Recently I had to answer several questions regarding how to connect an UI built with
the JavaFX SceneBuilder 1.0 Developer Preview to Java Code. So I figured out that a short overview might be helpful. But first, let me state
the obvious.
What is FXML?
To make it short, FXML is an XML based declaration format for JavaFX. JavaFX provides an FXML loader which will parse FXML files and from that construct a graph of Java object. It may sound complex when stated like that but it is actually quite simple. Here is an example of FXML file, which instantiate a StackPane and puts a Button inside it:
--
<?xml version="1.0" encoding="UTF-8"?>
<?import java.lang.*?>
<?import java.util.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>
<?import javafx.scene.paint.*?>
<StackPane prefHeight="150.0" prefWidth="200.0" xmlns:fx="http://javafx.com/fxml">
<children>
<Button mnemonicParsing="false" text="Button" />
</children>
</StackPane>
... and here is
the code I would have had to write if I had chosen to do
the same
thing programatically:
import javafx.scene.control.*;
import javafx.scene.layout.*;
...
final Button button = new Button("Button");
button.setMnemonicParsing(false);
final StackPane stackPane = new StackPane();
stackPane.setPrefWidth(200.0);
stackPane.setPrefHeight(150.0);
stacPane.getChildren().add(button);
As you can see - FXML is rather simple to understand - as it is quite close to
the JavaFX API. So OK FXML is simple, but why would I use it?Well, there are several answers to that - but my own favorite is: because you can make it with SceneBuilder.
What is SceneBuilder?
In short SceneBuilder is a layout tool that will let you graphically build JavaFX user interfaces by dragging and dropping JavaFX components from a library, and save it as an FXML file. SceneBuilder can also be used to load and modify JavaFX scenegraphs declared in FXML. Here is how I made
the small FXML file above:
Start
the JavaFX SceneBuilder 1.0 Developer Preview
In
the Library on
the left hand side, click on 'StackPane' and drag it on
the content view (the white
rectangle)
In
the Library, select a Button and drag it onto
the StackPane on
the content view.
In
the Hierarchy Panel on
the left hand side - select
the StackPane component, then invoke 'Edit > Trim To Selected' from
the menubar
That's it - you can now save, and you will obtain
the small FXML file shown above. Of course this is only a trivial sample, made for
the sake of
the example - and SceneBuilder will let you create much more complex UIs.
So, I have now an FXML file. But what do I do with it? How do I include it in my program? How do I write my main class?
Loading an FXML file with JavaFX
Well, that's
the easy part - because
the piece of code you need to write never changes. You can download and look at
the SceneBuilder samples if you need to get convinced, but here is
the short version:
Create a Java class (let's call it 'Main.java') which extends javafx.application.Application
In
the same directory copy/save
the FXML file you just created using SceneBuilder. Let's name it "simple.fxml"
Now here is
the Java code for
the Main class, which simply loads
the FXML file and puts it as root in a stage's scene.
/*
* Copyright (c) 2012, Oracle and/or its affiliates. All rights reserved.
*/
package simple;
import java.util.logging.Level;
import java.util.logging.Logger;
import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.scene.Scene;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;
public class Main extends Application {
/**
* @param args
the command line arguments
*/
public static void main(String[] args) {
Application.launch(Main.class, (java.lang.String[])null);
}
@Override
public void start(Stage primaryStage) {
try {
StackPane page = (StackPane) FXMLLoader.load(Main.class.getResource("simple.fxml"));
Scene scene = new Scene(page);
primaryStage.setScene(scene);
primaryStage.setTitle("FXML is Simple");
primaryStage.show();
} catch (Exception ex) {
Logger.getLogger(Main.class.getName()).log(Level.SEVERE, null, ex);
}
}
}
Great! Now I only have to use my favorite IDE to compile
the class and run it. But... wait... what does it do? Well nothing. It just displays a button in
the middle of a window. There's no logic attached to it. So how do we do that? How can I connect this button to my application logic? Here is how:
Connection to code
First let's define our application logic. Since this post is only intended to give a very brief overview - let's keep things simple. Let's say that
the only
thing I want to do is print a message on System.out when
the user clicks on my button. To do that, I'll need to register an action handler with my button. And to do that, I'll need to somehow get a handle on my button. I'll need some kind of controller logic that will get my button and add my action handler to it. So how do I get a handle to my button and pass it to my controller? Once again - this is easy: I just need to write a controller class for my FXML.
With each FXML file, it is possible to associate a controller class defined for that FXML. That controller class will make
the link between
the UI (the objects defined in
the FXML) and
the application logic. To each object defined in FXML we can associate an fx:id.
The value of
the id must be unique within
the scope of
the FXML, and is
the name of an instance variable inside
the controller class, in which
the object will be injected. Since I want to have access to my button, I will need to add an fx:id to my button in FXML, and declare an @FXML variable in my controller class with
the same name. In other words - I will need to add fx:id="myButton" to my button in FXML:
--
<Button fx:id="myButton" mnemonicParsing="false" text="Button" />
and declare @FXML private Button myButton in my controller class
@FXML
private Button myButton; // value will be injected by
the FXMLLoader
Let's see how to do this.
Add an fx:id to
the Button object
Load "simple.fxml" in SceneBuilder - if not already done
In
the hierarchy panel (bottom left), or directly on
the content view, select
the Button object.
Open
the Properties sections of
the inspector (right panel) for
the button object
At
the top of
the section, you will see a text field labelled fx:id. Enter myButton in that field and validate.
Associate a controller class with
the FXML file
Still in SceneBuilder, select
the top root object (in our case, that's
the StackPane), and open
the Code section of
the inspector (right hand side)
At
the top of
the section you should see a text field labelled Controller Class. In
the field, type simple.SimpleController. This is
the name of
the class we're going to create manually.
If you save at this point,
the FXML will look like this:
--
<?xml version="1.0" encoding="UTF-8"?>
<?import java.lang.*?>
<?import java.util.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>
<?import javafx.scene.paint.*?>
<StackPane prefHeight="150.0" prefWidth="200.0" xmlns:fx="http://javafx.com/fxml" fx:controller="simple.SimpleController">
<children>
<Button fx:id="myButton" mnemonicParsing="false" text="Button" />
</children>
</StackPane>
As you can see,
the name of
the controller class has been added to
the root object: fx:controller="simple.SimpleController"
Coding
the controller class
In your favorite IDE, create an empty SimpleController.java class. Now what does a controller class looks like? What should we put inside? Well - SceneBuilder will help you there: it will show you an example of controller skeleton tailored for your FXML. In
the menu bar, invoke View > Show Sample Controller Skeleton. A popup appears, displaying a suggestion for
the controller skeleton: copy
the code displayed there, and paste it into your SimpleController.java:
/**
* Sample Skeleton for "simple.fxml" Controller Class
* Use copy/paste to copy paste this code into your favorite IDE
**/
package simple;
import java.net.URL;
import java.util.ResourceBundle;
import javafx.fxml.FXML;
import javafx.fxml.Initializable;
import javafx.scene.control.Button;
public class SimpleController
implements Initializable {
@FXML // fx:id="myButton"
private Button myButton; // Value injected by FXMLLoader
@Override // This method is called by
the FXMLLoader when initialization is complete
public void initialize(URL fxmlFileLocation, ResourceBundle resources) {
assert myButton != null : "fx:id=\"myButton\" was not injected: check your FXML file 'simple.fxml'.";
// initialize your logic here: all @FXML variables will have been injected
}
}
Note that
the code displayed by SceneBuilder is there only for educational purpose: SceneBuilder does not create and does not modify Java files. This is simply a hint of what you can use, given
the fx:id present in your FXML file. You are free to copy all or part of
the displayed code and paste it into your own Java class.
Now at this point, there only remains to add our logic to
the controller class. Quite easy: in
the initialize method, I will register an action handler with my button:
() {
@Override
public void handle(ActionEvent event) {
System.out.println("That was easy, wasn't it?");
}
});
...
--
...
// initialize your logic here: all @FXML variables will have been injected
myButton.setOnAction(new EventHandler<ActionEvent>() {
@Override
public void handle(ActionEvent event) {
System.out.println("That was easy, wasn't it?");
}
});
...
That's it - if you now compile everything in your IDE, and run your application, clicking on
the button should print a message on
the console!
Summary
What happens is that in Main.java,
the FXMLLoader will load simple.fxml from
the jar/classpath, as specified by 'FXMLLoader.load(Main.class.getResource("simple.fxml"))'. When loading simple.fxml,
the loader will find
the name of
the controller class, as specified by
'fx:controller="simple.SimpleController"' in
the FXML. Upon finding
the name of
the controller class,
the loader will create an instance of that class, in which it will try to inject all
the objects that have an fx:id in
the FXML.
Thus, after having created '<Button fx:id="myButton" ... />',
the FXMLLoader will inject
the button instance into
the '@FXML private Button myButton;' instance variable found on
the controller instance. This is because
The instance variable has an @FXML annotation,
The name of
the variable exactly matches
the value of
the fx:id
Finally, when
the whole FXML has been loaded,
the FXMLLoader will call
the controller's initialize method, and our code that registers an action handler with
the button will be executed.
For a complete example, take a look at
the HelloWorld SceneBuilder sample. Also make sure to follow
the SceneBuilder Get Started guide, which will guide you through a much more complete example.
Of course, there are more elegant ways to set up an Event Handler using FXML and SceneBuilder. There are also many different ways to work with
the FXMLLoader. But since it's starting to be very late here, I think it will have to wait for another post.
I hope you have enjoyed
the tour!
--daniel
