Changing BorderPane contraints causes component to disappear

Question

Hello everyone,
I have this short snippet that is giving me a headache, don't know if it's something i'm doing wrong but the behavior seems weird to me:
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.BorderPane;
import javafx.scene.paint.Color;
import javafx.stage.Stage;
public class MainWindow extends Application {
     public static void main(String[] args) {
          Application.launch(args);
     public void start(Stage s) {
          BorderPane workArea = new BorderPane();
          Button button = new Button(".");
          Scene scene = new Scene(workArea, 800, 600, Color.TRANSPARENT);
          s.setScene(scene);
          s.show();
          workArea.setCenter(button);
          workArea.getChildren().remove(button);
          workArea.setLeft(button);
          workArea.getChildren().remove(button);
          workArea.setCenter(button);
}As you can see i move the same component back and forth between left and center. When you run this piece of code you'll get and empty stage.
I have a workaround (more like a hack) but it seems that i'm lacking something, what i do in order for this to work is creating a new instance of button and remove the original from the Pane, this works ok but then i would have to copy all the properties from the original object.
Any idea on if it's normal, why, and how to workaround this?
Thanks
Bruno

Answer 1

Hi Bruno,
Yea, weird this doesn't work - just ran it and got the same results as you. Strangely the following does work:
public void start(Stage stage) throws Exception
    BorderPane workArea = new BorderPane();
    Scene scene = new Scene(workArea, 800, 600);
    stage.setScene(scene);
    stage.show();
    Button button = new Button("Test");
    workArea.setLeft(button);
    workArea.setLeft(null);
    workArea.setRight(button);
    workArea.setRight(null);
    workArea.setCenter(button);
}So it looks like it is a problem specifically with the remove on BorderPane not doing something that the setLeft/Right/Center does. I can't see a bug for it so I would suggest raising one. The above code is painful as you would have to record where it was last added in order to remove it on the next move.
You could also use another layout. For my money, I highly recommend MigLayout. You can do very complex layouts simply in a single pane and it handles this dynamic adding stuff well:
public void start(Stage stage) throws Exception
    MigPane workArea = new MigPane("fill");
    Scene scene = new Scene(workArea, 800, 600);
    stage.setScene(scene);
    stage.show();
    Button button = new Button("Test");
    workArea.add(button, "left");
    workArea.getChildren().remove(button);
    workArea.add(button, "right");
    workArea.getChildren().remove(button);
    workArea.add(button, "center");
}You need the core MigLayout library from here: http://www.miglayout.com/
And the JFX wrapper is here: http://java.net/projects/miglayoutfx2/pages/Home
Start using Mig and you will never look back. I have used it in Swing heavily for commercial apps and it has never let me down.
If you can't bring yourself to use Mig, and you only want to change the anchor like in your simple example, here's a little cheat that seems to work:
public void start(Stage stage) throws Exception
    GridPane workArea = new GridPane();
    Scene scene = new Scene(workArea, 800, 600);
    stage.setScene(scene);
    stage.show();
    Button button = new Button("Test");
    workArea.add(button, 0, 0);
    workArea.setAlignment(Pos.CENTER_LEFT);
    workArea.setAlignment(Pos.CENTER_RIGHT);
    workArea.setAlignment(Pos.CENTER);
}This sidesteps the whole add/remove issue.
Cheers,
zonski