Find changed object in JavaFX ObservableArrayList - object

Question

It took me way too long to set up a listener on one property of the objects in my Observablelist and add a listener to it.
ObservableList<Track> observableResult = FXCollections.observableArrayList((Track tr)-> new Observable[]{tr.selectedProperty()});
observableResult.addListener(new ListChangeListener<Track>() {
#Override
public void onChanged(Change<? extends Track> c) {
c.next();
for(Track k : c.getAddedSubList()){
System.out.println(k.getTrackName());
}
}
});
But I can't seem to be able to locate the actual object the change has been made to. The Change class only appears to support added and removed members, which don't get triggered by the actual changes inside them.
I have a workaround for this, just calling another method that would loop trough the entire ObservableArrayList and get for example, only the selected items, but that gets pretty expensive after I have a couple of thousand objects. Finding the source members that got changed would allow me to just push them to another array and save a bunch of overhead.

Answer 1

You can call getFrom() on the change to get the index of the changed item. I don't think there's a way to actually figure out which property changed (if you have more than one property listed in the extractor) or get the old value, but maybe this is enough.
If you need more, you could consider registering your own listeners with the list to track, which would be tricky but not impossible.
Here's an SSCCE demonstrating the getFrom() call:
import java.util.Random;
import java.util.stream.IntStream;
import javafx.beans.Observable;
import javafx.beans.property.IntegerProperty;
import javafx.beans.property.SimpleIntegerProperty;
import javafx.collections.FXCollections;
import javafx.collections.ListChangeListener.Change;
import javafx.collections.ObservableList;
public class ListChangeListenerTest {
public static void main(String[] args) {
ObservableList<Item> itemList = FXCollections.observableArrayList(item -> new Observable[]{item.valueProperty()});
itemList.addListener((Change<? extends Item> c) -> {
while (c.next()) {
if (c.wasUpdated()) {
int index = c.getFrom();
System.out.println("Updated item at "+index+" new value is "+itemList.get(index).getValue());
}
}
});
IntStream.rangeClosed(1, 1000).mapToObj(Item::new).forEach(itemList::add);
Random rng = new Random();
itemList.get(rng.nextInt(itemList.size())).setValue(rng.nextInt(10000));
}
public static class Item {
private final IntegerProperty value = new SimpleIntegerProperty();
public Item(int value) {
setValue(value);
}
public final IntegerProperty valueProperty() {
return this.value;
}
public final int getValue() {
return this.valueProperty().get();
}
public final void setValue(final int value) {
this.valueProperty().set(value);
}
}
}
Here's a version that manages the listeners on the property manually. Note that
This doesn't use an extractor on the list
The property in the Item bean is constructed passing a reference to the bean that owns the property. This allows the listener on the property to get a reference to the Item (via a bit of ugly downcasting)
This gives a bit more flexibility; e.g. if you wanted to check modifications on multiple properties and perform different actions, this would allow this. As you can see, the listener can also access the old value.
import java.util.Random;
import java.util.stream.IntStream;
import javafx.beans.property.IntegerProperty;
import javafx.beans.property.Property;
import javafx.beans.property.SimpleIntegerProperty;
import javafx.beans.value.ChangeListener;
import javafx.collections.FXCollections;
import javafx.collections.ListChangeListener.Change;
import javafx.collections.ObservableList;
public class ListChangeListenerTest {
public static void main(String[] args) {
ChangeListener<Number> valueListener = (obs, oldValue, newValue) -> {
Item item = (Item) ((Property<?>) obs).getBean();
System.out.println("Value for "+item+" changed from " + oldValue + " to "+newValue);
};
ObservableList<Item> itemList = FXCollections.observableArrayList();
itemList.addListener((Change<? extends Item> change) -> {
while (change.next()) {
if (change.wasAdded()) {
for (Item item : change.getAddedSubList()) {
item.valueProperty().addListener(valueListener);
}
}
if (change.wasRemoved()) {
for (Item item : change.getRemoved()) {
item.valueProperty().removeListener(valueListener);
}
}
}
});
IntStream.rangeClosed(1, 1000).mapToObj(Item::new).forEach(itemList::add);
Random rng = new Random();
itemList.get(rng.nextInt(itemList.size())).setValue(rng.nextInt(10000));
}
public static class Item {
private final IntegerProperty value = new SimpleIntegerProperty(this, "value");
private final String id ;
public Item(int value) {
id = "Item "+value ;
setValue(value);
}
public final IntegerProperty valueProperty() {
return this.value;
}
public final int getValue() {
return this.valueProperty().get();
}
public final void setValue(final int value) {
this.valueProperty().set(value);
}
#Override
public String toString() {
return id ;
}
}
}
Finally, if you want to account for "bulk" updates, you need to implement ObservableList yourself. You can do this by subclassing ModifiableObservableListBase, and the basic idea is pretty straightforward. The implementation is made a bit tedious by having to create the Change object representing the update, but it's not too bad. Here's an example that allows updating a contiguous range:
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.function.Consumer;
import javafx.collections.ListChangeListener.Change;
import javafx.collections.ModifiableObservableListBase;
public class UpdatingObservableList<T> extends ModifiableObservableListBase<T> {
private final List<T> list ;
public UpdatingObservableList(List<T> list) {
this.list = list ;
}
public UpdatingObservableList() {
this(new ArrayList<>());
}
public void updateSublist(int start, int end, Consumer<T> updater) {
if (start < 0) throw new ArrayIndexOutOfBoundsException("Start ("+start+") cannot be < 0");
if (end < start) throw new IllegalArgumentException("End ("+end+") cannot be less than start ("+start+")");
if (end > size()) throw new ArrayIndexOutOfBoundsException("End ("+end+") cannot be greater than list size ("+size()+")");
for (T element : list.subList(start, end)) {
updater.accept(element);
}
fireChange(createUpdate(start, end));
}
#Override
public T get(int index) {
return list.get(index);
}
#Override
public int size() {
return list.size();
}
#Override
protected void doAdd(int index, T element) {
list.add(index, element);
}
#Override
protected T doSet(int index, T element) {
return list.set(index, element);
}
#Override
protected T doRemove(int index) {
return list.remove(index);
}
private Change<T> createUpdate(int start, int end) {
return new Change<T>(this) {
private boolean initialState = true ;
#Override
public boolean next() {
if (initialState) {
initialState = false ;
return true ;
}
return false ;
}
#Override
public void reset() {
initialState = true ;
}
#Override
public int getFrom() {
checkState();
return start ;
}
#Override
public int getTo() {
checkState();
return end ;
}
#Override
public List<T> getAddedSubList() {
checkState();
return Collections.emptyList();
}
#Override
public List<T> getRemoved() {
checkState();
return Collections.emptyList();
}
#Override
protected int[] getPermutation() {
checkState();
return new int[0];
}
#Override
public boolean wasAdded() {
checkState();
return false ;
}
#Override
public boolean wasRemoved() {
checkState();
return false ;
}
#Override
public boolean wasUpdated() {
return true ;
}
#Override
public boolean wasPermutated() {
checkState();
return false ;
}
#Override
public int getRemovedSize() {
checkState();
return 0 ;
}
#Override
public int getAddedSize() {
checkState();
return 0 ;
}
private void checkState() {
if (initialState) {
throw new IllegalStateException("Must call Change.next()");
}
}
};
}
}
and here's a version of the test class that uses this. Note that the update is performed via the list:
import java.util.Random;
import java.util.stream.IntStream;
import javafx.beans.property.IntegerProperty;
import javafx.beans.property.SimpleIntegerProperty;
import javafx.collections.ListChangeListener.Change;
public class ListChangeListenerTest {
public static void main(String[] args) {
UpdatingObservableList<Item> itemList = new UpdatingObservableList<Item>();
itemList.addListener((Change<? extends Item> change) -> {
while (change.next()) {
if (change.wasUpdated()) {
for (int i = change.getFrom() ; i < change.getTo() ; i++) {
System.out.println(itemList.get(i) + " updated - new value: "+itemList.get(i).getValue());
}
}
}
});
IntStream.rangeClosed(1, 1000).mapToObj(Item::new).forEach(itemList::add);
Random rng = new Random();
int start = rng.nextInt(itemList.size());
int end = Math.min(itemList.size(), start + 1 + rng.nextInt(15));
itemList.updateSublist(start, end, item -> item.setValue(rng.nextInt(10000)));
}
public static class Item {
private final IntegerProperty value = new SimpleIntegerProperty(this, "value");
private final String id ;
public Item(int value) {
id = "Item "+value ;
setValue(value);
}
public final IntegerProperty valueProperty() {
return this.value;
}
public final int getValue() {
return this.valueProperty().get();
}
public final void setValue(final int value) {
this.valueProperty().set(value);
}
#Override
public String toString() {
return id ;
}
}
}