Question

Overview

As a Swing developer of ten years, I’ve been thrilled with the features introduced with JavaFX 2.0, especially the rich, fluent, high-level data-binding facilities. This facility alone is worth the cost of learning a new API (which is much less since abandoning FX script). It’s going to have a direct impact on the readability and maintainably of my model/view synchronization code.

So far I’m having great success at first level and basic derived bindings, but am struggling to figure out the “JavaFX way” of binding one value to a value two or more levels of indirection in the data graph.

Problem

As shown in the code example below, I’m attempting to use javafx.beans.binding.Bindings.select() to synchronize the text value of a Label with one of the contained properties of the currently selected item in a ComboBox. This code is a simple example of something more complex I’m trying to do, so I understand that it’s not hard to do this with the lower level bindings API. I’d like to know if it’s possible with the higher-level fluent API, and if the select(...) method actually tracks changes in the indirect properties (i.e. update property if either the direct property or the selected subproperty change).

The documentation and examples on select(...) are sparse, so I’m hoping someone with advanced experience with this can tell me if I’m trying to use the API as designed, or if there’s another way to use the high-level binding API to do what I want.

Sample Code

Here’s the demo code. When run, there’s a ComboBox with two items in it, and then two labels. The first label shows the toString() version of the selected item. The second label attempts to display one of the properties of the selected item, but only displays null.

import static javafx.beans.binding.Bindings.*;
import javafx.application.Application;
import javafx.beans.property.ReadOnlyStringProperty;
import javafx.beans.property.SimpleStringProperty;
import javafx.beans.property.StringProperty;
import javafx.collections.FXCollections;
import javafx.scene.Scene;
import javafx.scene.control.ComboBox;
import javafx.scene.control.Label;
import javafx.scene.control.Tooltip;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

/** Testing cascading binding change triggers. */
public class SandboxTest extends Application {

    public static void main(String[] args) {
        launch(args);
    }

    @Override
    public void start(Stage stage) throws Exception {

        VBox root = new VBox(8);
        root.setStyle("-fx-padding: 8;");
        Scene s = new Scene(root);

        stage.setWidth(200);
        stage.setScene(s);

        ComboBox<MoPoJo> list = new ComboBox<SandboxTest.MoPoJo>();

        list.itemsProperty().set(FXCollections.observableArrayList(new MoPoJo("foo", "bar"), new MoPoJo("baz", "bat")));

        Label direct = new Label();
        direct.setTooltip(new Tooltip("Selected item to string"));
        Label withSelect = new Label();
        withSelect.setTooltip(new Tooltip("Second property of selected item"));

        direct.textProperty().bind(convert(list.getSelectionModel().selectedItemProperty()));
        withSelect.textProperty().bind(convert(select(list.getSelectionModel().selectedItemProperty(), "two")));

        root.getChildren().addAll(list, direct, withSelect);
        stage.show();
    }

    private static class MoPoJo {
        private StringProperty _one = new SimpleStringProperty();
        private StringProperty _two = new SimpleStringProperty();
        private StringProperty _name = new SimpleStringProperty();

        public MoPoJo(String o, String t) {
            _one.set(o);
            _two.set(t);
            _name.bind(format("{ %s, %s }", oneProperty(), twoProperty()));
        }

        public StringProperty oneProperty() {
            return _one;
        }

        public StringProperty twoProperty() {
            return _two;
        }

        public ReadOnlyStringProperty nameProperty() {
            return _name;
        }

        @Override
        public String toString() {
            return nameProperty().get();
        }
    }

}

Answer 1

Bindings.select can’t access private class. Make MoPoJo a public class and your code will work.

public static class MoPoJo {

P.S: I believe that fact worth to be mentioned in docs, so I filed http://javafx-jira.kenai.com/browse/RT-20640 on JavaFX javadoc.