Detection method for first time a Node is rendered

Question

Hello,
I need to lazily perform some computation until after a particular Node is shown to the User. So if the app has multiple tabs, the goal is to not do work for a tab that hasn't even been switched to yet.
I figured the VisibleProperty of Node would help but it's not acting as I'd expect.
final BorderPane tab1 = BorderPaneBuilder.create().build();
setOnFirstShown(tab1, new Runnable() {
  @Override
  public void run()
    _log.info("Show 1");
    tab1.setCenter(new Text("1"));
final BorderPane tab2 = BorderPaneBuilder.create().build();
setOnFirstShown(tab2, new Runnable() {
  @Override
  public void run()
    _log.info("Show 2");
    tab2.setCenter(new Text("2"));
tabs.getTabs().addAll(
  TabBuilder.create().text("One").content(tab1).build(),
  TabBuilder.create().text("Two").content(tab2).build());Here is the listener:
public static void setOnFirstShown( final Node node, final Runnable response )
  class Changer implements ChangeListener<Boolean>
    @Override
    public void changed( ObservableValue<? extends Boolean> obsVal, Boolean oldVal, Boolean newVal )
      _log.debug("Changed: " + newVal);
      if( newVal.booleanValue() )
        /** remove the listener b/c it's only used once */
        node.visibleProperty().removeListener( this );
        _log.debug( "Responding to first show for: " + node );
        try
          response.run();
        catch( Exception e )
          _log.error( "Failed to run response:" + response, e );
  node.visibleProperty().addListener(new Changer());
}So each tab should replace its content with a Text when it becomes visible. However in actuality it works like this:
1. Scene launches, nothing displayed in the first tab. The visibility flag is set to false but not to true
2. Click on the second tab and its content correctly gets a change (to true) in visibility and the Text node appears
3. Click back on the first tab and now it gets visible set to true and shows the Text node
So it seems like for what is initially shown, the Nodes' visible flag is not properly set.
Does anyone have any insight or a better way to delay reaction until a given Node is actually visible to the User?

Answer 1

The visible property of a Node doesn't quite do what you think. From the javadocs:
Specifies whether this Node and any subnodes should be rendered as part of the scene graph. A node may be visible and yet not be shown in the rendered scene if, for instance, it is off the screen or obscured by another Node.
The default value of the visible property is true.
So, your first tab content originally has visibility set to true. You add it as the content of the first tab, and it remains true. Since it never changes, your ChangeListener is never invoked.
The second tab's content also originally has visibility set to true. When you add it as the second tab, it's visibility becomes false to indicate it should not be rendered. When it is selected, the visibility of the first tab content changes to false and the visibility of the second tab content changes to true, invoking the listeners.
This example may help:
import javafx.application.Application;
import javafx.beans.value.ChangeListener;
import javafx.beans.value.ObservableValue;
import javafx.scene.Scene;
import javafx.scene.control.Tab;
import javafx.scene.control.TabPane;
import javafx.scene.layout.BorderPane;
import javafx.scene.text.Text;
import javafx.stage.Stage;
public class TabListenerTest extends Application {
  @Override
  public void start(Stage primaryStage) throws Exception {
    BorderPane root = new BorderPane();
    TabPane tabs = new TabPane();
    Tab tab1 = new Tab("Tab 1");
    Text text1 = new Text("This is tab 1");
    Tab tab2 = new Tab("Tab 2");
    Text text2 = new Text("This is tab 2");
    class VisibilityChangeListener implements ChangeListener<Boolean> {
      String content;
      VisibilityChangeListener(String content) {
        this.content = content ;
      @Override
      public void changed(ObservableValue<? extends Boolean> observable,
          Boolean oldValue, Boolean newValue) {
        System.out.println("Visibilty of "+content+" changed from "+oldValue+ " to "+newValue);
    text1.visibleProperty().addListener(new VisibilityChangeListener("Tab 1"));
    text2.visibleProperty().addListener(new VisibilityChangeListener("Tab 2"));
    tab1.setContent(text1);
    tab2.setContent(text2);
    tabs.getTabs().addAll(tab1, tab2);
    root.setCenter(tabs);
    Scene scene = new Scene(root, 400, 200);
    primaryStage.setScene(scene);
    primaryStage.sizeToScene();
    primaryStage.show();
  public static void main(String[] args) {
    launch(args);
}