vaadin-framework/documentation/components/components-customcomponent.asciidoc

---
title: Composition with Composite and CustomComponent
order: 33
layout: page
---

[[components.customcomponent]]
= Composition with Composite and CustomComponent

The ease of making new user interface components is one of the core features of
Vaadin. Typically, you simply combine existing built-in components to produce
composite components. In many applications, such composite components make up
the majority of the user interface.

As described earlier in
<<../application/application-architecture#application.architecture.composition,"Compositing
Components">>, you have two basic ways to create a composite - either by
extending a layout component or by using a composition component (a
[classname]#Composite# or a [classname]#CustomComponent#), which typically
wraps around a layout component.
The benefit of wrapping a layout composite is mainly encapsulation - hiding the
implementation details of the composition. Otherwise, a user of the composite
could rely on implementation details, which would create an unwanted dependency.

To create a composite, you need to inherit [classname]#Composite# or
[classname]#CustomComponent# and set the __composition root__ component in the
constructor. The composition root is typically a layout component that contains
other components.

The difference between the two composition classes is that a
[classname]#CustomComponent# introduces another layer in the DOM on the
browser, which can be used e.g. for styling but does require its own size
setting etc. On the other hand, a [classname]#Composite# is a more light-weight
version that has no visual representation in the browser, and is effectively
replaced by its contents.

For example:

[source, java]
----
class MyComposite extends CustomComponent {
    public MyComposite(String message) {
        // A layout structure used for composition
        Panel panel = new Panel("My Custom Component");
        VerticalLayout panelContent = new VerticalLayout();
        panel.setContent(panelContent);

        // Compose from multiple components
        Label label = new Label(message);
        panelContent.addComponent(label);
        panelContent.addComponent(new Button("Ok"));

        // The composition root MUST be set
        setCompositionRoot(panel);

        // Set the size as undefined at all levels
        panelContent.setSizeUndefined();
        panel.setSizeUndefined();
        // this is not needed for a Composite
        setSizeUndefined();
    }
}
----

Take note of the sizing when trying to make a customcomponent that shrinks to
fit the contained components. You have to set the size as undefined at all
levels. In the case of [classname]#CustomComponent#, the sizing of the composite
component and the composition root are separate, whereas [classname]#Composite#
delegates its size management to its composition root.

You can use the component as follows:

[source, java]
----
MyComposite mycomposite = new MyComposite("Hello");
----

The rendered component is shown in <<figure.components.customcomponent>>.

[[figure.components.customcomponent]]
.A custom composite component
image::img/customcomponent-example1.png[width=25%, scaledwidth=40%]

You can also inherit any other components, such as layouts, to attain similar
composition.
((("Google Web Toolkit")))
Even further, you can create entirely new low-level components, by integrating
pure client-side components or by extending the client-side functionality of
built-in components. Development of new components is covered in
<<../gwt/gwt-overview.asciidoc#gwt.overview,"Integrating
with the Server-Side">>.