fun String.addExt(amount: Int = 1)=this + "!".repeat(amount);
fun main(){
    val res="abc".addExt(4)
    println(res)
}

?使用場景：對沒有帶open的類無法進行集成修飾。

或者系統已經定義好的類無法進行常規的修改。所以使用擴展函數

字節碼

import kotlin.Metadata;
import kotlin.Unit;
import kotlin.jvm.internal.Intrinsics;
import kotlin.text.StringsKt;
import org.jetbrains.annotations.NotNull;

@Metadata(
   mv = {1, 6, 0},
   k = 2,
   d1 = {"\u0000\u0014\n\u0000\n\u0002\u0010\u0002\n\u0000\n\u0002\u0010\u000e\n\u0000\n\u0002\u0010\b\n\u0000\u001a\u0006\u0010\u0000\u001a\u00020\u0001\u001a\u0014\u0010\u0002\u001a\u00020\u0001*\u00020\u00032\b\b\u0002\u0010\u0004\u001a\u00020\u0005¨\u0006\u0006"},
   d2 = {"main", "", "addExt", "", "amount", "", "kotlin06"}
)
public final class ExtensionFuncKt {
   public static final void addExt(@NotNull String $this$addExt, int amount) {
      Intrinsics.checkNotNullParameter($this$addExt, "$this$addExt");
      (new StringBuilder()).append($this$addExt).append(StringsKt.repeat((CharSequence)"!", amount)).toString();
   }

   // $FF: synthetic method
   public static void addExt$default(String var0, int var1, int var2, Object var3) {
      if ((var2 & 1) != 0) {
         var1 = 1;
      }

      addExt(var0, var1);
   }

   public static final void main() {
      addExt("abc", 4);
      Unit res = Unit.INSTANCE;
      System.out.println(res);
   }

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

對Any進行擴展

fun String.addExt(amount: Int = 1) = this + "!".repeat(amount);
fun Any.easyPrint() = println(this)
fun main() {
    val res = "abc".addExt(4)
    println(res)
    res.easyPrint()
    15.easyPrint()
}

?

如果想完成這種的鏈式調用

我們就需要對泛型進行擴展

fun String.addExt(amount: Int = 1) = this + "!".repeat(amount);
fun  T.easyPrint(): T {
    println(this)
    return this
}

fun main() {
    val res = "abc".addExt(4)
    println(res)
    res.easyPrint().addExt(4).easyPrint()

}

?那我們再看看系統的let源碼

/**
 * 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.let(block: (T) -> R): R {
    contract {
        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
    }
    return block(this)
}

是不是感覺到了 有一點點的理解。。。