将展开动画添加到TreeView

我有TreeView的这个例子:

import java.util.Arrays;
import java.util.List;
import javafx.application.Application;
import javafx.beans.property.SimpleStringProperty;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.TextField;
import javafx.scene.control.TreeCell;
import javafx.scene.control.TreeItem;
import javafx.scene.control.TreeView;
import javafx.scene.input.KeyCode;
import javafx.scene.input.KeyEvent;
import javafx.scene.layout.VBox;
import javafx.scene.paint.Color;
import javafx.stage.Stage;
import javafx.util.Callback;

public class MainApp extends Application
{

    //private final Node rootIcon = new ImageView(new Image(getClass().getResourceAsStream("picture.png")));
    //private final Image depIcon = new Image(getClass().getResourceAsStream("picture.png"));
    List employees = Arrays.asList(
        new Employee("Ethan Williams", "Sales Department"),
        new Employee("Emma Jones", "Sales Department"),
        new Employee("Michael Brown", "Sales Department"),
        new Employee("Anna Black", "Sales Department"),
        new Employee("Rodger York", "Sales Department"),
        new Employee("Susan Collins", "Sales Department"),
        new Employee("Mike Graham", "IT Support"),
        new Employee("Judy Mayer", "IT Support"),
        new Employee("Gregory Smith", "IT Support"),
        new Employee("Jacob Smith", "Accounts Department"),
        new Employee("Isabella Johnson", "Accounts Department"));
    TreeItem rootNode = new TreeItem<>("MyCompany Human Resources");//, rootIcon);    // Set picture

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

    @Override
    public void start(Stage stage)
    {
        rootNode.setExpanded(true);
        for (Employee employee : employees)
        {
            TreeItem empLeaf = new TreeItem<>(employee.getName());
            boolean found = false;
            for (TreeItem depNode : rootNode.getChildren())
            {
                if (depNode.getValue().contentEquals(employee.getDepartment()))
                {
                    depNode.getChildren().add(empLeaf);
                    found = true;
                    break;
                }
            }
            if (!found)
            {
                TreeItem depNode = new TreeItem<>(
                    employee.getDepartment()//,new ImageView(depIcon)   // Set picture
                );
                rootNode.getChildren().add(depNode);
                depNode.getChildren().add(empLeaf);
            }
        }

        stage.setTitle("Tree View Sample");
        VBox box = new VBox();
        final Scene scene = new Scene(box, 400, 300);
        scene.setFill(Color.LIGHTGRAY);

        TreeView treeView = new TreeView<>(rootNode);
        //treeView.setEditable(true);
        treeView.setCellFactory(new Callback, TreeCell>()
        {
            @Override
            public TreeCell call(TreeView p)
            {
                return new TextFieldTreeCellImpl();
            }
        });

        box.getChildren().add(treeView);
        stage.setScene(scene);
        stage.show();
    }

    private final class TextFieldTreeCellImpl extends TreeCell
    {

        private TextField textField;

        public TextFieldTreeCellImpl()
        {
        }

        @Override
        public void startEdit()
        {
            super.startEdit();

            if (textField == null)
            {
                createTextField();
            }
            setText(null);
            setGraphic(textField);
            textField.selectAll();
        }

        @Override
        public void cancelEdit()
        {
            super.cancelEdit();
            setText((String) getItem());
            setGraphic(getTreeItem().getGraphic());
        }

        @Override
        public void updateItem(String item, boolean empty)
        {
            super.updateItem(item, empty);

            if (empty)
            {
                setText(null);
                setGraphic(null);
            }
            else
            {
                if (isEditing())
                {
                    if (textField != null)
                    {
                        textField.setText(getString());
                    }
                    setText(null);
                    setGraphic(textField);
                }
                else
                {
                    setText(getString());
                    setGraphic(getTreeItem().getGraphic());
                }
            }
        }

        private void createTextField()
        {
            textField = new TextField(getString());
            textField.setOnKeyReleased(new EventHandler()
            {

                @Override
                public void handle(KeyEvent t)
                {
                    if (t.getCode() == KeyCode.ENTER)
                    {
                        commitEdit(textField.getText());
                    }
                    else if (t.getCode() == KeyCode.ESCAPE)
                    {
                        cancelEdit();
                    }
                }
            });
        }

        private String getString()
        {
            return getItem() == null ? "" : getItem().toString();
        }
    }

    public static class Employee
    {

        private final SimpleStringProperty name;
        private final SimpleStringProperty department;

        private Employee(String name, String department)
        {
            this.name = new SimpleStringProperty(name);
            this.department = new SimpleStringProperty(department);
        }

        public String getName()
        {
            return name.get();
        }

        public void setName(String fName)
        {
            name.set(fName);
        }

        public String getDepartment()
        {
            return department.get();
        }

        public void setDepartment(String fName)
        {
            department.set(fName);
        }
    }
}

我想在展开树时添加动画.怎么做到这一点？