254 lines
5.9 KiB
Markdown
254 lines
5.9 KiB
Markdown
---
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title: kotlin中的apply run also with let函数
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date: 2020-06-08 16:27:10
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tags: [kotlin]
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categories: [kotlin]
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author: Karl
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---
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# apply
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```kotlin
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/**
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* Calls the specified function [block] with `this` value as its receiver and returns `this` value.
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*
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* For detailed usage information see the documentation for [scope functions](https://kotlinlang.org/docs/reference/scope-functions.html#apply).
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*/
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@kotlin.internal.InlineOnly
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public inline fun <T> T.apply(block: T.() -> Unit): T {
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contract {
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callsInPlace(block, InvocationKind.EXACTLY_ONCE)
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}
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block()
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return this
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}
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```
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我们从源码中可以知道 apply接受一个对象，返回一个对象。闭包中使用(this)指代对象
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一般使用于成员对象的操作
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官方文档对其解释为 apply the following assignments to the object.
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```kotlin
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class StudyOne {
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var karl: String ?= null
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var karlTwo: String ?= null
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}
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```
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```kotlin
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fun main(args: Array<String>) {
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val studyOne = StudyOne().apply {
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this.karl = "aa"
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this.karlTwo = "bb"
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}
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println("studyOne: ${studyOne.karl}")
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println("studyOne: ${studyOne.karlTwo}")
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}
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//运行结果
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//aa
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//bb
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```
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参考文档: [apply](https://kotlinlang.org/docs/reference/scope-functions.html#apply)
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# run
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```kotlin
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/**
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* Calls the specified function [block] with `this` value as its receiver and returns its result.
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*
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* For detailed usage information see the documentation for [scope functions](https://kotlinlang.org/docs/reference/scope-functions.html#run).
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*/
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@kotlin.internal.InlineOnly
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public inline fun <T, R> T.run(block: T.() -> R): R {
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contract {
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callsInPlace(block, InvocationKind.EXACTLY_ONCE)
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}
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return block()
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}
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```
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我们发现 run跟apply的差别只是apply返回对象本身，run一个返回lamda表达式(跟run做同样事情的还有with, let)
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run一般用于初始化和返回值的计算
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```kotlin
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fun testRun() {
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studyOne.run {
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println("karl")
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}
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}
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// 输出结果
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//karl
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```
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另一个run 非扩展性功能run
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```kotlin
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@kotlin.internal.InlineOnly
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public inline fun <R> run(block: () -> R): R {
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contract {
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callsInPlace(block, InvocationKind.EXACTLY_ONCE)
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}
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return block()
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}
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```
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执行闭包，返回闭包结果
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```kotlin
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fun main(args: Array<String>) {
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val studyOne = run {
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StudyOne()
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}
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println(studyOne)
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}
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//运行结果
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//StudyOne@7a81197d
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```
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参考文档: [run](https://kotlinlang.org/docs/reference/scope-functions.html#run)
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# also
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```kotlin
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/**
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* Calls the specified function [block] with `this` value as its argument and returns `this` value.
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*
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* For detailed usage information see the documentation for [scope functions](https://kotlinlang.org/docs/reference/scope-functions.html#also).
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*/
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@kotlin.internal.InlineOnly
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@SinceKotlin("1.1")
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public inline fun <T> T.also(block: (T) -> Unit): T {
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contract {
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callsInPlace(block, InvocationKind.EXACTLY_ONCE)
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}
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block(this)
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return this
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}
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```
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接受一个对象,返回一个对象本身,一般使用于不影响对象本身做某些事
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```kotlin
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fun main(args: Array<String>) {
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val numbers = mutableListOf("one", "two", "three")
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val alsoTest = numbers.also {
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println("kotlin真好用")
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println("c: $it")
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}
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.add("four")
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println("c: $numbers")
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}
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//运行结果
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//kotlin真好用
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//c: [one, two, three]
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//c: [one, two, three, four]
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```
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参考文档: [also](https://kotlinlang.org/docs/reference/scope-functions.html#also)
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# with
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```kotlin
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/**
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* Calls the specified function [block] with the given [receiver] as its receiver and returns its result.
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*
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* For detailed usage information see the documentation for [scope functions](https://kotlinlang.org/docs/reference/scope-functions.html#with).
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*/
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@kotlin.internal.InlineOnly
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public inline fun <T, R> with(receiver: T, block: T.() -> R): R {
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contract {
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callsInPlace(block, InvocationKind.EXACTLY_ONCE)
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}
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return receiver.block()
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}
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```
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非扩展功能，接受一个对象，返回lamda表达式结果，可以理解为 对这个对象执行以下操作
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```kotlin
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fun main(args: Array<String>) {
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val numbers = mutableListOf("one", "two", "three")
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with(numbers) {
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this.add("karl")
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this.add("帅的起飞")
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}
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println("numbers: $numbers")
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}
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//运行结果
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//numbers: [one, two, three, karl, 帅的起飞]
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```
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参考文档: [with](https://kotlinlang.org/docs/reference/scope-functions.html#with)
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# let
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```kotlin
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/**
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* Calls the specified function [block] with `this` value as its argument and returns its result.
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*
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* For detailed usage information see the documentation for [scope functions](https://kotlinlang.org/docs/reference/scope-functions.html#let).
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*/
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@kotlin.internal.InlineOnly
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public inline fun <T, R> T.let(block: (T) -> R): R {
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contract {
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callsInPlace(block, InvocationKind.EXACTLY_ONCE)
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}
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return block(this)
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}
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```
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接受一个对象，返回lamda表达式结果,或者指定return
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```kotlin
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fun main(args: Array<String>) {
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val numbers = mutableListOf("one", "two", "three")
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fun testLet(): Int {
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numbers.let {
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return if (Random().nextBoolean()) {
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it.add("five")
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println(it)
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1
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} else {
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it.add("six")
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println(it)
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2
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}
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}
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}
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}
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```
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# ?.let 对象不为空时执行
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参考文档: [let](https://kotlinlang.org/docs/reference/scope-functions.html#let)
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# taskIf
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满足block中条件，则返回当前值，否则返回null，block的返回值Boolean类型
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# takeUnless
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不满足block中条件，则返回当前值，否则返回null，block的返回值Boolean类型
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# 总结
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执行非空对象lamda: ?.let
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判断局部范围变化: let
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对象配置: apply
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对象配置和计算处理: run
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运行需要的lamda表达式: 非扩展功能 run
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附加: also
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对象分组调用: with
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