我们什么时候应该在 Kotlin 上使用 run、let、apply、also 和 with 的例子 [英] Example of when should we use run, let, apply, also and with on Kotlin

问题描述

我希望每个函数都有一个很好的例子 run, let, apply, also, with

I wish to have a good example for each function run, let, apply, also, with

我已阅读这篇文章 但还是缺少例子

I have read this article but still lack of an example

推荐答案

所有这些函数都是用于切换当前函数/变量的作用域.它们用于将属于一起的东西放在一个地方(主要是初始化).

All these functions are used for switching the scope of the current function / the variable. They are used to keep things that belong together in one place (mostly initializations).

以下是一些示例:

run - 返回任何你想要的东西，并将它使用的变量重新定义为 this

run - returns anything you want and re-scopes the variable it's used on to this

val password: Password = PasswordGenerator().run {
       seed = "someString"
       hash = {s -> someHash(s)}
       hashRepetitions = 1000

       generate()
   }

密码生成器现在被重新定义为 this，因此我们可以设置 seed、hash 和 hashRepetitions 而无需使用变量.generate() 将返回一个 Password 的实例.

The password generator is now rescoped as this and we can therefore set seed, hash and hashRepetitions without using a variable. generate() will return an instance of Password.

apply 类似，但会返回this:

val generator = PasswordGenerator().apply {
       seed = "someString"
       hash = {s -> someHash(s)}
       hashRepetitions = 1000
   }
val pasword = generator.generate()

这对于替代 Builder 模式特别有用，如果您想重新使用某些配置.

That's particularly useful as a replacement for the Builder pattern, and if you want to re-use certain configurations.

let - 主要用于避免空检查，但也可用作 run 的替代品.不同之处在于，this 仍将与之前相同，您可以使用 it 访问重新定义范围的变量:

let - mostly used to avoid null checks, but can also be used as a replacement for run. The difference is, that this will still be the same as before and you access the re-scoped variable using it:

val fruitBasket = ...

apple?.let {
  println("adding a ${it.color} apple!")
  fruitBasket.add(it)
}

上面的代码只有在它不为空时才会将苹果添加到篮子里.另请注意，it 现在 不再是可选的，因此您不会在这里遇到 NullPointerException(也就是说，您不需要使用 ?.代码>访问其属性)

The code above will add the apple to the basket only if it's not null. Also notice that it is now not optional anymore so you won't run into a NullPointerException here (aka. you don't need to use ?. to access its attributes)

also - 当你想使用 apply 时使用它，但不想隐藏 this

also - use it when you want to use apply, but don't want to shadow this

class FruitBasket {
    private var weight = 0

    fun addFrom(appleTree: AppleTree) {
        val apple = appleTree.pick().also { apple ->
            this.weight += apple.weight
            add(apple)
        }
        ...
    }
    ...
    fun add(fruit: Fruit) = ...
}

在这里使用 apply 会遮蔽 this，这样 this.weight 会引用苹果，而不是 > 到水果篮.

Using apply here would shadow this, so that this.weight would refer to the apple, and not to the fruit basket.

注意:我无耻地从我的博客取材

Note: I shamelessly took the examples from my blog

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