Callback and extractors for JavaFX ObservableList

Question

Here is the code:

package sample;

import javafx.beans.Observable;
import javafx.beans.property.IntegerProperty;
import javafx.beans.property.SimpleIntegerProperty;
import javafx.collections.FXCollections;
import javafx.collections.ListChangeListener;
import javafx.collections.ObservableList;
import javafx.util.Callback;

import java.util.List;

/**
 * Created by IDEA on 28/07/15.
 */
public class ListUpdateTest {
    public static void main(String[] args) {
        Callback<IntegerProperty, Observable[]> extractor = (IntegerProperty p) -> {
            System.out.println("The extractor is called.");
            Observable[] res = new Observable[]{p};
            System.out.println("Result from extractor: " + res);
            return res;
        };
        ObservableList<IntegerProperty> list = FXCollections.observableArrayList(extractor);
        System.out.printf("Before adding");
        IntegerProperty p1 = new SimpleIntegerProperty(10);
        IntegerProperty p2 = new SimpleIntegerProperty(20);
        list.addAll(p1, p2);
        System.out.println("After adding");
        list.addListener(new ListChangeListener<IntegerProperty>() {
            @Override
            public void onChanged(Change<? extends IntegerProperty> c) {
                System.out.println("List is " + c.getList());
                while (c.next()) {
                    if (c.wasUpdated()) {
                        System.out.println("An update is detected.");
                        int start = c.getFrom();
                        int end = c.getTo();
                        System.out.println(
                                String.format("Updated range: %d to %d",
                                        start, end)
                        );
                        List<? extends IntegerProperty> updatedElementsList;
                        updatedElementsList = c.getList().subList(start, end);
                        System.out.println("Updated elements: " + updatedElementsList);
                    }
                }
            }
        });
        p1.set(100);
    }
}

What is extractor for and why do we need it here?

Answer 1

An ObservableList will fire change events when items are added and removed from the list, whether or not it is created with an extractor.

However, if items in the list either are, or contain references to, observable properties, the list will fire updates when those properties change only if it is constructed with an extractor which returns an array containing references to those properties.

This should demonstrate the difference:

import javafx.beans.Observable;
import javafx.beans.property.IntegerProperty;
import javafx.beans.property.SimpleIntegerProperty;
import javafx.collections.FXCollections;
import javafx.collections.ListChangeListener;
import javafx.collections.ListChangeListener.Change;
import javafx.collections.ObservableList;


public class ListExtractorDemo {
    public static void main(String[] args) {
        ObservableList<IntegerProperty> listWithoutExtractor = 
                FXCollections.observableArrayList();

        ObservableList<IntegerProperty> listWithExtractor = 
                FXCollections.observableArrayList(p -> new Observable[]{p});

        listWithoutExtractor.addListener(createListener("listWithoutExtractor"));
        listWithExtractor.addListener(createListener("listWithExtractor"));

        IntegerProperty p1 = new SimpleIntegerProperty(1);
        IntegerProperty p2 = new SimpleIntegerProperty(2);

        // both lists will fire change events when items are added or removed:
        listWithoutExtractor.addAll(p1, p2);
        listWithExtractor.addAll(p1, p2);

        // only the list with the extractor will fire a change event when the observable value of an element changes:
        p2.set(3);
    }

    private static ListChangeListener<IntegerProperty> createListener(String listId) {
        return (Change<? extends IntegerProperty> c) -> {
            while (c.next()) {
                if (c.wasAdded()) {
                    System.out.println(listId + " added: "+c.getAddedSubList());
                }
                if (c.wasRemoved()) {
                    System.out.println(listId + " removed: "+c.getRemoved());
                }
                if (c.wasUpdated()) {
                    System.out.println(listId + " updated");
                }
            }
        };
    }
}

A typical use case is in @kleopatra's answer to JavaFX 2.0 Choice Box Issue. How to update a choiceBox, which represents a list of objects, when an object is updated?