JavaFX distance between two circle and keep updating property

Question

For assignment, I created 2 draggable circle and connect them with line with javaFX.

I need add text which calculate distance between two circle (or length of line) and that text need to keep updating when I drag circles, but that's where I stuck

    Circle circle1 = new Circle();
    circle1.setCenterX(40);
    circle1.setCenterY(40);
    circle1.setRadius(10);
    Circle circle2 = new Circle();
    circle2.setCenterX(120);
    circle2.setCenterY(150);
    circle2.setRadius(10);
    Line line = new Line ();

    line.startXProperty().bind(circle1.centerXProperty());
    line.startYProperty().bind(circle1.centerYProperty());
    line.endXProperty().bind(circle2.centerXProperty());
    line.endYProperty().bind(circle2.centerYProperty());

    circle1.setOnMousePressed(mousePressEventHandler);
    circle1.setOnMouseDragged(mouseDragEventHandler);
    circle2.setOnMousePressed(mousePressEventHandler);
    circle2.setOnMouseDragged(mouseDragEventHandler);

it's my two circles and line, and I tried

 Text distance = new Text();
 distance.textProperty().bind(circle1.centerXProperty()-circle2.centerXProperty() . . .);

However, as you know as, I can't normally calculate Property value, and I have no idea how should I do it.

Answer 1

You could create a DoubleProperty

DoubleProperty distanceProperty = new SimpleDoubleProperty();

and a ChangeListener in which you calculate the distance

ChangeListener<Number> changeListener = (observable, oldValue, newValue) -> {

  Point2D p1 = new Point2D(circle1.getCenterX(), circle1.getCenterY());
  Point2D p2 = new Point2D(circle2.getCenterX(), circle2.getCenterY());
  distanceProperty.set(p1.distance(p2));

};

assign the listener

circle1.centerXProperty().addListener( changeListener);
circle1.centerYProperty().addListener( changeListener);
circle2.centerXProperty().addListener( changeListener);
circle2.centerYProperty().addListener( changeListener);

and bind the distanceProperty to the text

Text text = new Text();
text.textProperty().bind(distanceProperty.asString());