Create a path transition with absolute coordinates for a StackPane object

Question

OrangeBlock is an orange block with text inside. It is implemented as a StackPane that contains text on top of a rectangle. (This approach is demonstrated in the documentation for StackPane.)

I've placed an OrangeBlock at coordinates (100, 80) and am now trying to make it travel smoothly to some target coordinates. Unfortunately I get a nasty bump in my path:

For some reason the coordinates in the PathElements are interpreted relative to the orange block.

Why is this? And how can I make my OrangeBlock travel along a path with absolute coordinates? Minimal working example below.

import javafx.animation.PathTransition;
import javafx.application.Application;
import javafx.scene.*;
import javafx.scene.layout.StackPane;
import javafx.scene.paint.Color;
import javafx.scene.shape.*;
import javafx.scene.text.Text;
import javafx.stage.Stage;
import javafx.util.Duration;

public class PathTransitionExample extends Application {
    @Override
    public void start(Stage primaryStage) throws Exception {
        Group root = new Group();

        OrangeBlock block = new OrangeBlock(60, 40);
        block.relocate(100, 80);
        root.getChildren().add(block);

        PathTransition transition = newPathTransitionTo(block, 460, 320);

        primaryStage.setScene(new Scene(root, 600, 400));
        primaryStage.show();
        transition.play();
    }

    private static PathTransition newPathTransitionTo(OrangeBlock block,
            double toX, double toY) {
        double fromX = block.getLayoutX();
        double fromY = block.getLayoutY();

        Path path = new Path();
        path.getElements().add(new MoveTo(fromX, fromY));
        path.getElements().add(new LineTo(toX, toY));

        PathTransition transition = new PathTransition();
        transition.setPath(path);
        transition.setNode(block);
        transition.setDelay(Duration.seconds(1));
        transition.setDuration(Duration.seconds(2));

        return transition;
    }

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

    private static class OrangeBlock extends StackPane {
        public OrangeBlock(int width, int height) {
            Rectangle rectangle = new Rectangle(width, height, Color.ORANGE);
            Text text = new Text("Block");
            getChildren().addAll(rectangle, text);
        }
    }
}

Answer 1

I debugged the JavaFX code out of curiosity. Seems like you are out of luck with a proper solution. Here's what happens:

The PathTransition code has a method interpolate(double frac) which includes:

cachedNode.setTranslateX(x - cachedNode.impl_getPivotX());
cachedNode.setTranslateY(y - cachedNode.impl_getPivotY());

The impl_getPivotX() and impl_getPivotY() methods contain this:

public final double impl_getPivotX() {
    final Bounds bounds = getLayoutBounds();
    return bounds.getMinX() + bounds.getWidth()/2;
}

public final double impl_getPivotY() {
    final Bounds bounds = getLayoutBounds();
    return bounds.getMinY() + bounds.getHeight()/2;
}

So the PathTransition always uses the center of your node for the calculation. In other words this works with e. g. a Circle node, but not with e. g. a Rectangle node. Moreover you need the layoutBounds, so the PathTransition must be created after the bounds were made available.

You can see in the PathTransition code that the calculations are all relative and already involve the layout position. So in your lineTo you have to consider this.

Worth noting is that the LineTo class has a method setAbsolut(boolean). However it doesn't solve your problem.

So my solution to your problem would be

• creating the PathTransition after the primary stage was made visible
• modification of the moveTo and lineTo parameters

This works for me (I added a Rectangle shape to identify the proper bounds visually):

public class PathTransitionExampleWorking2 extends Application {
    @Override
    public void start(Stage primaryStage) throws Exception {

        Group root = new Group();

        Rectangle rect = new  Rectangle( 100, 80, 460-100+60, 320-80+40);
        root.getChildren().add(rect);

        OrangeBlock block = new OrangeBlock(60, 40);
        block.relocate( 100, 80);

        root.getChildren().add(block);

        primaryStage.setScene(new Scene(root, 600, 400));
        primaryStage.show();

        // layout bounds are used in path transition => PathTransition creation must happen when they are available
        PathTransition transition = newPathTransitionTo(block, 460, 320);
        transition.play();
    }

    private static PathTransition newPathTransitionTo(OrangeBlock block, double toX, double toY) {

        double fromX = block.getLayoutBounds().getWidth() / 2;
        double fromY = block.getLayoutBounds().getHeight() / 2;

        toX -= block.getLayoutX() - block.getLayoutBounds().getWidth() / 2;
        toY -= block.getLayoutY() - block.getLayoutBounds().getHeight() / 2;

        Path path = new Path();
        path.getElements().add(new MoveTo(fromX, fromY));
        path.getElements().add(new LineTo(toX, toY));

        PathTransition transition = new PathTransition();
        transition.setPath(path);
        transition.setNode(block);
        transition.setDelay(Duration.seconds(1));
        transition.setDuration(Duration.seconds(2));

        return transition;
    }

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

    private static class OrangeBlock extends StackPane {
        public OrangeBlock(int width, int height) {
            Rectangle rectangle = new Rectangle(width, height, Color.ORANGE);
            Text text = new Text("Block");
            getChildren().addAll(rectangle, text);
        }
    }
}

edit: another solution would be to use this instead of MoveTo and LineTo:

public static class MoveToAbs extends MoveTo {

    public MoveToAbs( Node node) {
        super( node.getLayoutBounds().getWidth() / 2, node.getLayoutBounds().getHeight() / 2);
    }

}

public static class LineToAbs extends LineTo {

    public LineToAbs( Node node, double x, double y) {
        super( x - node.getLayoutX() + node.getLayoutBounds().getWidth() / 2, y - node.getLayoutY() + node.getLayoutBounds().getHeight() / 2);
    }

}

Note: You still have to create the PathTransition after the primaryStage was created.

edit: here's another example with the block moving to the position of the mouse-click:

public class PathTransitionExample extends Application {
    @Override
    public void start(Stage primaryStage) throws Exception {

        Group root = new Group();

        OrangeBlock block = new OrangeBlock(60, 40);
        block.relocate(100, 80);
        root.getChildren().add(block);

        Label label = new Label( "Click on scene to set destination");
        label.relocate(0, 0);
        root.getChildren().add(label);

        Scene scene = new Scene(root, 600, 400);

        scene.addEventFilter(MouseEvent.MOUSE_CLICKED, new EventHandler<Event>() {

            PathTransition transition;

            {
                transition = new PathTransition();
                transition.setNode(block);
                transition.setDuration(Duration.seconds(2));

            }

            @Override
            public void handle(Event event) {

                transition.stop();

                setPositionFixed(block.getLayoutX() + block.getTranslateX(), block.getLayoutY() + block.getTranslateY());

                double x = ((MouseEvent) event).getX();
                double y = ((MouseEvent) event).getY();

                Path path = new Path();
                path.getElements().add(new MoveToAbs( block));
                path.getElements().add(new LineToAbs( block, x, y));

                transition.setPath(path);
                transition.play();

            }

            private void setPositionFixed( double x, double y) {
                block.relocate(x, y);
                block.setTranslateX(0);
                block.setTranslateY(0);
            }

        });

        primaryStage.setScene( scene);
        primaryStage.show();

        PathTransition transition = newPathTransitionTo(block, 460, 320);
        transition.play();

    }

    private static PathTransition newPathTransitionTo(OrangeBlock block, double toX, double toY) {

        Path path = new Path();
        path.getElements().add(new MoveToAbs( block));
        path.getElements().add(new LineToAbs( block, toX, toY));

        PathTransition transition = new PathTransition();
        transition.setPath(path);
        transition.setNode(block);
        transition.setDelay(Duration.seconds(1));
        transition.setDuration(Duration.seconds(2));

        return transition;
    }

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

    private static class OrangeBlock extends StackPane {
        public OrangeBlock(int width, int height) {
            Rectangle rectangle = new Rectangle(width, height, Color.ORANGE);
            Text text = new Text("Block");
            getChildren().addAll(rectangle, text);
        }
    }

    public static class MoveToAbs extends MoveTo {

        public MoveToAbs( Node node) {
            super( node.getLayoutBounds().getWidth() / 2, node.getLayoutBounds().getHeight() / 2);
        }

    }

    public static class LineToAbs extends LineTo {

        public LineToAbs( Node node, double x, double y) {
            super( x - node.getLayoutX() + node.getLayoutBounds().getWidth() / 2, y - node.getLayoutY() + node.getLayoutBounds().getHeight() / 2);
        }

    }
}

Answer 2

The solution I'm using now is to simply offset layoutX and layoutY of the Path in the opposite direction.

private static void offsetPathForAbsoluteCoords(Path path, OrangeBlock block) {
    Node rectangle = block.getChildren().iterator().next();
    double width = rectangle.getLayoutBounds().getWidth();
    double height = rectangle.getLayoutBounds().getHeight();

    path.setLayoutX(-block.getLayoutX() + width / 2);
    path.setLayoutY(-block.getLayoutY() + height / 2);
}

Inserting a call to this method immediately after the Path instantiation fixes the problem.

I'm not really satisfied with this solution. I don't understand why layoutX and layoutY need to be involved at all. Is there a neater way?