Kotlin笔记：with/let/run/apply/also区别

Posted 2023-02-10 MichaelX_Blog

class Person(var name: String, var age: Int) 

    fun eat() 
        println("吃柠檬")
    

    fun work(hour: Int): Int 
        println("work $hour hour,earn ￥$hour * 60")
        return hour * 60
    

with

with()函数是一个内联函数，它把传入的对象作为接受者，在该函数内可以使用this指代该对象来访问其公有的属性和方法。该函数的返回值为函数块最后一行或指定的return表示式。

例：
fun main() 
    val person: Person = Person("hzh", 23)
    val result = with(person) 
        age = 24
        eat()
        work(8) // 返回480
    
    println("result is:$result")


运行结果：

吃柠檬
work 8 hour,earn ￥480
result is:480

return返回

fun main() 
    val person: Person = Person("hzh", 23)
    val result = with(person) 
        age = 24
        eat()
        work(8)
        return@with "HaHa" // return表达式，可以省略return@with直接返回最后一行
    
    println("result is:$result")


运行结果：

吃柠檬
work 8 hour,earn ￥480
result is:HaHa

with不做判空处理

fun main() 
    val person: Person? = Person("hzh", 23)
    val result = with(person) 
        age = 24
        eat()
        work(8)
    
    println("result is:$result")


注意📢：无法编译，person为可空对象，with()需要在函数块内判定对象是否为空
如：
val result = with(person) 
    this?.age = 24
    this?.eat()
    this?.work(8)

with()函数源码:

/**
 * Calls the specified function [block] with the given [receiver] as its receiver and returns its result.
 *
 * For detailed usage information see the documentation for [scope functions](https://kotlinlang.org/docs/reference/scope-functions.html#with).
 */
@kotlin.internal.InlineOnly
public inline fun <T, R> with(receiver: T, block: T.() -> R): R 
    contract 
        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
    
    return receiver.block()

let

let()函数是一个扩展对象函数，它可以对被扩展的对象做统一的判空处理，在函数块内使用it来指代该对象，可以访问对象的公有属性和方法。let()函数的返回值和with()函数一样，为函数块最后一行或指定的return表示式。

例：
fun main() 
    val person: Person? = Person("hzh", 23)
    val result = person?.let 
        it.age = 24
        it.eat()
        it.work(8) // 返回480
    
    println("result is:$result")


运行结果：

吃柠檬
work 8 hour,earn ￥480
result is:480

let()函数源码:

/**
 * Calls the specified function [block] with `this` value as its argument and returns its result.
 *
 * For detailed usage information see the documentation for [scope functions](https://kotlinlang.org/docs/reference/scope-functions.html#let).
 */
@kotlin.internal.InlineOnly
public inline fun <T, R> T.let(block: (T) -> R): R 
    contract 
        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
    
    return block(this)

run

run()函数是with()和let()函数的结合体，它可以像with()函数一样直接在函数块中使用this指代该对象，也可以像let()函数一样为对象做统一的判空处理。

例：
fun main() 
    val person: Person? = Person("hzh", 23)
    val result = person?.run 
        age = 24
        eat()
        work(8) // 返回480
    
    println("result is:$result")


运行结果：

吃柠檬
work 8 hour,earn ￥480
result is:480

run()函数的源码：

/**
 * Calls the specified function [block] with `this` value as its receiver and returns its result.
 *
 * For detailed usage information see the documentation for [scope functions](https://kotlinlang.org/docs/reference/scope-functions.html#run).
 */
@kotlin.internal.InlineOnly
public inline fun <T, R> T.run(block: T.() -> R): R 
    contract 
        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
    
    return block()

apply

apply()函数和run()函数相似，不同的是，run()函数是以闭包形式返回最后一行代码的值，而apply()函数返回的是传入的对象本身。

例：
fun main() 
    val person: Person? = Person("hzh", 23)
    println("person:$person")
    val result = person?.apply 
        age = 24
        eat()
        work(8) // 返回传入的person对象
    
    println("result is:$result")


运行结果：

person:Person@610455d6
吃柠檬
work 8 hour,earn ￥480
result is:Person@610455d6

apply()函数源码：

/**
 * Calls the specified function [block] with `this` value as its receiver and returns `this` value.
 *
 * For detailed usage information see the documentation for [scope functions](https://kotlinlang.org/docs/reference/scope-functions.html#apply).
 */
@kotlin.internal.InlineOnly
public inline fun <T> T.apply(block: T.() -> Unit): T 
    contract 
        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
    
    block()
    return this

also

also()函数和apply()函数相似，不同的是，also()函数在函数块中使用it指代该对象，而apply()函数在函数块中使用this指代该对象。

fun main() 
    val person: Person? = Person("hzh", 23)
    println("person:$person")
    val result = person?.also 
        it.age = 24
        it.eat()
        it.work(8) // 返回传入的person对象
    
    println("result is:$result")


运行结果：

person:Person@610455d6
吃柠檬
work 8 hour,earn ￥480
result is:Person@610455d6

also()函数源码：

/**
 * Calls the specified function [block] with `this` value as its argument and returns `this` value.
 *
 * For detailed usage information see the documentation for [scope functions](https://kotlinlang.org/docs/reference/scope-functions.html#also).
 */
@kotlin.internal.InlineOnly
@SinceKotlin("1.1")
public inline fun <T> T.also(block: (T) -> Unit): T 
    contract 
        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
    
    block(this)
    return this

区别

函数名	函数块内使用对象	返回值	是否扩展函数	适用场景
with	this	函数块最后一行或return表达式的值	否	适用于调用同一个类多个方法
let	it	函数块最后一行或return表达式的值	是	适用于对象统一处理不为空的情况
run	this	函数块最后一行或return表达式的值	是	适用with()、let()函数的任何场景
apply	this	该对象	是	适用于run()函数的任何场景，通产可用来在初始化一个对象实例时，操作对象属性并最终返回该对象。也可用于多个扩展函数链式调用
also	it	该对象	是	适用于let()函数的任何场景，一般可用于多个扩展函数链式调用

转载自

https://juejin.cn/post/6868179386344931342