Laziness in ScalaFX nodes - scala

Question

Here is an example from Pro ScalaFX:
package proscalafx.ch02.stagecoach
import scalafx.Includes._
import scalafx.application.JFXApp
import scalafx.application.JFXApp.PrimaryStage
import scalafx.beans.property.StringProperty
import scalafx.geometry.VPos
import scalafx.scene.control.{Button, CheckBox, Label, TextField}
import scalafx.scene.input.MouseEvent
import scalafx.scene.layout.{HBox, VBox}
import scalafx.scene.paint.Color
import scalafx.scene.shape.Rectangle
import scalafx.scene.text.Text
import scalafx.scene.{Group, Scene}
import scalafx.stage.{Screen, StageStyle}
/** Stage property example.
*
* Can be run with various command line parameters to control stage style:
* decorated - a solid white background and platform decorations (default).
* transparent - transparent background and no decorations.
* undecorated - a solid white background and no decorations.
* utility - a solid white background and minimal platform decorations used for a utility window.
* #author Rafael
*/
object StageCoachMain extends JFXApp {
val titleProperty = StringProperty("")
// Process command line parameters
val stageStyle = parameters.unnamed match {
case Seq("transparent") => StageStyle.TRANSPARENT
case Seq("undecorated") => StageStyle.UNDECORATED
case Seq("utility") => StageStyle.UTILITY
case _ => StageStyle.DECORATED
}
lazy val textStageX = new Text {
textOrigin = VPos.Top
}
lazy val textStageY = new Text {
textOrigin = VPos.Top
}
lazy val textStageW = new Text {
textOrigin = VPos.Top
}
lazy val textStageH = new Text {
textOrigin = VPos.Top
}
lazy val textStageF = new Text {
textOrigin = VPos.Top
}
lazy val checkBoxResizable = new CheckBox {
text = "resizable"
// disable = stageStyle == StageStyle.TRANSPARENT || stageStyle == StageStyle.UNDECORATED
}
lazy val checkBoxFullScreen = new CheckBox {
text = "fullScreen"
}
lazy val titleTextField = new TextField {
text = "Stage Coach"
prefColumnCount = 15
}
stage = new PrimaryStage {
resizable = false
title <== titleProperty
scene = new Scene(370, 370) {
fill = Color.Transparent
root = new Group {
children = List(
new Rectangle {
width = 350
height = 350
arcWidth = 50
arcHeight = 50
fill = Color.SkyBlue
},
new VBox {
layoutX = 30
layoutY = 20
spacing = 10
children = List(
textStageX,
textStageY,
textStageW,
textStageH,
textStageF,
checkBoxResizable,
checkBoxFullScreen,
new HBox {
spacing = 10
children = List(
new Label("title:"),
titleTextField)
},
new Button {
text = "toBack()"
onAction = handle {stage.toBack()}
},
new Button {
text = "toFront()"
onAction = handle {stage.toFront()}
},
new Button {
text = "close()"
onAction = handle {stage.close()}
}
)
}
)
}
}
}
//when mouse button is pressed, save the initial position of screen
val rootGroup = stage.scene().content(0)
var dragAnchorX = 0.0
var dragAnchorY = 0.0
rootGroup.onMousePressed = (me: MouseEvent) => {
dragAnchorX = me.screenX - stage.x.value
dragAnchorY = me.screenY - stage.y.value
}
rootGroup.onMouseDragged = (me: MouseEvent) => {
stage.x = me.screenX - dragAnchorX
stage.y = me.screenY - dragAnchorY
}
textStageX.text <== new StringProperty("x: ") + stage.x.asString
textStageY.text <== new StringProperty("y: ") + stage.y.asString
textStageW.text <== new StringProperty("width: ") + stage.width.asString
textStageH.text <== new StringProperty("height: ") + stage.height.asString
textStageF.text <== new StringProperty("focused: ") + stage.focused.asString
stage.resizable = false
// NOTE: Due to a bug in JavaFX (2.2.3+) Stage.resizableProperty(), cannot directly use binding here,
// see http://javafx-jira.kenai.com/browse/RT-25942
// TODO: Revert to binding once JavaFX bug is corrected
// stage.resizable <==> checkBoxResizable.selected
checkBoxResizable.selected.onChange {
// To avoid using resizableProperty, use delegate.setResizable()
// stage.resizable = checkBoxResizable.selected.get
stage.delegate.setResizable(checkBoxResizable.selected())
}
checkBoxFullScreen.onAction = handle {
stage.fullScreen = checkBoxFullScreen.selected()
}
stage.title <== titleTextField.text
stage.initStyle(stageStyle)
stage.onCloseRequest = handle {println("Stage is closing")}
val primScreenBounds = Screen.primary.visualBounds
stage.x = (primScreenBounds.width - stage.width()) / 2
stage.y = (primScreenBounds.height - stage.height()) / 2
}
If I remove lazy before these Text objects, the app seems to works exactly the same as before. For example the textStageX object should show x-coordinate of the stage in real-time, and it still does without being lazy. So, what purpose does lazy serve here?
Another problem is that these lines of code
val primScreenBounds = Screen.primary.visualBounds
stage.x = (primScreenBounds.width - stage.width()) / 2
stage.y = (primScreenBounds.height - stage.height()) / 2
seems to intend to place the window at the center of the primary screen, but fails to do so (on OS X 10.10, with Java 1.8 and Scala 2.11.7).

Answer 1

lazy in unnecessary here. It is used in JFXApp if there are initialization order issues.
For the centering to work. The stage has to be shown first. I am not sure it this new in JavaFX 8 or there was a bug in original StageCoachMain code. Just add:
stage.show()
before the last three lines. You can also replace them with simply:
stage.centerOnScreen()
Thanks for pointing this out. I cleaned up the code in ProScalaFX repo.