JavaFX get mouse events while drag in drop in progress

Question

I am adding some drag and drop behavior to my application where the user can DnD stuff onto a canvas with custom logic and rendering code. I need to get the mouse's position relative to the canvas (i.e. 0,0 being top-left) but the DragEvent does not let me do this, and mouse events do not get delivered to the canvas during the DnD.

Is it possible to allow the mouse events to be delivered while a drag-and-drop is in progress?

Answer 1

You can do this in a dragOver handler, which is invoked any time the mouse moves on the node on which the handler is registered during the drag.

Simple example:

import java.util.stream.Stream;

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.canvas.Canvas;
import javafx.scene.canvas.GraphicsContext;
import javafx.scene.control.Label;
import javafx.scene.input.ClipboardContent;
import javafx.scene.input.Dragboard;
import javafx.scene.input.TransferMode;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.Pane;
import javafx.scene.layout.VBox;
import javafx.scene.paint.Color;
import javafx.scene.shape.Circle;
import javafx.stage.Stage;

public class DragToCanvas extends Application {

    private Color draggingColor ;

    @Override
    public void start(Stage primaryStage) {
        Pane dragFromPane = new Pane();
        Canvas canvas = new Canvas(600, 600);
        GraphicsContext gc = canvas.getGraphicsContext2D();
        gc.setFill(Color.ANTIQUEWHITE);
        gc.fillRect(0, 0, 600, 600);

        Label coordinateLabel = new Label("[ ]");
        coordinateLabel.setStyle("-fx-font-size: 24;");

        Color[] colors = {Color.RED, Color.GREEN, Color.BLUE, Color.BLACK} ;
        for (int i = 0;  i < colors.length; i++) {
            drawCircle(colors[i], dragFromPane, i);
        }

        canvas.setOnDragOver(e -> {
            if (draggingColor != null 
                    && e.getDragboard().hasString() 
                    && e.getDragboard().getString().equals("circle")) {
                e.acceptTransferModes(TransferMode.COPY);
            }
            coordinateLabel.setText(String.format("[%.1f, %.1f]", e.getX(), e.getY()));
        });

        canvas.setOnDragDropped(e -> {
            if (e.getDragboard().hasString() 
                    && e.getDragboard().getString().equals("circle")) {
                gc.setFill(draggingColor);
                gc.fillOval(e.getX()-25, e.getY()-25, 50, 50);
                draggingColor = null ;
                e.setDropCompleted(true);
            }
        });

        BorderPane root = new BorderPane();
        root.setCenter(canvas);
        root.setLeft(dragFromPane);
        root.setBottom(coordinateLabel);
        Scene scene = new Scene(root);
        primaryStage.setScene(scene);
        primaryStage.show();
    }

    private void drawCircle(Color c, Pane dragFromPane, int index) {
        Circle circle = new Circle(60, 60*(index+1), 25);
        circle.setFill(c);
        dragFromPane.getChildren().add(circle);
        circle.setOnDragDetected(e -> {
            Dragboard db = circle.startDragAndDrop(TransferMode.COPY);
            db.setDragView(circle.snapshot(null, null));
            ClipboardContent cc = new ClipboardContent();
            cc.putString("circle");
            db.setContent(cc);
            draggingColor = c ;
        });

    }

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