javafx 8 listview first and last row - javafx

Question

Is there a way to determine the first and last visible row of a listview? In other words I'm looking for two indexes into an array that populates a listview which represent the top and the bottom row of the 'display window'.

Answer 1

You could get the VirtualFlow of the ListView which has methods for getting the first and last rows.
Example:
import javafx.application.Application;
import javafx.collections.FXCollections;
import javafx.collections.ObservableList;
import javafx.event.Event;
import javafx.scene.Scene;
import javafx.scene.control.IndexedCell;
import javafx.scene.control.ListView;
import javafx.scene.control.ScrollBar;
import javafx.scene.layout.Priority;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;
import com.sun.javafx.scene.control.skin.VirtualFlow;
public class ListViewSample extends Application {
#Override
public void start(Stage stage) {
VBox box = new VBox();
ListView<Integer> list = new ListView<>();
ObservableList<Integer> items = FXCollections.observableArrayList();
for( int i=0; i < 100; i++) {
items.add(i);
}
list.setItems(items);
box.getChildren().add(list);
VBox.setVgrow(list, Priority.ALWAYS);
Scene scene = new Scene(box, 200, 200);
stage.setScene(scene);
stage.show();
VirtualFlow flow = (VirtualFlow) list.lookup( ".virtual-flow");
flow.addEventFilter(Event.ANY, event -> {
IndexedCell first = flow.getFirstVisibleCellWithinViewPort();
IndexedCell last = flow.getLastVisibleCellWithinViewPort();
System.out.println( list.getItems().get( first.getIndex()) + " - " + list.getItems().get( last.getIndex()) );
});
}
public static void main(String[] args) {
launch(args);
}
}
You see the fully visible first and last items in the console.
ps: I leave the no data check and event handling to you
Alternate version without css lookup:
import javafx.application.Application;
import javafx.collections.FXCollections;
import javafx.collections.ObservableList;
import javafx.event.Event;
import javafx.scene.Node;
import javafx.scene.Scene;
import javafx.scene.control.IndexedCell;
import javafx.scene.control.ListView;
import javafx.scene.control.ScrollBar;
import javafx.scene.layout.Priority;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;
import com.sun.javafx.scene.control.skin.VirtualFlow;
public class ListViewSample extends Application {
ListView<String> list = new ListView<String>();
#Override
public void start(Stage stage) {
VBox box = new VBox();
ListView<Integer> list = new ListView<>();
ObservableList<Integer> items = FXCollections.observableArrayList();
for( int i=0; i < 100; i++) {
items.add(i);
}
list.setItems(items);
box.getChildren().add(list);
VBox.setVgrow(list, Priority.ALWAYS);
Scene scene = new Scene(box, 200, 200);
stage.setScene(scene);
stage.show();
VirtualFlow virtualFlow = null;
for( Node node: list.getChildrenUnmodifiable()) {
if( node instanceof VirtualFlow) {
virtualFlow = (VirtualFlow) node;
}
}
final VirtualFlow flow = virtualFlow;
flow.addEventFilter(Event.ANY, event -> {
IndexedCell first = flow.getFirstVisibleCellWithinViewPort();
IndexedCell last = flow.getLastVisibleCellWithinViewPort();
System.out.println( list.getItems().get( first.getIndex()) + " - " + list.getItems().get( last.getIndex()) );
});
}
public static void main(String[] args) {
launch(args);
}
}

Answer 2

UPDATE
VirtualFlow is available only after the ListView has been rendered, because it uses Layout parameters which are not available until after the ListView is visible on the stage. So I had to make sure that I got the VirtualFlow when it was certain that the ListView had been rendered. Since I was manipulating the list with various methods I call this method at the end of each method:
private VirtualFlow flow;
private void updateListView(int centreIndex) {
if (flow == null)
flow = (VirtualFlow) myListView.lookup(".virtual-flow");
if (flow != null){
IndexedCell first = flow.getFirstVisibleCellWithinViewPort();
IndexedCell last = flow.getLastVisibleCellWithinViewPort();
System.out.println(first.getIndex() + " - " + last.getIndex());
}
// Now the list can be selectively 'redrawn' using the scollTo() method,
// and using the .getSelectionModel().select(centreIndex) to set the
// desired cell
}
It's bit of a hack, but it works. Using layout parameters does have a drawback though that needs to be considered. If the height of the ListView is only 1 pixel less than the total height of all rows, n number of rows will be visible, but the flow will report n-1 rows which will appear to be a discrepancy at first. Hence keeping a fixed layout height is imperative. At least now by using scrollTo(..) I have control over the position of the selected item in the list (I want to keep it centred in the list display when an item is dragged through the list). This solution leaves me feeling uneasy, but it seems to be the only 'simple' way.
Just a note on the odd-looking logic. It seems that getting the flow takes time, while the program keeps executing. The second (flow != null) is necessary to avoid a NullPointerException.

Answer 3

UPDATE 2
My hack turns out not to work. The whole hack is dependent on timing. Rendering is done on a different thread and as soon as I changed the order of instantiation of classes in my app, I got a NullPointerException again. I turned to the Java doc:
"JavaFX is not thread safe and all JavaFX manipulation should be run on the JavaFX processing thread. If you allow a JavaFX application to interact with a thread other than the main processing thread, unpredictable errors will occur"
And they do! So forget the above - it does not work and will make you scratch your head (and more!) trying to debug it ;-)