How to pass ScrollPane event KEY_PRESSED, KeyCode.LEFT to parent pane? - javafx

Question

I have a StackPane with a single child - ScrollPane. I am trying to handle KEY_PRESSED, KeyCode.LEFT (just an example, I want to handle every arrow key) event on StackPane.
As I am concerned the particular event is consumed on ScrollPane. I would like to prevent that but cannot find any reasonable way.

Answer 1

import javafx.application.Application;
import javafx.event.Event;
import javafx.event.EventDispatcher;
import javafx.scene.Scene;
import javafx.scene.control.ScrollPane;
import javafx.scene.input.KeyCode;
import javafx.scene.input.KeyEvent;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;
public class ScrollPaneDispatcherApp extends Application {
public static void main(String[] args) {
launch(args);
}
#Override
public void start(Stage stage) throws Exception {
StackPane stackPane = new StackPane();
stackPane.setStyle("-fx-backgound-color: red");
ScrollPane scrollPane = new ScrollPane();
scrollPane.setStyle("-fx-background-color: yellow");
stackPane.getChildren().add(scrollPane);
Scene scene = new Scene(stackPane);
stage.setScene(scene);
stage.show();
scrollPane.requestFocus();
EventDispatcher sceneEventDispatcher = scene.getEventDispatcher();
EventDispatcher stackPaneEventDispatcher = stackPane.getEventDispatcher();
EventDispatcher scrollPaneEventDispatcher = scrollPane.getEventDispatcher();
scene.setEventDispatcher((event, tail) -> {
if (KeyEvent.ANY.equals(event.getEventType().getSuperType())) {
System.out.println("DISPATCH\tScene\t\tevent=" + event.getEventType());
}
return sceneEventDispatcher.dispatchEvent(event, tail);
});
stackPane.setEventDispatcher((event, tail) -> {
if (KeyEvent.ANY.equals(event.getEventType().getSuperType())) {
System.out.println("DISPATCH\tStackPane\tevent=" + event.getEventType());
}
return stackPaneEventDispatcher.dispatchEvent(event, tail);
});
scrollPane.setEventDispatcher((event, tail) -> {
if (KeyEvent.ANY.equals(event.getEventType().getSuperType())) {
System.out.println("DISPATCH\tScrollPane\tevent=" + event.getEventType());
}
Event eventToDispatch = scrollPaneEventDispatcher.dispatchEvent(event, tail);
if (KeyEvent.KEY_PRESSED.equals(event.getEventType())) {
if (KeyCode.LEFT.equals(((KeyEvent) event).getCode()) || KeyCode.RIGHT.equals(((KeyEvent) event).getCode())) {
if (eventToDispatch == null) {
return event;
}
}
}
return eventToDispatch;
});
scene.addEventFilter(KeyEvent.ANY,
event -> System.out.println("FILTER\t\tScene\t\tevent=" + event.getEventType()));
stackPane.addEventFilter(KeyEvent.ANY,
event -> System.out.println("FILTER\t\tStackPane\tevent=" + event.getEventType()));
scrollPane.addEventFilter(KeyEvent.ANY,
event -> System.out.println("FILTER\t\tScrollPane\tevent=" + event.getEventType()));
scene.addEventHandler(KeyEvent.ANY,
event -> System.out.println("HANDLER\t\tScene\t\tevent=" + event.getEventType()));
stackPane.addEventHandler(KeyEvent.ANY,
event -> System.out.println("HANDLER\t\tStackPane\tevent=" + event.getEventType()));
scrollPane.addEventHandler(KeyEvent.ANY,
event -> System.out.println("HANDLER\t\tScrollPane\tevent=" + event.getEventType()));
}
}
Proposed solution overrides LEFT, RIGHT arrows KEY_PRESSED events consumption behaviour for ScrollPane. The clue is the new EventDispatcher for ScrollPane. Rest of the code is only for debugging purposes.

Answer 2

By default, the ScrollPane will consume the key press event and the StackPane will not.
If you want to intercept a key press using a parent pane event handler, then you should use an event filter (https://docs.oracle.com/javafx/2/events/filters.htm) to do so (to intercept the event during the capturing phase rather than the bubbling phase).
Read up on event processing (https://docs.oracle.com/javafx/2/events/processing.htm) if you need to understand the capturing versus bubbling concepts.
From docs.oracle.com/javafx/2/events/filters.htm : Event filters enable the parent node to provide common processing for its child nodes or to intercept an event and prevent child nodes from acting on the event