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