Mouse Enter/Exit Events

Question

Hi,
I've got a question regarding mouse enter/exit events. Specifically, I would like to know why so many mouse enter/exit events are generated. Suppose you stack multiple Panes like this:
@Override
public void start(Stage frame) throws Exception {
     Pane root = new Pane();
     frame.setScene(new Scene(root));
     addEnterExitHandlers(root, "root");
     Pane fst = new Pane();
     root.getChildren().add(fst);
     addEnterExitHandlers(fst, "first");
     Pane snd = new Pane();
     fst.getChildren().add(snd);
     addEnterExitHandlers(snd, "second");
     Rectangle rect = new Rectangle(0, 0, 100, 100);
     snd.getChildren().add(rect);
     addEnterExitHandlers(rect, "rect");
     frame.show();
The addEnterExitHandlers() method simply registers event handlers for MouseEvent.MOUSE_ENTERED_TARGET and MouseEvent.MOUSE_EXITED_TARGET which print the event type's name together with the specified string.
The resulting output looks like this:
root MOUSE_ENTERED
first MOUSE_ENTERED
root MOUSE_ENTERED_TARGET
second MOUSE_ENTERED
first MOUSE_ENTERED_TARGET
root MOUSE_ENTERED_TARGET
rect MOUSE_ENTERED
second MOUSE_ENTERED_TARGET
first MOUSE_ENTERED_TARGET
root MOUSE_ENTERED_TARGET
root MOUSE_EXITED
first MOUSE_EXITED
root MOUSE_EXITED_TARGET
second MOUSE_EXITED
first MOUSE_EXITED_TARGET
root MOUSE_EXITED_TARGET
rect MOUSE_EXITED
second MOUSE_EXITED_TARGET
first MOUSE_EXITED_TARGET
root MOUSE_EXITED_TARGET
This output is not accurate, because I inserted empty lines to visually group related elements.
As you can see, a MOUSE_ENTERED event is fired for every node and all parent nodes receive MOUSE_ENTER_TARGET events. I just thought that it would be sufficient to only fire the last group of enter/exit events, respectively, i.e.:
rect MOUSE_ENTERED
second MOUSE_ENTERED_TARGET
first MOUSE_ENTERED_TARGET
root MOUSE_ENTERED_TARGET
rect MOUSE_EXITED
second MOUSE_EXITED_TARGET
first MOUSE_EXITED_TARGET
root MOUSE_EXITED_TARGET
Is there a specific reason for the behavior?
Best regards,
Matthias

Answer 1

Hi Matthias,
yes, there is a reason for this. In vast majority of use-cases, the entered/exited handlers are added on a node to handle the case when mouse enters/exits the node. The _TARGET variants tell you "this node or some of its children was entered/exited", which is not quite it. So right now, if you want to check that fst was entered, you just do:
fst.setOnMouseEntered(new EventHandler<MouseEvent>() {
  @Override public void handle(MouseEvent e) {
    // your code
If we generated only the last group of events as you suggest, you would have to do:
fst.addEventHandler(MouseEvent.MOUSE_ENTERED_TARGET, new EventHandler<MouseEvent>() {
  @Override public void handle(MouseEvent e) {
    if (e.getTarget() == fst) {
      // your code
Even if we added the convenience handlers for the _TARGET variants (setOnMouseEnteredTarget), having to always check the target would be pretty annoying.
Cheers,
Pavel