Kotlin Extension — Declaring Extensions as Members

由于官网的例子很全面，因此这里直接套用Kotlin官网的例子：

open class D {
}

class D1 : D() {
}

open class C {
    open fun D.foo() {
        println("D.foo in C")
    }

    open fun D1.foo() {
        println("D1.foo in C")
    }

    fun caller(d: D) {
        d.foo()   // call the extension function
    }
}

class C1 : C() {
    override fun D.foo() {
        println("D.foo in C1")
    }

    override fun D1.foo() {
        println("D1.foo in C1")
    }
}

C().caller(D())   // prints "D.foo in C"
C1().caller(D())  // prints "D.foo in C1" - dispatch receiver is resolved virtually
C().caller(D1())  // prints "D.foo in C" - extension receiver is resolved statically

例子很简单，无非就是Extension的声明和如何调用Extension方法（注意：这里调用Members' Extension 通过了实例方法的转接，可以跟Kotlin Extension - Method中提到的无法在.kt文件中从实例或者外界直接拿到Extension方法引用联系起来）
这里Kotlin定义了两个重要的概念：

dispatch receiver: 指的便是例子中执行caller() 方法的C或者C1实例对象

extension receiver: 指的便是例子中caller() 方法定义中执行Extension方法foo() 的对象

接下来看看三个例子给我们揭示的一些重要现象

1. C().caller(D1())

很明显，我们这里往caller()方法里面传的是extension receiver类型是D1，但是为何最后调用的是父类D的Extension方法呢?
通过反编译生成的.class文件我们找到的原因所在

//C.class
...
public final void caller(com.maxtropy.viewtest.D);
    Code:
       0: aload_1
       1: ldc           #39                 // String d
       3: invokestatic  #15                 // Method kotlin/jvm/internal/Intrinsics.checkParameterIsNotNull:(Ljava/lang/Object;Ljava/lang/String;)V
       6: aload_0
       7: aload_1
       8: invokevirtual #41                 // Method foo:(Lcom/maxtropy/viewtest/D;)V
      11: return
...

//.class where C().caller(D1()) was executed
...
      32: invokespecial #20                 // Method com/maxtropy/viewtest/C."":()V
      35: new           #29                 // class com/maxtropy/viewtest/D1
      38: dup
      39: invokespecial #30                 // Method com/maxtropy/viewtest/D1."":()V
      42: checkcast     #22                 // class com/maxtropy/viewtest/D
      45: invokevirtual #27                 // Method com/maxtropy/viewtest/C.caller:(Lcom/maxtropy/viewtest/D;)V
...

在caller()的入参中，d的参数类型是D，因此在调用caller()时，入参实例的类型就已经被上转型成D了。

2. C1().caller(D())

程序执行的结果很明显的告诉我们这是动态选择的结果，执行的是C1中的Extension方法，而不是父类C中的Extension方法。
哇！在 top-level Extension反编译的例子中很明显的看到Extension方法被编译成了静态方法，这里怎么又跟动态选择有关了呢？？？
同样施以反编译的魔法：

//C.class
Compiled from "C.kt"
public class com.maxtropy.viewtest.C {
  public void foo(com.maxtropy.viewtest.D);
    Code:
       0: aload_1
       1: ldc           #9                  // String $receiver
       3: invokestatic  #15                 // Method kotlin/jvm/internal/Intrinsics.checkParameterIsNotNull:(Ljava/lang/Object;Ljava/lang/String;)V
       6: ldc           #17                 // String D.foo in C
       8: astore_2
       9: getstatic     #23                 // Field java/lang/System.out:Ljava/io/PrintStream;
      12: aload_2
      13: invokevirtual #29                 // Method java/io/PrintStream.println:(Ljava/lang/Object;)V
      16: return

  public void foo(com.maxtropy.viewtest.D1);
    Code:
       0: aload_1
       1: ldc           #9                  // String $receiver
       3: invokestatic  #15                 // Method kotlin/jvm/internal/Intrinsics.checkParameterIsNotNull:(Ljava/lang/Object;Ljava/lang/String;)V
       6: ldc           #35                 // String D1.foo in C
       8: astore_2
       9: getstatic     #23                 // Field java/lang/System.out:Ljava/io/PrintStream;
      12: aload_2
      13: invokevirtual #29                 // Method java/io/PrintStream.println:(Ljava/lang/Object;)V
      16: return

  public final void caller(com.maxtropy.viewtest.D);
    Code:
       0: aload_1
       1: ldc           #39                 // String d
       3: invokestatic  #15                 // Method kotlin/jvm/internal/Intrinsics.checkParameterIsNotNull:(Ljava/lang/Object;Ljava/lang/String;)V
       6: aload_0
       7: aload_1
       8: invokevirtual #41                 // Method foo:(Lcom/maxtropy/viewtest/D;)V
      11: return

  public com.maxtropy.viewtest.C();
    Code:
       0: aload_0
       1: invokespecial #45                 // Method java/lang/Object."":()V
       4: return
}

//C1.class
Compiled from "C.kt"
public final class com.maxtropy.viewtest.C1 extends com.maxtropy.viewtest.C {
  public void foo(com.maxtropy.viewtest.D);
    Code:
       0: aload_1
       1: ldc           #9                  // String $receiver
       3: invokestatic  #15                 // Method kotlin/jvm/internal/Intrinsics.checkParameterIsNotNull:(Ljava/lang/Object;Ljava/lang/String;)V
       6: ldc           #17                 // String D.foo in C1
       8: astore_2
       9: getstatic     #23                 // Field java/lang/System.out:Ljava/io/PrintStream;
      12: aload_2
      13: invokevirtual #29                 // Method java/io/PrintStream.println:(Ljava/lang/Object;)V
      16: return

  public void foo(com.maxtropy.viewtest.D1);
    Code:
       0: aload_1
       1: ldc           #9                  // String $receiver
       3: invokestatic  #15                 // Method kotlin/jvm/internal/Intrinsics.checkParameterIsNotNull:(Ljava/lang/Object;Ljava/lang/String;)V
       6: ldc           #35                 // String D1.foo in C1
       8: astore_2
       9: getstatic     #23                 // Field java/lang/System.out:Ljava/io/PrintStream;
      12: aload_2
      13: invokevirtual #29                 // Method java/io/PrintStream.println:(Ljava/lang/Object;)V
      16: return

  public com.maxtropy.viewtest.C1();
    Code:
       0: aload_0
       1: invokespecial #40                 // Method com/maxtropy/viewtest/C."":()V
       4: return
}

原来声明在Member中的Extension方法全部被编译成了Member所在类型的实例方法！
这实例方法有什么用呢？（再次重复这些实例方法在写.kt时无法通过实例直接拿到引用，也就c.foo()是不能通过编译的）
玄妙便在call()方法的第6、行, 第6行的aload_0指的便是this, 也即是caller()被执行时真正的实例对象dispatch receiver。正是this，让Extension方法具备了动态选择的能力。