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 and why is it needed here?

Answer 1

An ObservableList lights up change events when items are added and removed from the list, regardless of whether it is created or not using the extractor.

However, if the items in the list are or contain links to observable properties, the list will trigger updates if these properties are changed only if they are constructed using an extractor that returns an array containing links to these 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 @kleopatra's answer to the JavaFX 2.0 Choice Box Issue. How to update choiceBox, which presents a list of objects when an object is updated?