Scala - instantiation of a class with curly braces

[Erwin Ramírez Millán]

I am starting with Scala and with ScalaFX, I understand most of the code but I don't understand this code using for the examples in ScalaFx;

where instantiate an anonymous class follow it by curly braces, How this works???

import scalafx.application.JFXApp
import scalafx.application.JFXApp.PrimaryStage
import scalafx.geometry.Insets
import scalafx.scene.Scene
import scalafx.scene.effect.DropShadow
import scalafx.scene.layout.HBox
import scalafx.scene.paint.Color._
import scalafx.scene.paint.{LinearGradient, Stops}
import scalafx.scene.text.Text

object ScalaFXHelloWorld extends JFXApp {

  stage = new PrimaryStage {

    title = "ScalaFX Hello World"

    scene = new Scene {

      fill = Black

      content = new HBox {

        padding = Insets(20)

        children = Seq(
          new Text {
            text = "Hello"
            style = "-fx-font-size: 48pt"
            fill = new LinearGradient(
              endX = 0,
              stops = Stops(PaleGreen, SeaGreen)
            )
          },
          new Text {
            text = "World!!!"
            style = "-fx-font-size: 48pt"
            fill = new LinearGradient(
              endX = 0,
              stops = Stops(Cyan, DodgerBlue)
            )
            effect = new DropShadow {
              color = DodgerBlue
              radius = 25
              spread = 0.25

            }
          }
        )

      }

    }

  }

}

the part I don't understand is why in the creation of an anonymous class is follow by curly braces (with some more statements)(Scene is not a trail to be filling the abstract parts of that class) and even fill or content are functions not a variables and for instant Black for fill is a val meaning that this line

fill = Black

is doing calling a function fill and assigning a val to it(don't make sense for me ), this is fill definition 

def fill: ObjectProperty[jfxsp.Paint] = delegate.fillProperty

and this is Black 

val Black = new Color(jfxsp.Color.BLACK)

how works this instantiation of a new object with curly braces please help, want to understand. This is because ScalaFx is wrapping JavaFx and something special is going on here?. Thank you guys.

[Oliver Ruebenacker]

     Hello,

  You can do it just like this:

scala> class A { var x: Int = 0 }

defined class A

scala> val a = new A { x = 2 }

a: A = $anon$1@1787bc24

  So, new A {...} creates, as you pointed out, an anonymous class. It extends A. Here, constructor of A assigns 0 to x, and then the constructor of the anonymous class re-assigns 2 to x.

     Best, Oliver

--
You received this message because you are subscribed to the Google Groups "scala-user" group.
To unsubscribe from this group and stop receiving emails from it, send an email to scala-user+...@googlegroups.com.
For more options, visit https://groups.google.com/d/optout.

--

Oliver Ruebenacker

Solutions Architect at Altisource Labs

Be always grateful, but never satisfied.

[Jon Pretty]

Hi Erwin,

I'm not familiar with ScalaFX, but I'll try to answer.

You're right that there are a variety of anonymous classes here. In your example, no `val`s, `var`s or `def`s are getting (re)defined, so everything that appears between the curly braces of the anonymous class is side-effecting code that is run during instantiation of those classes.

It *looks* like these calls are reassigning new values to `var`s, but given you've just shown me that, for example, `fill` is implemented by a `def`, this means that it can't be a `var`. A call to `someIdentifier = value` will get translated by the compiler into a call to `someIdentifier_=(value)` if `someIdentifier` is not a `var`. You should find that there's a method defined something like `fill_=(c: Color)`. So where you're seeing `fill = Black`, that's becoming `fill_=(Black)`.

This sort of side-effecting code in constructors is discouraged, though. Not many projects are using this style any more.

Cheers,
Jon