Test.kt

import javafx.animation.AnimationTimer
import javafx.application.Application
import javafx.scene.Group
import javafx.scene.Scene
import javafx.scene.SnapshotParameters
import javafx.scene.image.ImageView
import javafx.scene.image.WritableImage
import javafx.scene.paint.Color
import javafx.scene.shape.ClosePath
import javafx.scene.shape.LineTo
import javafx.scene.shape.MoveTo
import javafx.scene.shape.Path
import javafx.stage.Stage

class Test : Application() {
    override fun start(stage: Stage?) {
        val width = 960.0
        val height = 600.0

        val group = Group()

        val wi = WritableImage(width.toInt(), height.toInt())

        group.children += ImageView(wi)

        val path = Path()
        group.children += path

        val params = SnapshotParameters()
        params.fill = Color.TRANSPARENT

        val t = object : AnimationTimer() {
            override fun handle(now: Long) {
                group.snapshot(params, wi)

                path.elements.clear()
                path.stroke = Color.BLACK
                path.elements += MoveTo(0.0, 0.0)
                path.elements += LineTo(0.0, height)
                path.elements += LineTo(width, height)
                path.elements += LineTo(width, 0.0)
                path.elements += LineTo(0.0, 0.0)
                path.elements += ClosePath()
            }
        }
        t.start()

        val scene = Scene(group)

        stage?.width = width
        stage?.height = height

        stage?.scene = scene
        stage?.show()
    }
}

fun main(args: Array<String>) {
    Application.launch(Test::class.java, *args)
}