I am confused by this javafx ActionEvent code - javafx

Question

According to this code, button("enlarge") will trigger an action which is calling the method enlarge(), which will do: circle.setRadius(circle.getRadius() + 2);
which is just merely changing the radius value. what I don't understand is that how merely changing the radius will somehow make the program redraw the entire circle.
import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.StackPane;
import javafx.scene.layout.HBox;
import javafx.scene.layout.BorderPane;
import javafx.scene.paint.Color;
import javafx.scene.shape.Circle;
import javafx.stage.Stage;
public class ControlCircle extends Application {
private CirclePane1 circlePane = new CirclePane1();
#Override
public void start(Stage primaryStage) {
HBox hBox = new HBox();
hBox.setSpacing(10);
hBox.setAlignment(Pos.CENTER);
Button btEnlarge = new Button("Enlarge");
Button btShrink = new Button("Shrink");
hBox.getChildren().add(btEnlarge);
hBox.getChildren().add(btShrink);
btEnlarge.setOnAction(new EnlargeHandler());
btShrink.setOnAction(new ShrinkHandler());
BorderPane borderPane = new BorderPane();
borderPane.setCenter(circlePane);
borderPane.setBottom(hBox);
BorderPane.setAlignment(hBox, Pos.CENTER);
Scene scene = new Scene(borderPane, 200, 150);
primaryStage.setTitle("ControlCircle");
primaryStage.setScene(scene);
primaryStage.show();
}
class EnlargeHandler implements EventHandler<ActionEvent> {
#Override
public void handle(ActionEvent e) {
circlePane.enlarge();
}
}
class ShrinkHandler implements EventHandler<ActionEvent>{
#Override
public void handle(ActionEvent e) {
circlePane.shrink();
}
}
}
class CirclePane1 extends StackPane{
private Circle circle = new Circle(50);
public CirclePane1() {
getChildren().add(circle);
circle.setStroke(Color.BLACK);
circle.setFill(Color.WHITE);
}
public void enlarge() {
circle.setRadius(circle.getRadius() + 2);
}
public void shrink() {
circle.setRadius(circle.getRadius() > 2 ? circle.getRadius() - 2
: circle.getRadius());
}
}

Answer 1

Looking at the source code of Circle you can see how the radius property is defined:
private final DoubleProperty radius = new DoublePropertyBase() {
#Override
public void invalidated() {
NodeHelper.markDirty(Circle.this, DirtyBits.NODE_GEOMETRY);
NodeHelper.geomChanged(Circle.this);
}
#Override
public Object getBean() {
return Circle.this;
}
#Override
public String getName() {
return "radius";
}
};
When you change the value of the radius property its invalidated() method is called. And this is where the "magic" happens, it calls these two methods:
NodeHelper.markDirty(Circle.this, DirtyBits.NODE_GEOMETRY);
NodeHelper.geomChanged(Circle.this);
Now, I don't know what these methods do specifically - nor do I have the time/desire to study it - but they tell the JavaFX runtime that the Circle needs to be redrawn. This means that the next time a rendering pulse occurs, which may be triggered by these methods, the Circle will be drawn with its new radius.
This is all very optimized and will cause a redraw only when applicable (such as only when part of ascene-graph).