On Sun, 4 Oct 2020 09:24:16 GMT, Jeanette Winzenburg <faste...@openjdk.org>
wrote:
>> the problem was (and still is) in MultipleSelectionModelBase:
>>
>> selectedItems = new
>> SelectedItemsReadOnlyObservableList<T>(selectedIndices, () ->
>> getItemCount()) {
>> @Override protected T getModelItem(int index) {
>> return MultipleSelectionModelBase.this.getModelItem(index);
>> }
>> };
>>
>> meaning that the selectedItems change during the removal of items (that's
>> plain wrong!) which shows when removing the
>> items from the start:
>>
>> // removeAll before fix of
>> for (int i = 0; i < size(); ++i) {
>> if (c.contains(get(i))) {
>> remove(i);
>> --i;
>> modified = true;
>> }
>> }
>>
>> doing so is basically valid (note the decremented i). Just the selectedItems
>> look into the underlying model, so **c**
>> is changing during the removal which is a no-go.
>> Returning to the old (pre
>> [JDK-8093144](https://bugs.openjdk.java.net/browse/JDK-8093144)) still makes
>> the tests
>> against it fail (ListViewTest.test_rt35857 f.i.).
>> Still wondering why the detour over the BitSet was choosen as fix (vs. the
>> more natural remove-from-last). The listView
>> test is passing for the bitSet and for the back-to-front approach. Can we
>> imagine a context where the broken
>> selectedItems impl would add wreckage to the latter?
>
>> The listView test is passing for the bitSet and for the back-to-front
>> approach. Can we imagine a context where the
>> broken selectedItems impl would add wreckage to the latter?
>
> To answer my own question: yes. A failing test with the back-to-front
> approach (the existing test in ListViewTest
> modified in having the last item selected)
> @Test public void test_rt35857Back() {
> ObservableList<String> fxList =
> FXCollections.observableArrayList("A", "B", "C");
> final ListView<String> listView = new ListView<String>(fxList);
>
> listView.getSelectionModel().select(2);
>
> ObservableList<String> selectedItems =
> listView.getSelectionModel().getSelectedItems();
> assertEquals(1, selectedItems.size());
> assertEquals("C", selectedItems.get(0));
>
> listView.getItems().removeAll(selectedItems);
> assertEquals(2, fxList.size());
> assertEquals("A", fxList.get(0));
> assertEquals("B", fxList.get(1));
> }
>
> Feels like coming nearer to the why of the BitSet approach: it guards against
> the scenario where changing the current
> list implicitly changes the list given as parameter - in that case, it's
> unsafe to query the parameter list while
> removing items (c.contains simply reports nonsense). This may happen
> whenever the parameter list does some kind of
> live lookup into the current list (such as f.i.
> SelectedItemsReadOnlyObservableList) - which is not as evil as I
> thought yesterday (did it myself in custom selectionModels ;) The BitSet
> solves it by a two-pass approach: first
> collect all items to remove/retain without (that is keep the parameter list
> in a valid state, allowing to safely access
> it), then do the removal without further accessing the (then invalid)
> parameter list. The fix at that time was a
> deliberate decision by the designer of the collections, so the context when
> it happens was deemed a valid use-case.
> Looks like we should **not** revert it.
Nice catch! So now it's clear that the current approach was adopted because the
source list itself can change as a
result of removing an element from the target list in some cases, such as the
one you pointed out above. I filed
[JDK-8254040](https://bugs.openjdk.java.net/browse/JDK-8254040) to add the test
case you listed above so we avoid a
possible future regression.
So a two-pass algorithm is still needed: the first one to collect the elements
to be removed, the second to actually
remove them. While it isn't necessary to use a BitSet to collect the indexes to
be removed, that does seems a
reasonable approach. Unless there is a good reason to change it to some other
two-pass algorithm, it's probably best to
leave it as-is, in which case this PR and the associated JBS issue can be
withdrawn.
-------------
PR: https://git.openjdk.java.net/jfx/pull/305
