Evaluation context

By default, expressions are evaluated against the root element of the FXML document. This can be used to bind properties of controls to custom properties defined in the code-behind class. For other use cases, alternative evaluation contexts can be specified:

Selector	Evaluates against
(no notation)	root element, or `fx:context` if set
`:context`	explicit selector for the no-notation default context
`:root`	root element, regardless of `fx:context`
`:element`	current element
`:parent(0)`	current element, equivalent to `:element`
`:parent` or `:parent(1)`	immediate parent of the current element
`:parent(N)`	element `N` levels above the current element
`:parent<MyType>`	nearest ancestor assignable to `MyType`
`:parent<MyType>(0)`	current element, provided it is assignable to `MyType`
`:parent<MyType>(N)`	`N`th matching ancestor, where the nearest matching ancestor is `1`

Context selectors are specified as part of the expression path:

<Rectangle height="${:element.width}"/>

A context selector is a complete expression primary. For example:

:context returns the default evaluation context object
:element returns the current element
:parent returns the immediate parent element
:parent === owner is an identity comparison with another object

The type qualifier and depth can be used independently or together: :parent<Pane> selects the nearest Pane ancestor, while :parent<Pane>(2) selects the second matching Pane ancestor. A depth of 0 always denotes the current element and succeeds for a typed selector only when the current element has the requested type.

Using fx:Evaluate with :element or :parent selectors may lead to unexpected results, since the evaluated value may depend on the order of element initialization.

Consider the following example:
<Pane prefWidth="123">
    <Label prefWidth="$:parent.prefWidth"/>
</Pane>
Perhaps surprisingly, Label.prefWidth will be -1.0 instead of 123.0. The reason for this behavior is that child elements are initialized before parent elements, which means that when the fx:Evaluate expression is evaluated, Pane.prefWidth still has its default value of -1.0.

In cases like these, an observable binding expression should be preferred.

Changing the default evaluation context with `fx:context`

The default evaluation context is the root element of the FXML document. This can be changed with the fx:context attribute, which can be bound to an arbitrary object:

com/sample/MyControl.java

public class MyControl extends MyControlBase {
    final MyBindingContext myContext;

    MyControl() {
        myContext = new MyBindingContext();
        initializeComponent();
    }
}

class MyBindingContext {
    ObjectProperty<User> userProperty();
}

com/sample/MyControl.fxml

<StackPane xmlns="http://javafx.com/javafx" xmlns:fx="http://jfxcore.org/fxml/2.0"
           fx:subclass="com.sample.MyControl"
           fx:context="$myContext">
    <!-- "user.name" will be evaluated against "myContext" -->
    <Label text="${user.name}"/>
</StackPane>

fx:context can be set not only to a specific object, but also be bound to an ObservableValue if the evaluation context is expected to change. Note that this will incur listener management overhead.

Evaluation context

Changing the default evaluation context with fx:context

Changing the default evaluation context with `fx:context`