Kotlin常用函数 let，with，apply，also,run

标签：u0002 run u0010 Kotlin also str main public block

前言

kotlin 开发中常用的几个函数，let，with，run，apply，also ，他们都是范围函数，具体使用场景迷糊，开发中不知道如何使用。这里罗列出来泛型表达式，反编译之后字节码进行对比，方便记忆

函数名	定义inline的结构	函数体内使用的对象	返回值	是否是扩展函数	适用的场景
let	fun <T, R> T.let(block: (T) -> R): R = block(this)	it指代当前对象	闭包	是	适用于处理不为null的操作场景
with	fun <T, R> with(receiver: T, block: T.() -> R): R = receiver.block()	this指代当前对象或者省略	闭包	否	类对象连续调用类属性和方法的位置
run	fun <T, R> T.run(block: T.() -> R): R = block()	this指代当前对象或者省略	闭包	是	适用于let,with函数任何场景。
apply	fun T.apply(block: T.() -> Unit): T { block(); return this }	this指代当前对象或者省略	this	是	builder模式使用地方
also	fun T.also(block: (T) -> Unit): T { block(this); return this }	it指代当前对象	this	是	builder模式使用地方

注意kotlin lambda语法：

block: (T) -> R lambda 表达式声明。
block: T.() -> R 带接收者的lambda表达式声明，这种写法可以在 block 中使用 this 关键字代表 T实参使用

fun main() {

   val str=123
//①    val combinStr=str.let { "$str world" }
//②   val combinStr= with(str) {
//        "$str world"
//    }
//③ val combinStr=str.run { "$str world " }

//④    val combinStr=str.apply { "$str world" }

//⑤    val combinStr=str.also { "$str world" }
    println(combinStr)

}

//泛型 T/R, [入参T返回 R]

//① fun <T, R> T.let(block: (T) -> R): R = block(this)
//② fun <T, R> with(receiver: T, block: T.() -> R): R = receiver.block()
//③ fun <T, R> T.run(block: T.() -> R): R = block()
//④ fun T.apply(block: T.() -> Unit): T { block(); return this }
//⑤ fun T.also(block: (T) -> Unit): T { block(this); return this }

let

//① fun <T, R> T.let(block: (T) -> R): R = block(this)

fun main() {
   val str=123
   val combinStr=str.let { "$str world" }
    println(combinStr)
   }


package letrunwithalsoapply;

import kotlin.Metadata;
import kotlin.jvm.functions.Function1;
import kotlin.jvm.internal.Intrinsics;
import org.jetbrains.annotations.NotNull;

@Metadata(
   mv = {1, 6, 0},
   k = 2,
   xi = 2,
   d1 = {"\u0000\u0012\n\u0000\n\u0002\u0010\u0002\n\u0002\b\u0004\n\u0002\u0018\u0002\n\u0002\b\u0002\u001a\u0006\u0010\u0000\u001a\u00020\u0001\u001a/\u0010\u0002\u001a\u0002H\u0003\"\u0004\b\u0000\u0010\u0004\"\u0004\b\u0001\u0010\u0003*\u0002H\u00042\u0012\u0010\u0005\u001a\u000e\u0012\u0004\u0012\u0002H\u0004\u0012\u0004\u0012\u0002H\u00030\u0006¢\u0006\u0002\u0010\u0007¨\u0006\b"},
   d2 = {"main", "", "let", "R", "T", "block", "Lkotlin/Function1;", "(Ljava/lang/Object;Lkotlin/jvm/functions/Function1;)Ljava/lang/Object;", "untitled2"}
)
public final class LetRunAlsoWithApplyKt {
   public static final void main() {
      final int str = 123;
      String combinStr = (String)let(Integer.valueOf(str), (Function1)(new Function1() {
         // $FF: synthetic method
         // $FF: bridge method
         public Object invoke(Object var1) {
            return this.invoke(((Number)var1).intValue());
         }

         @NotNull
         public final String invoke(int it) {
            return str + " world";
         }
      }));
      System.out.println(combinStr);
   }

   // $FF: synthetic method
   public static void main(String[] var0) {
      main();
   }

   public static final Object let(Object $this$let, @NotNull Function1 block) {
      Intrinsics.checkNotNullParameter(block, "block");
      return block.invoke($this$let);
   }

}

with

//② fun <T, R> with(receiver: T, block: T.() -> R): R = receiver.block()

fun main() {
   val str=123
   val combinStr= with(str) {
        "$str world"
    }
    println(combinStr)
   }



package letrunwithalsoapply;

import kotlin.Metadata;
import kotlin.jvm.functions.Function1;
import kotlin.jvm.internal.Intrinsics;
import org.jetbrains.annotations.NotNull;

@Metadata(
   mv = {1, 6, 0},
   k = 2,
   xi = 2,
   d1 = {"\u0000\u0016\n\u0000\n\u0002\u0010\u0002\n\u0002\b\u0005\n\u0002\u0018\u0002\n\u0002\u0018\u0002\n\u0002\b\u0002\u001a\u0006\u0010\u0000\u001a\u00020\u0001\u001a8\u0010\u0002\u001a\u0002H\u0003\"\u0004\b\u0000\u0010\u0004\"\u0004\b\u0001\u0010\u00032\u0006\u0010\u0005\u001a\u0002H\u00042\u0017\u0010\u0006\u001a\u0013\u0012\u0004\u0012\u0002H\u0004\u0012\u0004\u0012\u0002H\u00030\u0007¢\u0006\u0002\b\b¢\u0006\u0002\u0010\t¨\u0006\n"},
   d2 = {"main", "", "with", "R", "T", "receiver", "block", "Lkotlin/Function1;", "Lkotlin/ExtensionFunctionType;", "(Ljava/lang/Object;Lkotlin/jvm/functions/Function1;)Ljava/lang/Object;", "untitled2"}
)
public final class LetRunAlsoWithApplyKt {
   public static final void main() {
      final int str = 123;
      String combinStr = (String)with(Integer.valueOf(str), (Function1)(new Function1() {
         // $FF: synthetic method
         // $FF: bridge method
         public Object invoke(Object var1) {
            return this.invoke(((Number)var1).intValue());
         }

         @NotNull
         public final String invoke(int $this$with) {
            return str + " world";
         }
      }));
      System.out.println(combinStr);
   }

   // $FF: synthetic method
   public static void main(String[] var0) {
      main();
   }

   public static final Object with(Object receiver, @NotNull Function1 block) {
      Intrinsics.checkNotNullParameter(block, "block");
      return block.invoke(receiver);
   }
}

run

```kotlin

//③ fun <T, R> T.run(block: T.() -> R): R = block()

fun main() {
   val str=123
   val combinStr=str.run { "$str world " }
    println(combinStr)
   }



package letrunwithalsoapply;

import kotlin.Metadata;
import kotlin.jvm.functions.Function1;
import kotlin.jvm.internal.Intrinsics;
import org.jetbrains.annotations.NotNull;

@Metadata(
   mv = {1, 6, 0},
   k = 2,
   xi = 2,
   d1 = {"\u0000\u0016\n\u0000\n\u0002\u0010\u0002\n\u0002\b\u0004\n\u0002\u0018\u0002\n\u0002\u0018\u0002\n\u0002\b\u0002\u001a\u0006\u0010\u0000\u001a\u00020\u0001\u001a4\u0010\u0002\u001a\u0002H\u0003\"\u0004\b\u0000\u0010\u0004\"\u0004\b\u0001\u0010\u0003*\u0002H\u00042\u0017\u0010\u0005\u001a\u0013\u0012\u0004\u0012\u0002H\u0004\u0012\u0004\u0012\u0002H\u00030\u0006¢\u0006\u0002\b\u0007¢\u0006\u0002\u0010\b¨\u0006\t"},
   d2 = {"main", "", "run", "R", "T", "block", "Lkotlin/Function1;", "Lkotlin/ExtensionFunctionType;", "(Ljava/lang/Object;Lkotlin/jvm/functions/Function1;)Ljava/lang/Object;", "untitled2"}
)
public final class LetRunAlsoWithApplyKt {
   public static final void main() {
      final int str = 123;
      String combinStr = (String)run(Integer.valueOf(str), (Function1)(new Function1() {
         // $FF: synthetic method
         // $FF: bridge method
         public Object invoke(Object var1) {
            return this.invoke(((Number)var1).intValue());
         }

         @NotNull
         public final String invoke(int $this$run) {
            return str + " world ";
         }
      }));
      System.out.println(combinStr);
   }

   // $FF: synthetic method
   public static void main(String[] var0) {
      main();
   }

   public static final Object run(Object $this$run, @NotNull Function1 block) {
      Intrinsics.checkNotNullParameter(block, "block");
      return block.invoke($this$run);
   }
}

apply

//④ fun T.apply(block: T.() -> Unit): T { block(); return this }

fun main() {
   val str=123
   val combinStr=str.apply { "$str world" }
    println(combinStr)
   }


import kotlin.Metadata;

@Metadata(
   mv = {1, 6, 0},
   k = 2,
   xi = 2,
   d1 = {"\u0000\b\n\u0000\n\u0002\u0010\u0002\n\u0000\u001a\u0006\u0010\u0000\u001a\u00020\u0001¨\u0006\u0002"},
   d2 = {"main", "", "untitled2"}
)
public final class LetRunAlsoWithApplyKt {
   public static final void main() {
      int str = 123;
      int var4 = false;
      (new StringBuilder()).append(str).append(" world").toString();
      System.out.println(str);
   }

   // $FF: synthetic method
   public static void main(String[] var0) {
      main();
   }
}

also

//⑤fun T.also(block: (T) -> Unit): T { block(this); return this }

fun main() {
   val str=123
    val combinStr=str.also { "$str world" }
    println(combinStr)
   }



package letrunwithalsoapply;

import kotlin.Metadata;

@Metadata(
   mv = {1, 6, 0},
   k = 2,
   xi = 2,
   d1 = {"\u0000\b\n\u0000\n\u0002\u0010\u0002\n\u0000\u001a\u0006\u0010\u0000\u001a\u00020\u0001¨\u0006\u0002"},
   d2 = {"main", "", "untitled2"}
)
public final class LetRunAlsoWithApplyKt {
   public static final void main() {
      int str = 123;
      int var4 = false;
      (new StringBuilder()).append(str).append(" world").toString();
      System.out.println(str);
   }

   // $FF: synthetic method
   public static void main(String[] var0) {
      main();
   }
}

总结

基本上 apply 和 with 所做的事情就是对提供给它的接收者调用扩展函数类型的参数。 apply 函数被声明为这个接收者的扩展函数，而 with 函数把却把它作为第一个参数。另外， apply 返回接收者本身，而 with 返回调用 lambda 后的结果。如果你并不关心返回值，这两个方式是可以互换使用的

Kotlin系列之let、with、run、apply、also函数的使用this还是it？合理使用with操作符

标签：u0002,run,u0010,Kotlin,also,str,main,public,block
From： https://blog.51cto.com/u_15861646/5823433

Kotlin常用函数 let，with，apply，also,run

let

with

run

apply

also

总结

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