Introducing the Binding API

The Binding API allows you to simplify listeners to just one line of code:

// chapter3/basics/WidthBinding.java
public void start(Stage stage) {
    Label lblWidth = new Label();
 
    lblWidth.textProperty().bind(stage.widthProperty().asString());

    stage.setScene(new Scene(new StackPane(lblWidth), 200, 150));
    stage.show();
}

Let's take a closer look at the binding code:

lblWidth               // object we want to change
  .textProperty()      // property of the object to be changed
  .bind(               // bind() call
     stage             // object we want to monitor
     .widthProperty()  // property of the monitored object we want to track
     .asString());     // assigning Double value to a String property

The bind() method comes from the base Property interface, which has the following API methods:

• bind (ObservableValue<? extends T> observable): Binds a Property to Observable.
• unbind (): Stops binding.
• boolean isBound (): Checks whether a Property is already bound. It's important as only one binding connection can be set for a Property, although this connection can be made as complex as you need, as will be shown later in the chapter.
• bindBidirectional (Property<T> other): Ties two properties together in both directions.
• unbindBidirectional (Property<T> other): Stops the bidirectional binding.

Bidirectional binding connects two properties, essentially making them always have the same value.

Consider the following example:

// chapter3/basics/BidirectionalBindingDemo.java
public void start(Stage stage) {
    Slider s1 = new Slider(0, 100, 40);
    Slider s2 = new Slider(0, 100, 40);
    s2.setOrientation(Orientation.VERTICAL);
    s1.valueProperty().bindBidirectional(s2.valueProperty());

    VBox root = new VBox(5, s1, s2);
    root.setAlignment(Pos.CENTER);
    root.setPadding(new Insets(20));

    stage.setScene(new Scene(root, 200, 150));
    stage.show();
}

The preceding code creates two sliders. You can drag either of them and the other one will mimic the movement: