Scene Graph behaviour when adding node to multiple scenes//panes - javafx-2

Question

I am currently experimenting with JavaFX's scene graph. I came across a strange problem that i can't really explain. Have a look at the following code:
final BorderPane bp1 = new BorderPane();
final Button button = new Button("CLICK ME");
Scene sc1 = new Scene(bp1,100,100);
bp1.setCenter(button);
stage1.setScene(sc1);
stage1.show();
Stage stage2 = new Stage();
final BorderPane bp2 = new BorderPane();
Scene sc2 = new Scene(bp2,100,100);
stage2.setScene(sc2);
stage2.setX(250);
stage2.show();
bp2.setCenter(button);
bp1.setCenter(button);
What i am trying to do here is to add a node (Button) to a Borderpane which lies within a scene on stage1. At the end of the code i try to add the node to a different scene(pane) on stage2. This actually works. But the last line does not. The button remains on stage2. The strange thing is that if i replace the borderpane with a gridpane, the code works as intended. Why does the borderpane behave different from the gridpane in this situation?
Another question which came to my mind in this case: I assume that there is exactly 1 scene graph per scene (or stage). Is that correct?

Answer 1

I guess you found a bug in the BorderPane implementation. The BorderPane has a bit of peculiar way how it manages its children. You can find the BorderPane implementation here. I suggest you file a bug report at the javafx jira
My testcase:
final BorderPane bp1 = new BorderPane();
final BorderPane bp2 = new BorderPane();
System.out.println("bp1 "+bp1);
System.out.println("bp2 "+bp2);
final Button button = new Button("CLICK ME");
button.parentProperty().addListener(new ChangeListener<Parent>() {
#Override
public void changed(ObservableValue<? extends Parent> observableValue, Parent parent, Parent parent2) {
System.out.println("changed");
System.out.println(button.getParent());
}
});
bp1.centerProperty().addListener(new InvalidationListener() {
#Override
public void invalidated(Observable observable) {
System.out.println("bp1 center invalidated ");
}
});
bp2.centerProperty().addListener(new InvalidationListener() {
#Override
public void invalidated(Observable observable) {
System.out.println("bp2 center invalidated ");
}
});
// bp1.centerProperty().addListener(new ChangeListener<Node>() {
// #Override
// public void changed(ObservableValue<? extends Node> observableValue, Node node, Node node2) {
// System.out.println("bp1 center changed "+node+" -> "+node2);
// }
// });
// bp2.centerProperty().addListener(new ChangeListener<Node>() {
// #Override
// public void changed(ObservableValue<? extends Node> observableValue, Node node, Node node2) {
// System.out.println("bp2 center changed "+node+" -> "+node2);
// }
// });
Scene sc1 = new Scene(bp1,100,100);
bp1.setCenter(button);
primaryStage.setScene(sc1);
primaryStage.show();
Stage stage2 = new Stage();
Scene sc2 = new Scene(bp2,100,100);
stage2.setScene(sc2);
stage2.setX(250);
stage2.show();
bp2.setCenter(button);
bp1.setCenter(button);
output:
bp1 BorderPane#48a80c67
bp2 BorderPane#10c66375
changed
BorderPane#48a80c67[styleClass=root]
bp1 center invalidated
changed
null
changed
BorderPane#10c66375[styleClass=root]
bp2 center invalidated
Explanation:
I found that bp1's center property is not invalidated and thus not updated when the button is added to the second scene. Adding a node to another scene should normally cause it to be removed from the previous scene. However, since BorderPane uses a peculiar way to hold its values (object properties for center, left and so on), after the button is added to the second scene, the button is not removed from the center property of the BorderPane. Setting the button a second time on the same BorderPane doesn't add the button to its children because it thinks that it is already set as the center node.
To your second question:
yes, only one scene graph per stage/scene.