JavaFX TableView Column CheckBox listener - javafx

Question

I have TableColumn with CheckBox. I have to do this listener and I wonder why the listener doubles after every click.
selectedColumn.setCellFactory(column -> new CheckBoxTableCell<>());
selectedColumn.setCellValueFactory(cellData -> {
Dir dir = cellData.getValue();
BooleanProperty property = dir.isSelectedProperty();
property.addListener((ObservableValue<? extends Boolean> observable, Boolean oldValue, Boolean newValue) ->{
System.out.println(newValue);
});
return property;
});
First click in checkbox i row return:
True
True
Second unselect return:
False
False
False
False
Thrid select return:
True
True
True
True
True
True
why ? :)

Answer 1

This is happening because controls like TableView (and ListView etc) virtualizes its contents. From TableView Javadoc:
The TableView control is designed to visualize an unlimited number of
rows of data, broken out into columns.
The TableView may hold a large number of items. However, on the screen, you could probably see 10-30 rows (each corresponding to a single item) at any point in time.
Without virtualization, if you have 1 million items, it would create 1 million TableRow objects (we haven't talked about TableCell), with each TableRow maintaining all the states/values. This takes a lot of memory and processing power.
On the other hand, with virtualization, if your View can only display 10 rows, TableView will only create, for example, 12 TableRow instances. As you scroll through the list, some of the items disappear from your sight. These TableRow instances are immediately reused for items that enters your sight.
This is why setCellFactory() and setCellValueFactory() methods are of Callback type. Each time a row was reused, it will call this callback object to update the TableCell. This is why your listener is being added repeatedly, causing this.
Depending on what you need, it may be possible to add a ListChangedListener to the list of items.
ObservableList<Dir> list = FXCollections.observableArrayList(item -> new javafx.beans.Observable[] {item.isSelectedProperty()});
list.addAll(DirList.getDirList());
dirList.setItems(list);
list.addListener(new ListChangeListener<Dir>() {
#Override public void onChanged(javafx.collections.ListChangeListener.Change<? extends Dir> c) {
while (c.next()) {
if (c.wasUpdated()) {
// Do something
}
}
}
});

Answer 2

I see you have a model named Dir with a BooleanProperty named selected
You can simply let the cellValueFactory just:
selectedColumn.setCellValueFactory(cellData -> cellData.getValue().isSelectedProperty());
This would update the model's property if you check/un-check the CheckBox in the table cell.
If you use this selected/deselected state of the checkbox, you can use the model, and add the listener there, you will get the same result. Then the listener will be added just once.
You should have instances of Dir created then you can do the following for ex. in initialize:
dir1.selectedProperty().addListener((observable, oldValue, newValue) -> {
dir2.selectedProperty().set(oldValue);
});
Or anything you want, then you are sure that the listener is added just once.
As #Jai mentioned, the cell data is used many time and every time the CallBack is called, the listener is added to the property, so don't use it inside that method if you want to avoid adding the listener many times to the property.