Application.scala

package org.mucode.scim

import scalafx.Includes._

import scalafx.application.JFXApp
import scalafx.event.ActionEvent
import scalafx.scene.Scene
import scalafx.scene.layout.BorderPane

object Application extends JFXApp {
  stage = new JFXApp.PrimaryStage {
    title.value = "scim"
    scene = new Scene {
      content = new BorderPane {
        top = new MainMenu {
          onBeforeExit = (ae: ActionEvent) => {
            println("BEFORE EXIT")
          }
        }
      }
    }
  }
}

MainMenu.scala

package org.mucode.scim

import javafx.beans.property.ObjectPropertyBase
import javafx.event.EventHandler

import scalafx.Includes._
import scalafx.application.Platform
import scalafx.event.ActionEvent
import scalafx.scene.control.{MenuItem, Menu, MenuBar}

class MainMenu extends MenuBar {
  useSystemMenuBar = true

  lazy val beforeExit = new ObjectPropertyBase[EventHandler[javafx.event.ActionEvent]] {
    def getName = "onBeforeExit"
    def getBean = MainMenu.this
  }

  def onBeforeExit = beforeExit

  def onBeforeExit_=(v: EventHandler[javafx.event.ActionEvent]) {
    onBeforeExit() = v
  }

  menus = List(
    new Menu("File") {
      items = List(
        new MenuItem("Exit") {
          onAction = (ae: ActionEvent) => {
            if(onBeforeExit() != null) onBeforeExit().handle(ae)
            println("EXIT")
            Platform.exit()
          }
        }
      )
    },
    new Menu("View") {},
    new Menu("Help") {}
  )
}