CODEWITHSUNDEEP
javafxparent-child
Marius
Marius

Reputation: 1050

parent node losing its child

I have observed some weird behavior while dealing with some java fx components. I am trying to dynamically add children to a parent node via the controller, once a button is pressed on the GUI. Here is a reproducible scenario of the problem:

    VBox vbox = new VBox();
    HBox entry = new HBox();
    Button button = new Button();
    TextField text = new TextField();

    entry.getChildren().add(text);
    entry.getChildren().add(button);

    vbox.getChildren().add(entry);

    System.out.println(vbox.getChildren().isEmpty() ? "empty" : "not empty"); //prints out "not empty"
    HBox newEntry = new HBox(entry);
    System.out.println(vbox.getChildren().isEmpty() ? "empty" : "not empty"); //prints out "empty"

Can you please enlighten me why vbox is losing entry as a child when I pass entry to a constructor? I am passing entry to the constructor in order to copy all the other object attributes/properties that are already set via the FXML form. In my actual code, each element has a different identifier, set via the node.setId() method, so duplicate IDs are not the problem. Am I not aware of some underlying mechanism? Thank you.

Upvotes: 0

Views: 310

Answers (1)

fabian
fabian

Reputation: 82451

I am passing entry to the constructor in order to copy all the other object attributes/properties that are already set via the FXML form.

It doesn't work this way. That constructor adds the same instance as child of the newly created HBox. Since a Node can only have a single parent and a Node occurs in the child list of it's Parent, JavaFX has to fix the state by removing the Node from the child list of it's former parent.

Note that quite a few Panes allow you to pass children to one of the constructor. Those are not copy constructors, but simply "shortcuts" for that allow you to add the children without using pane.getChildren().addAll(children); In fact I'm not aware of any copy constructor for Nodes in the JavaFX API.

HBox newEntry = new HBox(entry);

Creates a new HBox containing entry as it's only child.

Instead of trying to copy a part of the scene graph, it's most often easier to create a helper method that creates the part of the scene graph (which can also be done using a fxml file).

(There is no functionality in the JavaFX API allowing you to create a copy of a Node hierarchy.)

Upvotes: 3

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