correct way to move a node by dragging in javafx 2?


Question

I'm converting a Swing/Graphics2D app with a lot of custom painting to a JavaFX2 app. Although I absolutely love the new API, I seem to have a performance problem when painting an ellipse that I want to paint below the mouse cursor wherever the mouse is moved. When I move my mouse in a steady way, not ridicously fast, I notice the ellipse is always drawn a few centimeters behind on the mouse trail, and only catches up when I stop moving the cursor. This in a scenegraph with only a handful nodes. In my Swing app I didn't have that problem.

I'm wondering if this is the correct approach for drawing a shape where the mousecursor is?

import javafx.application.Application;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.SceneBuilder;
import javafx.scene.input.MouseEvent;
import javafx.scene.layout.Pane;
import javafx.scene.paint.Color;
import javafx.scene.shape.Ellipse;
import javafx.scene.shape.EllipseBuilder;
import javafx.stage.Stage;

public class TestApp extends Application {
public static void main(String[] args) {
    launch(args);
}

@Override
public void start(Stage primaryStage) throws Exception {
    Pane p = new Pane();

    final Ellipse ellipse = EllipseBuilder.create().radiusX(10).radiusY(10).fill(Color.RED).build();
    p.getChildren().add(ellipse);

    p.setOnMouseMoved(new EventHandler<MouseEvent>() {
        public void handle(MouseEvent event) {
            ellipse.setCenterX(event.getX());
            ellipse.setCenterY(event.getY());
        }
    });

    Scene scene = SceneBuilder.create().root(p).width(1024d).height(768d).build();
    primaryStage.setScene(scene);

    primaryStage.show();
}
}

Small update: I upgraded to JavaFX 2.2 and Java7u6 (on Windows 7 64bit), doesn't seem to make a difference though.

1
22
8/16/2012 7:05:21 PM

Accepted Answer

The lag that you're describing (between your mouse and the dragged shape) is a known JavaFX bug:

https://bugs.openjdk.java.net/browse/JDK-8087922

You can work around it (on Windows, at least) by using an undocumented JVM flag:

-Djavafx.animation.fullspeed=true

This flag is normally for internal performance testing, which is why it is undocumented, but we've been using it for months and haven't had any problems with it so far.

EDIT:

There's another, similar way to workaround this bug that might be a little easier on CPU usage. Simply turn off Prism's vertical sync:

-Dprism.vsync=false

In our app, either of these workarounds solves the lag; there's no need to do both.

7
4/13/2016 4:54:23 PM

Here is some code I use to allow a Label to be dragged around in a Pane. I don't notice any significant lag behind the mouse trail with it.

// allow the label to be dragged around.
final Delta dragDelta = new Delta();
label.setOnMousePressed(new EventHandler<MouseEvent>() {
  @Override public void handle(MouseEvent mouseEvent) {
    // record a delta distance for the drag and drop operation.
    dragDelta.x = label.getLayoutX() - mouseEvent.getSceneX();
    dragDelta.y = label.getLayoutY() - mouseEvent.getSceneY();
    label.setCursor(Cursor.MOVE);
  }
});
label.setOnMouseReleased(new EventHandler<MouseEvent>() {
  @Override public void handle(MouseEvent mouseEvent) {
    label.setCursor(Cursor.HAND);
  }
});
label.setOnMouseDragged(new EventHandler<MouseEvent>() {
  @Override public void handle(MouseEvent mouseEvent) {
    label.setLayoutX(mouseEvent.getSceneX() + dragDelta.x);
    label.setLayoutY(mouseEvent.getSceneY() + dragDelta.y);
  }
});
label.setOnMouseEntered(new EventHandler<MouseEvent>() {
  @Override public void handle(MouseEvent mouseEvent) {
    label.setCursor(Cursor.HAND);
  }
});

. . .

// records relative x and y co-ordinates.
class Delta { double x, y; }

Here is a small complete example app using the above code.

Update The above example, will still lag the object being dragged behind the cursor when the objects being dragged are small.

An alternate approach is to use an ImageCursor comprising of a MousePointer superimposed over the an image representation of the node being dragged, then hide and show the actual node at the start and completion of the drag. This means that the node drag rendering will not lag the cursor (as the image representation of the node is now the cursor). However this approach does have drawbacks => there are restrictions on the size and format of ImageCursors, plus you need to convert your Node to an Image to place it in an ImageCursor, for which you may need advanced Node => Image conversion operations only to available in JavaFX 2.2+.


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