toFront() throwing UnsupportedOperationException

Question

I want to create a StackPane where, whatever is added, a node of my choice is always at the front (rather than having to be careful about the order I add things in, or remembering to call toFront() on that particular node whenever anything is added.)

In order to do this, I simply place a listener on the child list of the relevant StackPane object, so that whenever anything changes, it calls toFront() on the relevant node, for example:

public class Test extends Application {

    @Override
    public void start(Stage stage) {
        StackPane root = new StackPane();
        final Rectangle r1 = new Rectangle(50, 50);
        root.getChildren().add(r1);
        root.getChildren().addListener(new ListChangeListener<Node>() {
            @Override
            public void onChanged(ListChangeListener.Change<? extends Node> change) {
                try {
                    while(change.next()) {
                        if(change.wasAdded()) {
                            r1.toFront();
                        }
                    }
                }
                catch(Exception ex) {
                    ex.printStackTrace();
                }
            }
        });
        root.getChildren().add(new Rectangle(50, 50));

        stage.setScene(new Scene(root));
        stage.show();
    }
}

In Java 7, this works just fine. However, in JFX8 (latest build downloaded just now), it fails with the following:

java.lang.UnsupportedOperationException
        at java.util.Collections$UnmodifiableList.add(Collections.java:1374)
        at javafx.collections.ListChangeBuilder.nextRemove(ListChangeBuilder.java:208)
        at javafx.collections.ObservableListBase.nextRemove(ObservableListBase.java:150)
        at javafx.collections.ModifiableObservableListBase.remove(ModifiableObservableListBase.java:181)
        at com.sun.javafx.collections.VetoableListDecorator.remove(VetoableListDecorator.java:284)
        at com.sun.javafx.collections.VetoableListDecorator.remove(VetoableListDecorator.java:209)
        at javafx.scene.Parent.impl_toFront(Parent.java:624)
        at javafx.scene.Node.toFront(Node.java:1713)
        at test.Test$1.onChanged(Test.java:34)
        at com.sun.javafx.collections.ListListenerHelper$Generic.fireValueChangedEvent(ListListenerHelper.java:315)
        at com.sun.javafx.collections.ListListenerHelper.fireValueChangedEvent(ListListenerHelper.java:72)
        at com.sun.javafx.collections.VetoableListDecorator$1.onChanged(VetoableListDecorator.java:77)
        at com.sun.javafx.collections.ListListenerHelper$Generic.fireValueChangedEvent(ListListenerHelper.java:315)
        at com.sun.javafx.collections.ListListenerHelper.fireValueChangedEvent(ListListenerHelper.java:72)
        at javafx.collections.ObservableListBase.fireChange(ObservableListBase.java:233)
        at javafx.collections.ListChangeBuilder.commit(ListChangeBuilder.java:482)
        at javafx.collections.ListChangeBuilder.endChange(ListChangeBuilder.java:541)
        at javafx.collections.ObservableListBase.endChange(ObservableListBase.java:205)
        at javafx.collections.ModifiableObservableListBase.add(ModifiableObservableListBase.java:155)
        at java.util.AbstractList.add(AbstractList.java:108)
        at com.sun.javafx.collections.VetoableListDecorator.add(VetoableListDecorator.java:200)
        at test.Test.start(Test.java:41)
        at com.sun.javafx.application.LauncherImpl$8.run(LauncherImpl.java:837)
        at com.sun.javafx.application.PlatformImpl$7.run(PlatformImpl.java:331)
        at com.sun.javafx.application.PlatformImpl$6$1.run(PlatformImpl.java:297)
        at com.sun.javafx.application.PlatformImpl$6$1.run(PlatformImpl.java:294)
        at java.security.AccessController.doPrivileged(Native Method)
        at com.sun.javafx.application.PlatformImpl$6.run(PlatformImpl.java:294)
        at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
        at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
        at com.sun.glass.ui.win.WinApplication.access$300(WinApplication.java:39)
        at com.sun.glass.ui.win.WinApplication$4$1.run(WinApplication.java:112)
        at java.lang.Thread.run(Thread.java:744)

And yes, test.Test$1.onChanged(Test.java:34) does indeed refer to r1.toFront();.

Is this to be considered a bug, or am I breaking some rule I'm unaware of by trying to achieve things this way? I did wonder whether the list was still being changed while the onChanged() method was executing, and toFront() would also change the list contents, hence the exception - but the Javadoc to onChanged() clearly says:

Called after a change has been made to an ObservableList.

(Bolding is mine.)

EDIT: At this point I'm more certain that it's a bug, so the related bug report is here.

Answer 1

It seems that you are not allowed to modify a (JavaFX) list inside an event handler that is currently handling another (previous) modification event of the same list. Although this seems reasonable it is not self-evident, so there should be a more obvious exception in that case.

Unfortunately non-speeking exceptions are very common in JavaFX.

Fortunately the solution/workaround is pretty easy: Call your modifying code (here: r1.toFront) by Platform.runLater(), it will delay your modification to happen after the originating event:

root.getChildren().addListener(new ListChangeListener<Node>() {
    @Override
    public void onChanged(ListChangeListener.Change<? extends Node> change) {
        Platform.runLater(new Runnable() {
            public void run() { r1.toFront(); }                 
        });
    }
});

Sidenote: toFront does nothing, if the component is already at front. This prevents infinite loops. Nevertheless, as this is not explicitely mentioned in the documentation, you might not rely on that.