How to understand and use `<fx:root>` , in JavaFX?

Question

It says tag fx:root has been added to javafx 2.2, but I don't understand how to use it, although with this sample: http://docs.oracle.com/javafx/2/fxml_get_started/whats_new2.htm

main.xml

<?import javafx.scene.layout.GridPane?>
<GridPane fx:controller="sample.Controller"
          xmlns:fx="http://javafx.com/fxml" alignment="center" hgap="10" vgap="10">
    <fx:include fx:id="editorPane" source="editor.fxml"/>
</GridPane>

editor.fxml without fx:root:

<?import javafx.scene.control.TextArea?>
<TextArea fx:id="editor" prefWidth="500" prefHeight="400" 
   fx:controller="sample.EditorController"
   xmlns:fx="http://javafx.com/fxml"/>

editor.fxml with fx:root:

<fx:root type="javafx.scene.control.TextArea"
     fx:id="editor" prefWidth="500" prefHeight="400"
     fx:controller="sample.EditorController"
     xmlns:fx="http://javafx.com/fxml"/>

Actually, I can't find any difference for the two kinds of code. Do I miss anything?

Answer 1

<fx:root> provides a solution to the issue of defining a reusable component with FXML.

As an example, imagine you want to define a simple custom component consisting of a TextField and Button contained in an HBox. You need this to be represented by a subclass of Node, so you can write code like

VBox vbox = new VBox();
vbox.getChildren().add(new MyComponent());

The issue is you need a Java class which is a subclass of Node, as well as the FXML. In pure Java (no FXML), you could do this with:

public class MyComponent extends HBox {
    private TextField textField ;
    private Button button ;

    public MyComponent() {
        textField = new TextField();
        button = new Button();
        this.getChildren().addAll(textField, button);
    }
}

Using FXML to define the custom component without the <fx:root> element presents a problem, because you need the FXML to be some kind of node, and then another node instance to represent the class wrapping it:

<HBox>
<TextField fx:id="textField"/>
<Button fx:id="button" />
</HBox>

and

public class MyComponent extends HBox {
    @FXML
    private TextField textField ;
    @FXML
    private Button button ;
    public MyComponent() {
        try {
            FXMLLoader loader = new FXMLLoader(getClass().getResource("MyComponent.fxml"));
            loader.setController(this);
            HBox hbox = loader.load();
            this.getChildren().add(hbox);
        } catch (IOException exc) {
            // handle exception
        }
    }
}

This results in MyComponent consisting of an HBox wrapping an HBox wrapping the TextField and Button. The additional, redundant HBox is a result of needing one Node for the FXML root and one Node to represent the component.

<fx:root> gives a mechanism to create the Node as the component (the Java class) and then to instruct the FXML file to use that node as its root:

<fx:root type="javafx.scene.layout.HBox">
<TextField fx:id="textField" />
<Button fx:id="button" />
</fx:root>

and

public class MyComponent extends HBox {
    @FXML 
    private TextField textField ;
    @FXML
    private Button button ;
    public MyComponent() {
        try {
            FXMLLoader loader = new FXMLLoader(getClass().getResource("MyComponent.fxml"));
            loader.setController(this);
            loader.setRoot(this);
            loader.load();
        } catch (IOException exc) {
            // handle exception
        }
    }
}

Now MyComponent has the same structure as the original all-Java version, an HBox containing a TextField and a Button. You can't do this using FXML without the <fx:root> element.