Kotlin常用函數 let，with，apply，also,run

前言

kotlin 開發中常用的幾個函數，let，with，run，apply，also ，他們都是範圍函數，具體使用場景迷糊，開發中不知道如何使用。這裡羅列出來泛型表達式，反編譯之後位元組碼進行對比，友善記憶

函數名	定義inline的結構	函數體内使用的對象	傳回值	是否是擴充函數	适用的場景
let	fun <T, R> T.let(block: (T) -> R): R = block(this)	it指代目前對象	閉包	是	适用于處理不為null的操作場景
with	fun <T, R> with(receiver: T, block: T.() -> R): R = receiver.block()	this指代目前對象或者省略	閉包	否	類對象連續調用類屬性和方法的位置
run	fun <T, R> T.run(block: T.() -> R): R = block()	this指代目前對象或者省略	閉包	是	适用于let,with函數任何場景。
apply	fun T.apply(block: T.() -> Unit): T { block(); return this }	this指代目前對象或者省略	this	是	builder模式使用地方
also	fun T.also(block: (T) -> Unit): T { block(this); return this }	it指代目前對象	this	是	builder模式使用地方

注意kotlin lambda文法：

block: (T) -> R lambda 表達式聲明。

block: T.() -> R 帶接收者的lambda表達式聲明，這種寫法可以在 block 中使用 this 關鍵字代表 T實參使用

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, [入參T傳回 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();
   }
}

Kotlin常用函數 let，with，apply，also,run

let

with

run

apply

also

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