A label doesn't show up in two layouts

Vaadin 6 / 7 / 8
1

Hi all,
I was expecting that a single label would show up in two different layouts if I want it to. But it doesn’t, see the code below.


	protected void init(VaadinRequest request) {
		final VerticalLayout layout = new VerticalLayout();
		layout.setMargin(true);
		setContent(layout);

		Label label = new Label("Will this show twice?");
		CssLayout l1 = new CssLayout();
		l1.addComponent(label); // this one doesn't show up
		layout.addComponent(l1);

		CssLayout l2 = new CssLayout();
		l2.addComponent(label);
		layout.addComponent(l2);
	}

With this code only one label shows up on my screen. I think this is a bug but please correct me if I’m wrong. Occurs at least with 7.x series.
Thanks & cheers,
Ville

2

All components including Label can have only one parent container.

3

Syam is right, a component instance is only allowed to reside in one parent container at a time. Adding a component to a parent will implicitly remove it from any previous parent container.

You could create a property of the content of your label and bind that to the labels if you wish for them to always contain the exact same string of text.

4

The component hierarchy is a tree, just like the DOM hierarchy on the client side is a tree. I don’t know of any UI library/framework where components could have multiple parents (making the hierarchy a DAG - directed acyclic graph). It might certainly allow some nice things but would also significantly increase the all-around complexity of the system.

5

On the other hand, why is no exception thrown if trying to add a component to another parent without removing it first?

6

Indeed. Probably because someone once thought it’s “convenient” the way it is. And now we can’t change it without breaking
a lot
 of stuff.