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Kotlin常用函数 let，with，apply，also,run
2022-06-21 16:05:00 【灯塔@kuaidao】
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
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