JavaFX: handle KeyEvents during Drag&Drop?

Question

I have a JavaFX application that uses drag & drop (as described in this tutorial). This application also uses a KeyEvent-filter to change its state when a certain key is pressed or released.

For example, I use this example from the official Drag & Drop tutorial and add the following handlers during the start method:

public class HelloDragAndDrop extends Application {

    // ...

    @Override public void start(Stage stage) {

        // ...

        stage.addEventFilter(KeyEvent.KEY_PRESSED, event -> {
            if (event.getCode() == KeyCode.R)
                scene.setFill(Color.RED);
        });

        stage.addEventFilter(KeyEvent.KEY_RELEASED, event -> {
            if (event.getCode() == KeyCode.R)
                scene.setFill(Color.LIGHTGREEN);
        });

        // ...
    }

    // ...
}

In general, this will make the background turn red while I hold down the R key and green again when I release it. But here's the problem: no KeyEvents are dispatched during the drag & drop action. Suppose the following user actions:

user presses R -> background turns red
user starts dragging
user releases R -> background remains red
user finishes dragging -> background remains red

The only way to get back to the regular color is to press and release R again.

Main question: is there any way to receive those KeyEvents during the drag&drop action?

WorkAroundQuestion: in case I have to work with the fact that those KeyEvents might be "lost" - is there a way I can do something like boolean isKeyDown(KeyCode keyCode) so i can at least adjust the application's state after the drag&drop action is finished?

(P.S.: this is not the same question as that question as this is about drag&drop while that question is about general drag gestures.)

T. Klein T. Klein · Accepted Answer · 2017-03-17T09:17:32

I would write this as a comment, but unfortunately I lack the reputation for that - with the following code, I can perform the sequence of actions that you suggested, but in step 3, the background switches back to green. Does that already solve your problem? I guess it just has something to do with the placement of the event handlers. What does not work is starting the drag and pressing r, as the background stays as it is if I do that.

import javafx.application.Application;
import javafx.event.EventHandler;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.input.*;
import javafx.scene.paint.Color;
import javafx.scene.text.Text;
import javafx.stage.Stage;

/**
 * Demonstrates a drag-and-drop feature.
 */
public class HelloDragAndDrop extends Application {

    @Override public void start(Stage stage) {
        stage.setTitle("Hello Drag And Drop");

        Group root = new Group();
        Scene scene = new Scene(root, 400, 200);
        scene.setFill(Color.LIGHTGREEN);

        final Text source = new Text(50, 100, "DRAG ME");
        source.setScaleX(2.0);
        source.setScaleY(2.0);

        final Text target = new Text(250, 100, "DROP HERE");
        target.setScaleX(2.0);
        target.setScaleY(2.0);

        stage.addEventFilter(KeyEvent.KEY_PRESSED, event -> {
            if (event.getCode() == KeyCode.R)
                scene.setFill(Color.RED);
        });

        stage.addEventFilter(KeyEvent.KEY_RELEASED, event -> {
            if (event.getCode() == KeyCode.R)
                scene.setFill(Color.LIGHTGREEN);
        });

        source.setOnDragDetected(new EventHandler <MouseEvent>() {
            public void handle(MouseEvent event) {
                /* drag was detected, start drag-and-drop gesture*/
                System.out.println("onDragDetected");

                /* allow any transfer mode */
                Dragboard db = source.startDragAndDrop(TransferMode.ANY);

                /* put a string on dragboard */
                ClipboardContent content = new ClipboardContent();
                content.putString(source.getText());
                db.setContent(content);

                event.consume();
            }
        });

        target.setOnDragOver(new EventHandler <DragEvent>() {
            public void handle(DragEvent event) {
                /* data is dragged over the target */
                System.out.println("onDragOver");

                /* accept it only if it is  not dragged from the same node 
                 * and if it has a string data */
                if (event.getGestureSource() != target &&
                        event.getDragboard().hasString()) {
                    /* allow for both copying and moving, whatever user chooses */
                    event.acceptTransferModes(TransferMode.COPY_OR_MOVE);
                }

                event.consume();
            }
        });

        target.setOnDragEntered(new EventHandler <DragEvent>() {
            public void handle(DragEvent event) {
                /* the drag-and-drop gesture entered the target */
                System.out.println("onDragEntered");
                /* show to the user that it is an actual gesture target */
                if (event.getGestureSource() != target &&
                        event.getDragboard().hasString()) {
                    target.setFill(Color.GREEN);
                }

                event.consume();
            }
        });

        target.setOnDragExited(new EventHandler <DragEvent>() {
            public void handle(DragEvent event) {
                /* mouse moved away, remove the graphical cues */
                target.setFill(Color.BLACK);

                event.consume();
            }
        });

        target.setOnDragDropped(new EventHandler <DragEvent>() {
            public void handle(DragEvent event) {
                /* data dropped */
                System.out.println("onDragDropped");
                /* if there is a string data on dragboard, read it and use it */
                Dragboard db = event.getDragboard();
                boolean success = false;
                if (db.hasString()) {
                    target.setText(db.getString());
                    success = true;
                }
                /* let the source know whether the string was successfully 
                 * transferred and used */
                event.setDropCompleted(success);

                event.consume();
            }
        });

        source.setOnDragDone(new EventHandler <DragEvent>() {
            public void handle(DragEvent event) {
                /* the drag-and-drop gesture ended */
                System.out.println("onDragDone");
                /* if the data was successfully moved, clear it */
                if (event.getTransferMode() == TransferMode.MOVE) {
                    source.setText("");
                }

                event.consume();
            }
        });

        root.getChildren().add(source);
        root.getChildren().add(target);
        stage.setScene(scene);
        stage.show();
    }

    public static void main(String[] args) {
        Application.launch(args);
    }
}

JavaFX: handle KeyEvents during Drag&Drop?

1 Answers