Chaining value class calls

Question

Is it possible to chain values classes without them actually being instantiated? For example:

import scala.swing._

object SwingPlus {
  implicit class RichComponent(val self: Component) extends AnyVal {
    def clientProps: ClientProperties = new ClientProperties(self)
  }

  implicit class ClientProperties(val self: Component) extends AnyVal {
    def += (kv: (String, Any)): Component = {
      self.peer.putClientProperty(kv._1, kv._2)
      self
    }
  }
}

Use case:

import SwingPlus._
val b = Button("Foo") { println("bar") }
b.clientProps += "JButton.buttonType" -> "textured"
val f = new Frame { contents = but; pack(); open() }

In the call b.clientProps +=, does ClientProperties get instantiated, or are these just pure static (optimised) method calls?

Answer 1

They are static calls. That's what value classes do: http://vptheron.github.io/blog/2013/11/02/a-word-of-implicit-conversions-and-value-classes/

Answer 2

It looks that way:

val b = Button("Foo") { println("bar") }
b.clientProps += "JButton.buttonType" -> "textured"

disassembled:

   18:  invokevirtual   #33; //Method scala/swing/Button$.apply:(Ljava/lang/String;Lscala/Function0;)Lscala/swing/Button;
   21:  putfield    #10; //Field b:Lscala/swing/Button;
   24:  getstatic   #38; //Field SwingPlus$ClientProperties$.MODULE$:LSwingPlus$ClientProperties$;
   27:  getstatic   #43; //Field SwingPlus$RichComponent$.MODULE$:LSwingPlus$RichComponent$;
   30:  getstatic   #48; //Field SwingPlus$.MODULE$:LSwingPlus$;
   33:  aload_0
   34:  invokevirtual   #50; //Method b:()Lscala/swing/Button;
   37:  invokevirtual   #54; //Method SwingPlus$.RichComponent:(Lscala/swing/Component;)Lscala/swing/Component;
   40:  invokevirtual   #57; //Method SwingPlus$RichComponent$.clientProps$extension:(Lscala/swing/Component;)Lscala/swing/Component;
   43:  getstatic   #62; //Field scala/Predef$ArrowAssoc$.MODULE$:Lscala/Predef$ArrowAssoc$;
   46:  getstatic   #67; //Field scala/Predef$.MODULE$:Lscala/Predef$;
   49:  ldc #69; //String JButton.buttonType
   51:  invokevirtual   #73; //Method scala/Predef$.any2ArrowAssoc:(Ljava/lang/Object;)Ljava/lang/Object;
   54:  ldc #75; //String textured
   56:  invokevirtual   #79; //Method scala/Predef$ArrowAssoc$.$minus$greater$extension:(Ljava/lang/Object;Ljava/lang/Object;)Lscala/Tuple2;
   59:  invokevirtual   #83; //Method SwingPlus$ClientProperties$.$plus$eq$extension:(Lscala/swing/Component;Lscala/Tuple2;)Lscala/swing/Component;