如何在Java FX中绑定HBox或VBox内容

Question

我有一个带有默认形状的HBox和两个按钮。每个对象都可以生成一个三角形并将其放置在SHAPE ObjectProperty中。我的问题是我没有办法将HBox内容绑定到ObjectProperty。请问有人对如何实现这一目标有想法吗？

package stackOverflow;

    import javafx.application.Application;
    import javafx.application.Platform;
    import javafx.beans.property.ObjectProperty;
    import javafx.beans.property.SimpleObjectProperty;
    import javafx.scene.Scene;
    import javafx.scene.control.Button;
    import javafx.scene.layout.GridPane;
    import javafx.scene.layout.HBox;
    import javafx.scene.paint.Color;
    import javafx.scene.shape.Circle;
    import javafx.scene.shape.Polygon;
    import javafx.stage.Stage;

    public class BoxContent extends Application
    {
        // property accessible to buttons
        ObjectProperty SHAPE = new SimpleObjectProperty();
        // shape method
        Polygon getDownGreenTriangle() {
            Polygon polygon = new Polygon(0.0, 0.0, 15.0, 0.0, 7.5, 15.0);
            polygon.setFill(Color.LIGHTGREEN);                
            return polygon; }
        // shape method
        Polygon getUpRedTriangle() {
            Polygon polygon = new Polygon(0.0, 0.0, 15.0, 0.0, 7.5, 15.0);
            polygon.setFill(Color.RED);
            SHAPE.set(polygon);
            return polygon; }
        // default circle
        Circle circle = new Circle(10, Color.GREY);

        @Override
        public void start(Stage stage) throws Exception {

                // box not available to buttons (how to bind content?)
                HBox hbox = new HBox();
                hbox.getChildren().add(circle);
                // buttons           
                Button GREEN = new Button("Green");
                Button RED = new Button("Red");

                // actions
                GREEN.setOnAction(e -> { 
                    Platform.runLater(() -> {                             
                        SHAPE.set(getDownGreenTriangle());
                    }); }); 
                RED.setOnAction(e -> { 
                    Platform.runLater(() -> { 
                        SHAPE.set(getDownGreenTriangle());
                    }); }); 


                // pane
                GridPane root = new GridPane();
                root.setHgap(10);
                root.setVgap(10);
                root.add(hbox, 1, 1);
                root.add(RED, 2, 1);
                root.add(GREEN, 3, 1);

                Scene scene = new Scene(root, 160, 45);
                stage.setScene(scene);
                stage.show();
            } // end start

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