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kotlin 作用域函数 : let、run、with、apply、 also、takeIf、takeUnless

1.官方文档

　　英文： https://kotlinlang.org/docs/reference/scope-functions.html

　　中文： https://www.kotlincn.net/docs/reference/scope-functions.html

2.简介

在kotlin标准库的Standard.kt 文件中，定义了一系列函数模板。其中的 [ run 、with、let、apply、aloso 、takeIf、takeUnless] 称作用域函数。
它们有个共同点是，最后一个参数都是一个函数指针，当使用 lambda 表达式方式调用这些函数时，在{ } 内部，可以访问调用者对象而无需其名称，所以叫它们作用域函数。
它们的作用就是让对象上执行一个额外代码，在某些情况下，可以简化代码。
它们生成对象类型的扩展函数(with除外)

　　部分代码定义如下：

 1 public inline fun <T> T.apply(block: T.() -> Unit): T {
 2     contract {
 3         callsInPlace(block, InvocationKind.EXACTLY_ONCE)
 4     }
 5     block()
 6     return this
 7 }
 8 
 9 /**
10  * Calls the specified function [block] with `this` value as its argument and returns `this` value.
11  *
12  * For detailed usage information see the documentation for [scope functions](https://kotlinlang.org/docs/reference/scope-functions.html#also).
13  */
14 @kotlin.internal.InlineOnly
15 @SinceKotlin("1.1")
16 public inline fun <T> T.also(block: (T) -> Unit): T {
17     contract {
18         callsInPlace(block, InvocationKind.EXACTLY_ONCE)
19     }
20     block(this)
21     return this
22 }
23 
24 /**
25  * Calls the specified function [block] with `this` value as its argument and returns its result.
26  *
27  * For detailed usage information see the documentation for [scope functions](https://kotlinlang.org/docs/reference/scope-functions.html#let).
28  */
29 @kotlin.internal.InlineOnly
30 public inline fun <T, R> T.let(block: (T) -> R): R {
31     contract {
32         callsInPlace(block, InvocationKind.EXACTLY_ONCE)
33     }
34     return block(this)
35 }

3.内置的几个函数对照表

3.1 对比表

简称	原型	作用	返回值类型	对象的引用名
run	inline fun <R> run(block: () -> R): R	在需要表达式的地方运行语句无参数运行一段代码	代码块的返回类型	-
run	inline fun <T, R> T.run(block: T.() -> R): R	对象配置并且计算结果	代码块的返回类型	this
with	inline fun <T, R> with(receiver: T, block: T.() -> R): R	以对象为参数执行一段代码	代码块的返回类型	this
let	inline fun <T, R> T.let(block: (T) -> R): R	对一个非空（non-null）对象执行代码将表达式作为变量引入为局部作用域中	代码块的返回类型	it
also	inline fun <T> T.also(block: (T) -> Unit): T	给对象添加附加效果	对象类型	it
apply	inline fun <T> T.apply(block: T.() -> Unit): T	对象配置	对象类型	this
takeIf	inline fun <T> T.takeIf(predicate: (T) -> Boolean): T?	predicate(对象) 执行结果为true返回对象，相反null	对象类型或 null	it
takeUnless	inline fun <T> T.takeUnless(predicate: (T) -> Boolean): T?	!predicate(对象) 执行结果为fase返回对象，相反null	对象类型或 null	it

also,apply，takeIf,takeUnless , a和 t 开头的返回对象类型，其它的返回代码块类型。
also,let , takeIf,takeUnless 引用对象用的是it,其它用this

3.2 注意事项

　　适当的场景下,使用作用域函数可以使代码变简洁，但请：

避免过度使用：这会降低代码的可读性并可能导致错误。
避免嵌套使用
避免链式调用：此时很容易对当前上下文对象及 this 或 it 的值感到困惑。

4.let

4.1 作用

 1 /**
 2  * Calls the specified function [block] with `this` value as its argument and returns its result.
 3  *
 4  * For detailed usage information see the documentation for [scope functions](https://kotlinlang.org/docs/reference/scope-functions.html#let).
 5  */
 6 @kotlin.internal.InlineOnly
 7 public inline fun <T, R> T.let(block: (T) -> R): R {
 8     contract {
 9         callsInPlace(block, InvocationKind.EXACTLY_ONCE)
10     }
11     return block(this)
12 }

对一个非空（non-null）对象执行 lambda 表达式
将表达式作为变量引入为局部作用域中

4.2 测试

最后一行是返回值

1     class Scope2(var name : String)
2 
3     //1.最后一行是返回值
4     val num = 12.let {
5         3
6         Log.e(ScopeTAG, "testLet it = $it")
7     }
8     Log.e(ScopeTAG, "testLet num = $num")

结果：

testLet it = 12
testLet num = 31

函数式调用

1     //2.函数式调用
2     fun <T> leeeeeeeeeeeet1(it : T){
3         Log.e(ScopeTAG,"testLet let1<T> it = $it")
4     }
5     13f.let(::leeeeeeeeeeeet1)

结果

testLet let1<T> it = 13.0

非空值调用 let

1     //3.非空值调用 let
2     val flt = 13f.let { 16 }
3     Log.e(ScopeTAG, "testLet flt = $flt")

结果

testLet flt = 16

非空对象调用

1     //4.非空对象调用
2     val fullName = Scope2("not-").let {
3         val last = "null"
4         it.name += last
5         Log.e(ScopeTAG, "testLet name append $last ")
6         it.name
7     }
8     Log.e(ScopeTAG, "testLet fullName = $fullName ")

结果

testLet name append null 
testLet fullName = not-null

空对象调用

1     //5.空对象调用
2     val sc2 : Scope2 ? = null
3     val name2 = sc2  ?. let {
4         it.name += "fff" //虽然sc2 为 null ,但是由于是sc2 ?. 调用，所以在{}内不用以 ?.方式使用it
5     }
6     Log.e(ScopeTAG, "testLet name2 = $name2 ")

结果

testLet name2 = null

将表达式作为变量引入为局部作用域中

1     //6.将表达式作为变量引入为局部作用域中
2     1 + (2 * 300) .let {
3         Log.e(ScopeTAG, "testLet 1 + (2 * 300) = $it ")
4     }

结果

testLet 1 + (2 * 300) = 600

静态类或者伴生对象.let,访问静态成员

1     //7.静态类或者伴生对象.let,访问静态成员
2     Int.let {
3         //对Int的伴生对象调用let
4         Log.e(ScopeTAG, "testLet Int.MIN_VALUE = ${it.MIN_VALUE} , SIZE_BYTES = ${it.SIZE_BYTES} , it = $it")
5     }

结果

testLet Int.MIN_VALUE = -2147483648 , SIZE_BYTES = 4 , it = kotlin.jvm.internal.IntCompanionObject@e99cd85

5.apply

5.1 作用

 1 /**
 2  * Calls the specified function [block] with `this` value as its receiver and returns `this` value.
 3  *
 4  * For detailed usage information see the documentation for [scope functions](https://kotlinlang.org/docs/reference/scope-functions.html#apply).
 5  */
 6 @kotlin.internal.InlineOnly
 7 public inline fun <T> T.apply(block: T.() -> Unit): T {
 8     contract {
 9         callsInPlace(block, InvocationKind.EXACTLY_ONCE)
10     }
11     block()
12     return this
13 }

　　主要用来对象配置

5.2 测试

配置对象

 1     class Scope3 {
 2         var name    = ""
 3         var age     = 0
 4         var first   = ""
 5         var last    = ""
 6         var address = ""
 7         var phone   = ""
 8 
 9 
10         override fun toString(): String {
11             return "name = $name,age = $age,first = $first,last = $last,address = $address,phone = $phone"
12         }
13     }
14     //1.主要用来配置对象
15     val sc3 = Scope3().apply {
16         name    = "eot"
17         age     = 12
18         first   = "li"
19         last    = "per"
20         address = "ooeoeooe"
21         phone   = "13xxxxxxx"
22     }
23 
24     Log.e("scope_test","testApply sc3 : $sc3")

结果

testApply sc3 : name = eot,age = 12,first = li,last = per,address = ooeoeooe,phone = 13xxxxxxx

静态类或有内部静态类可以使用类名.apply{} ,没有静态内部类的不支持

 1 class Apply{
 2     companion object{
 3         var value = 2046
 4         override fun toString(): String {
 5             return "value = $value"
 6         }
 7     }
 8 }
 9 object Apply2{
10     var value = 99
11     override fun toString(): String { return """value = $value""" }
12 }
13 class Apply3{
14     object Static1{
15         var value1 = 99
16         override fun toString(): String { return """value1 = $value1""" }
17     }
18     object Static2{
19         var value2 = 99
20         override fun toString(): String { return """value2 = $value2""" }
21     }
22 }
23 
24     //2.静态类或有内部静态类可以使用类名.apply{} ,没有静态内部类的不支持
25     val a0 = Apply.apply { value = 11 }
26     Log.e(ScopeTAG,"testApply a0 : $a0")
27 
28     val a4 = Apply2.apply { value = 55 }
29     Log.e(ScopeTAG,"testApply s4 : $a4")
30 
31     val a3 = Apply3.Static1.apply { value1 = 66 }
32     Log.e(ScopeTAG,"testApply a3 : $a3")

结果

testApply a0 : value = 11
testApply s4 : value = 55
testApply a3 : value1 = 66

kotlin内置常用类都有静态内部类

1     val a1 = Int.apply {
2         123
3         12 * 23
4     }
5     val a2 = String.apply { """hello"""}
6     Log.e(ScopeTAG,"testApply a1 : $a1 ,a2 : $a2")

结果

testApply a1 : kotlin.jvm.internal.IntCompanionObject@6a122e8 ,a2 : kotlin.jvm.internal.StringCompanionObject@2aebd01

没有静态内部类的不支持,编译不过

1     //3.没有静态内部类的不支持
2     class Apply4 { var value = 22}
3     val a6 = Apply4.apply { value = 77 }
4     Log.e(ScopeTAG,"testApply a6 : $a6")

结果：编译不过

6.run

6.1 作用

　　这个函数有两个版本

　　全局函数版本

 1 /**
 2  * Calls the specified function [block] and returns its result.
 3  *
 4  * For detailed usage information see the documentation for [scope functions](https://kotlinlang.org/docs/reference/scope-functions.html#run).
 5  */
 6 @kotlin.internal.InlineOnly
 7 public inline fun <R> run(block: () -> R): R {
 8     contract {
 9         callsInPlace(block, InvocationKind.EXACTLY_ONCE)
10     }
11     return block()
12 }

　　扩展函数版本

 1 /**
 2  * Calls the specified function [block] with `this` value as its receiver and returns its result.
 3  *
 4  * For detailed usage information see the documentation for [scope functions](https://kotlinlang.org/docs/reference/scope-functions.html#run).
 5  */
 6 @kotlin.internal.InlineOnly
 7 public inline fun <T, R> T.run(block: T.() -> R): R {
 8     contract {
 9         callsInPlace(block, InvocationKind.EXACTLY_ONCE)
10     }
11     return block()
12 }

　　全局函数版本：运行一段代码，无参数。

　　扩展函数版本 : 对象配置并且计算结果

6.2 测试

全局函数版

1     //1.全局函数版
2     run {
3         Log.e(ScopeTAG,"testRun 全局函数版: 运行代码")
4     }
5     val i3 = 122 * 2 + run { Log.e(ScopeTAG,"testRun 全局函数版: 在表达式中运行语句"); 23 } + 34
6     Log.e(ScopeTAG,"testRun 全局函数版: i3 = $i3 (122 * 2 + 23 + 34)")

结果

testRun 全局函数版: 运行代码
testRun 全局函数版: 在表达式中运行语句
testRun 全局函数版: i3 = 301 (122 * 2 + 23 + 34)

扩展函数版本

1     //2.扩展函数版本
2     class A(var name : String){ fun query() : String = "name = $name" }
3 
4     val result = A("null").run {
5         this.name = "r1"
6         query()
7     }
8     Log.e(ScopeTAG,"testRun result = $result")

结果

testRun result = name = r1

7.with

7.1 作用

 1 /**
 2  * Calls the specified function [block] with the given [receiver] as its receiver and returns its result.
 3  *
 4  * For detailed usage information see the documentation for [scope functions](https://kotlinlang.org/docs/reference/scope-functions.html#with).
 5  */
 6 @kotlin.internal.InlineOnly
 7 public inline fun <T, R> with(receiver: T, block: T.() -> R): R {
 8     contract {
 9         callsInPlace(block, InvocationKind.EXACTLY_ONCE)
10     }
11     return receiver.block()
12 }

　以对象为参数，执行以下操作

7.2 测试

 1 fun testWith(){
 2     //1.全局函数版
 3     with(3) {
 4         Log.e(ScopeTAG,"testWith this = $this")
 5     }
 6 
 7     //2.扩展函数版本
 8     class A(var name : String)
 9 
10     val w1 = A("null")
11     Log.e(ScopeTAG,"testWith r1.name = ${w1.name}")
12     with(w1) {
13         this.name = "w1"
14         Log.e(ScopeTAG,"testWith name = $name")
15     }
16 
17     //3.同理，对静态类或者有伴生对象的类
18     with(Int){
19         Log.e(ScopeTAG, "testWith Int.MIN_VALUE = ${this.MIN_VALUE} , SIZE_BYTES = ${this.SIZE_BYTES} , it = $this")
20     }
21     with(Apply2){
22         Log.e(ScopeTAG, "testWith Apply2.value = ${this.value} , this = $this")
23     }
24 
25     //4.空对象
26     val nullInt : Int? = null
27     with(nullInt){
28         Log.e(ScopeTAG, "testWith nullInt = $this")
29     }
30 
31     //5.在表达式中
32     val ret = 12 + 29 * with(3){ //注意优先级 12 + (29 * 6) = 186
33         this * 2
34     }
35     Log.e(ScopeTAG, "testWith ret = $ret")
36 
37 }

　结果

testWith this = 3
testWith r1.name = null
testWith name = w1
testWith Int.MIN_VALUE = -2147483648 , SIZE_BYTES = 4 , it = kotlin.jvm.internal.IntCompanionObject@3222e75
testWith Apply2.value = 55 , this = value = 55
testWith nullInt = null
testWith ret = 186

8.also

8.1 作用

 1 /**
 2  * Calls the specified function [block] with `this` value as its argument and returns `this` value.
 3  *
 4  * For detailed usage information see the documentation for [scope functions](https://kotlinlang.org/docs/reference/scope-functions.html#also).
 5  */
 6 @kotlin.internal.InlineOnly
 7 @SinceKotlin("1.1")
 8 public inline fun <T> T.also(block: (T) -> Unit): T {
 9     contract {
10         callsInPlace(block, InvocationKind.EXACTLY_ONCE)
11     }
12     block(this)
13     return this
14 }

　　当你在代码中看到 also 时，可以将其理解为“并且用该对象执行以下操作”。

8.2 测试

 1 object Also{ var value = 1 ; override fun toString(): String { return "value = $value" } }
 2 
 3 fun testAlso(){
 4     //1,对象并且执行的
 5     val name = "testAslo-hello"
 6     val sub = name.substringAfter("-").also {
 7         it.toUpperCase()
 8     }
 9     Log.e(ScopeTAG, "testAlso sub = $sub")
10 
11     //2,静态类、伴生对象
12     val a1 = Int.also { 23 }
13     Log.e(ScopeTAG, "testAlso a1 = $a1")
14     val a2 = Also.apply { value = 39 }
15     Log.e(ScopeTAG, "testAlso a2 = $a2")
16 
17     //3,空对象
18     val nil : Also? = null
19     nil?.also {
20         it.value = 122
21     }
22     Log.e(ScopeTAG, "testAlso nil = $nil")
23 
24 }

结果

testAlso sub = hello
testAlso a1 = kotlin.jvm.internal.IntCompanionObject@6a122e8
testAlso a2 = value = 39
testAlso nil = null

9. takeIf、takeUnless

9.1 作用

 1 /**
 2  * Returns `this` value if it satisfies the given [predicate] or `null`, if it doesn't.
 3  */
 4 @kotlin.internal.InlineOnly
 5 @SinceKotlin("1.1")
 6 public inline fun <T> T.takeIf(predicate: (T) -> Boolean): T? {
 7     contract {
 8         callsInPlace(predicate, InvocationKind.EXACTLY_ONCE)
 9     }
10     return if (predicate(this)) this else null
11 }
12 
13 /**
14  * Returns `this` value if it _does not_ satisfy the given [predicate] or `null`, if it does.
15  */
16 @kotlin.internal.InlineOnly
17 @SinceKotlin("1.1")
18 public inline fun <T> T.takeUnless(predicate: (T) -> Boolean): T? {
19     contract {
20         callsInPlace(predicate, InvocationKind.EXACTLY_ONCE)
21     }
22     return if (!predicate(this)) this else null
23 }

　　takeIf 以对象为参数，判断它是否满足某个条件，满足返回对象本身，否则返回null

　　takeUnless 与takeIf相反，判断是否不满足

9.2 测试

 1 fun testTake(){
 2 
 3     val now = SystemClock.elapsedRealtime()
 4     val tif = now.takeIf {
 5         Log.e(ScopeTAG,"testTake takeIf now = $it")
 6         it % 2  == 0L
 7     }
 8     Log.e(ScopeTAG,"testTake tif = $tif")
 9 
10     val tus = now.takeUnless {
11         Log.e(ScopeTAG,"testTake takeUnless now = $it")
12         it % 2 == 0L
13     }
14     Log.e(ScopeTAG,"testTake tus = $tus")
15 }

结果

testTake takeIf now = 5769124
testTake tif = 5769124
testTake takeUnless now = 5769124
testTake tus = null

对象配置并且计算结果

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原文地址：https://www.cnblogs.com/mhbs/p/12113034.html