Is there any alternative solution for Vaadin 8's ComponentAttach and Detach

When we used Vaadin 8, we were able to get informed about attach and detach events for ordinary components, by registering ComponentAttach and Detach listeners to container components. Thus, we were able to gain some control on the lifecycle of any component which is created anywhere and added to the component tree. Is there any replacement or an alternative solution for this feature in Vaadin Flow? How can we get informed when a new component is added to or removed from the component tree?

Yes, you can do for example:

public class MainView extends VerticalLayout {
    protected void onAttach(AttachEvent attachEvent) {


Thanks for your quick reply. Yes, you are right, we can do it by extending components or registering attach/detach event listeners, but actually what I want to do is something like this:

public class MyUI extends UI {

    protected void init(VaadinRequest vaadinRequest) {

		final VerticalLayout content = new VerticalLayout();
        content.addComponentAttachListener(new MyComponentAttachListener());
        // for demonstration
		VerticalLayout layout1 = new VerticalLayout();
		HorizontalLayout layout2 = new HorizontalLayout();
		VerticalLayout layout3 = new VerticalLayout();
		VerticalLayout layout4 = new VerticalLayout();
		layout1.addComponents(layout2, layout3);
		layout2.addComponents(layout4, new TextField());
		layout3.addComponent(new Label());
		layout4.addComponent(new CheckBox());
    public static class MyComponentAttachListener implements ComponentAttachListener {

		public void componentAttachedToContainer(ComponentAttachEvent event) {
    		HasComponents parentComponent = event.getContainer();
    		Component attachedComponent = event.getAttachedComponent();
    		if(attachedComponent instanceof ComponentAttachDetachNotifier) {
    			((ComponentAttachDetachNotifier) attachedComponent).addComponentAttachListener(this);
    		// Do whatever you want! For example register attach/detach listeners, traverse component tree, apply extensions, etc.
    		System.out.println("Someone has attached a new component to the component tree and I have been notified without forcing him to register any listener explicitly!");    		
    @WebServlet(urlPatterns = "/*", name = "MyUIServlet", asyncSupported = true)
    @VaadinServletConfiguration(ui = MyUI.class, productionMode = false)
    public static class MyUIServlet extends VaadinServlet {

If you run the code above, you will see that, our listener has been notified for each component. We didn’t register any listener for these components, but we got informed when they were attached. Furthermore, we could pass on this behavior to child components implicitly. By this way, we can get control on ordinary components. AFAIK, this feature has been withdrawn in Vaadin flow. Is there any alternative solution for this?

What you are you doing in the Attach ?

I am asking since, in some cases BeforeEnterObserver or AfterNavigationObserver may be better fit.

Unlike Vaadin 8, there is not exactly that listener in containers.

When we were using Vaadin 8, we handled some i18n, validation, permission checking and dependency injection requirements by using this feature. It gave us ability and flexibility to manage unmanaged (created individually without help of any DI solution) Vaadin components. By using this feature with extension mechanism, we were able to add extra capabilities to legacy Vaadin components. Before looking for alternative solutions case by case, I’ve just wanted to check if there is any fully-compatible replacement.

If a navigation is triggered then these observers may be appropriate, but otherwise, as you say, there is no alternative in Vaadin Flow. I hope there will be in upcoming releases. I know, you’ve redesigned whole framework bottom-up and may be you have strong reasons to exclude these kind of features, but for extensibility, IMHO, you should provide a clean mechanism to observe changes on server-side component tree, and an extension mechanism for legacy components.

I’m quite new in Vaadin Flow and keep discovering day by day. Thanks for your attention.