> Unit) {
}

class A {
    init {
        "anythingA".ifNull { it, s -> }
    }
}

class B {
    init {
        "anythingB".ifNull { it, s -> }
    }
}

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