How to get JavaFX TreeView to behave consistently upon node expansion? - javafx

Question

I have a JavaFX TreeView with an invisible root and a handful of 'folder' TreeItems that have many 'file' TreeItems as children. The 'folder' TreeItems typically fit inside the TreeView without there being any scrollbars.
invisible-root/
folder/
folder/
folder/
file
file
file
...
file
Sometimes, when I expand a 'folder' TreeItem, the scrollbars appear but the scroll position remains the same. (This is what I want!) However, sometimes, expanding a TreeItem causes the scrollbars appear and the TableView scrolls to the last child of the expanded TreeItem!
This is very unexpected and surprising, especially since I have difficulty predicting which of the two behaviors I will see: (1) stay put, or (2) scroll to last item. Personally, I think behavior (1) is less surprising and preferable.
Any thoughts on how to deal with this?
I see this behavior on Java8u31.

Answer 1

The problem is in VirtualFlow. In layoutChildren() there is this section:
if (lastCellCount != cellCount) {
// The cell count has changed. We want to keep the viewport
// stable if possible. If position was 0 or 1, we want to keep
// the position in the same place. If the new cell count is >=
// the currentIndex, then we will adjust the position to be 1.
// Otherwise, our goal is to leave the index of the cell at the
// top consistent, with the same translation etc.
if (position == 0 || position == 1) {
// Update the item count
// setItemCount(cellCount);
} else if (currentIndex >= cellCount) {
setPosition(1.0f);
// setItemCount(cellCount);
} else if (firstCell != null) {
double firstCellOffset = getCellPosition(firstCell);
int firstCellIndex = getCellIndex(firstCell);
// setItemCount(cellCount);
adjustPositionToIndex(firstCellIndex);
double viewportTopToCellTop = -computeOffsetForCell(firstCellIndex);
adjustByPixelAmount(viewportTopToCellTop - firstCellOffset);
}
The problem arises if position is 1.0 (== scrolled to bottom), because in that case there is no recalculation. A workaround would be to override the TreeViewSkin to provide your own VirtualFlow and fix the behavior there.
The code below is meant to illustrate the problem, it's not a real solution, just a starting point if you really want to fix it:
import com.sun.javafx.scene.control.skin.TreeViewSkin;
import com.sun.javafx.scene.control.skin.VirtualFlow;
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.IndexedCell;
import javafx.scene.control.Skin;
import javafx.scene.control.TreeItem;
import javafx.scene.control.TreeView;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;
public class TreeViewScrollBehaviour extends Application {
#Override
public void start(Stage primaryStage) {
TreeView treeView = new TreeView() {
#Override
protected Skin createDefaultSkin() {
return new TTreeViewSkin(this); //To change body of generated methods, choose Tools | Templates.
}
};
TreeItem<String> treeItem = new TreeItem<String>("Root");
for (int i = 0; i < 20; i++) {
TreeItem<String> treeItem1 = new TreeItem<>("second layer " + i);
treeItem.getChildren().add(treeItem1);
for (int j = 0; j < 20; j++) {
treeItem1.getChildren().add(new TreeItem<>("Third Layer " + j));
}
}
treeView.setRoot(treeItem);
StackPane root = new StackPane();
root.getChildren().addAll(treeView);
Scene scene = new Scene(root, 300, 250);
primaryStage.setTitle("Hello World!");
primaryStage.setScene(scene);
primaryStage.show();
}
/**
* #param args the command line arguments
*/
public static void main(String[] args) {
launch(args);
}
class TTreeViewSkin<T extends IndexedCell> extends TreeViewSkin<T> {
public TTreeViewSkin(TreeView treeView) {
super(treeView);
}
#Override
protected VirtualFlow createVirtualFlow() {
return new TVirtualFlow<T>(); //To change body of generated methods, choose Tools | Templates.
}
}
class TVirtualFlow<T extends IndexedCell> extends VirtualFlow<T> {
#Override
public double getPosition() {
double position = super.getPosition();
if (position == 1.0d) {
return 0.99999999999;
}
return super.getPosition(); //To change body of generated methods, choose Tools | Templates.
}
#Override
public void setPosition(double newPosition) {
if (newPosition == 1.0d) {
newPosition = 0.99999999999;
}
super.setPosition(newPosition); //To change body of generated methods, choose Tools | Templates.
}
}
}