JavaFX ListChangeListener: getPermutation() not working - javafx

Question

I use ListChangeListener to listen to changes in Tab Pane.
private final TabPane tabBar = new TabPane();
...
tabBar.getTabs().addListener(this::tabsChanged);
I'm trying to listen to tab move events with the following code:
private void tabsChanged(ListChangeListener.Change<? extends Tab> change) {
while (change.next()) {
if (change.wasPermutated()) {
for (int i = change.getFrom(); i < change.getTo(); i++) {
System.out.println(i + " -> " + change.getPermutation(i));
}
}
}
}
As JavaFX documentation says:
In order to get the new position of an element, you must call:
change.getPermutation(oldIndex). Returns: the new index of the same
element.
But in my case change.getPermutation(i) always returns just i.
For example, I have 4 tabs.
Their indexes are: 0, 1, 2, 3.
Then I move the 4th tab to the first position.
I expect the following output:
0 -> 1
1 -> 2
2 -> 3
3 -> 0
But I get:
0 -> 0
1 -> 1
2 -> 2
3 -> 3
How can I make it work as I need?

Answer 1

As already noted in the comments: the behavior you observe is a bug just reported as JDK-8278062 - the doc and your expectation based on the doc is correct, the notification (implemented in the internal class TabObservableList) is wrong.
Normally, if we want to find the newIndex, a listChangeListener would do something like:
for (int oldIndex = c.getFrom(); oldIndex < c.getTo(); ++oldIndex) {
int newIndex = c.getPermutation(oldIndex);
...
}
To work around the issue, we could manually keep a copy of the tabs, lookup the tab at the old index and find its new index in the re-ordered tabs:
for (int oldIndex = c.getFrom(); oldIndex < c.getTo(); ++oldIndex) {
Tab tab = copy.get(oldIndex);
int newIndex = c.getList().indexOf(tab);
...
}
// update the copy
Or we could have some fun and implement a TransformationList around the original tabs that does the work for us :) It jumps in when it detects a permutation and fires the correct notification. Note that the only internal class used below is SourceChangeAdapter, we either need to relax encapsulation or c&p its content (it is doing nothing but pass on notifications on behalf of the wrapper)
public class TabObservableListWrapper extends TransformationList<Tab, Tab> {
// copy of source used to build the correct permutation
private ObservableList<Tab> copy = FXCollections.observableArrayList();
public TabObservableListWrapper(ObservableList<Tab> source) {
super(source);
updateCopy();
}
#Override
protected void sourceChanged(Change<? extends Tab> c) {
// TBD: cope with a change that has
// - a mixture of permutation and other subchanges
// - multiple subchanges of type permutation
boolean isPermutation = false;
// check if the change is a permutation
while (c.next()) {
if (c.wasPermutated()) {
isPermutation = true;
break;
}
}
c.reset();
if (isPermutation) {
beginChange();
updatePermutation(c);
endChange();
} else {
// assuming other change type notifications are correct, just delegate
fireChange(new SourceAdapterChange<>(this, c));
}
// keep copy sync'ed to source
updateCopy();
}
/**
* Converts the incorrect permutation notification from source
* into a correct one and let super fire the appropriate change.
*
* Note: this method must be called inside a begin/endChange block.
* #param c a change with a single subChange of type wasPermutated
*/
private void updatePermutation(Change<? extends Tab> c) {
c.next();
int from = c.getFrom();
int to = c.getTo();
int permSize = to - from;
int[] perm = new int[permSize];
// fill the perm
for(int i = 0; i < permSize; i++) {
int oldIndex = from + i;
Tab tab = copy.get(oldIndex);
perm[i] = c.getList().indexOf(tab);
}
nextPermutation(from, to, perm);
}
// keep copy sync'ed
private void updateCopy() {
copy.setAll(getSource());
}
// implement public methods by delegating 1:1 to source
#Override
public int getSourceIndex(int index) {
return index;
}
#Override
public int getViewIndex(int index) {
return index;
}
#Override
public Tab get(int index) {
return getSource().get(index);
}
#Override
public int size() {
return getSource().size();
}
}
To use, wrap it around a tabPane's tab list and listen to the wrapper instead of directly to original list, something like:
TabObservableListWrapper wrapper = new TabObservableListWrapper(tabPane.getTabs());
wrapper.addListener((ListChangeListener<Tab>)change -> {
while (change.next()) {
if (change.wasPermutated()) {
System.out.println("from wrapper:");
for (int oldIndex = change.getFrom(); oldIndex < change.getTo(); oldIndex++) {
System.out.println(oldIndex + " -> " + change.getPermutation(oldIndex));
}
}
}
});