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----
-title: Component and UI Extensions
-order: 7
-layout: page
----
-
-[[gwt.extension]]
-= Component and UI Extensions
-
-Adding features to existing components by extending them by inheritance creates
-a problem when you want to combine such features. For example, one add-on could
-add spell-check to a [classname]#TextField#, while another could add client-side
-validation. Combining such add-on features would be difficult if not impossible.
-You might also want to add a feature to several or even to all components, but
-extending all of them by inheritance is not really an option. Vaadin includes a
-component plug-in mechanism for these purposes. Such plug-ins are simply called
-__extensions__.
-
-Also a UI can be extended in a similar fashion. In fact, some Vaadin features
-such as the JavaScript execution are UI extensions.
-
-Implementing an extension requires defining a server-side extension class and a
-client-side connector. An extension can have a shared state with the connector
-and use RPC, just like a component could.
-
-[[gwt.extension.server-side]]
-== Server-Side Extension API
-
-The server-side API for an extension consists of class that extends (in the Java
-sense) the [classname]#AbstractExtension# class. It typically has an
-__extend()__ method, a constructor, or a static helper method that takes the
-extended component or UI as a parameter and passes it to __super.extend()__.
-
-For example, let us have a trivial example with an extension that takes no
-special parameters, and illustrates the three alternative APIs:
-
-
-----
-public class CapsLockWarning extends AbstractExtension {
-    // You could pass it in the constructor
-    public CapsLockWarning(PasswordField field) {
-        super.extend(field);
-    }
-
-    // Or in an extend() method
-    public void extend(PasswordField field) {
-        super.extend(field);
-    }
-
-    // Or with a static helper
-    public static addTo(PasswordField field) {
-        new CapsLockWarning().extend(field);
-    }
-}
-----
-
-The extension could then be added to a component as follows:
-
-
-----
-PasswordField password = new PasswordField("Give it");
-
-// Use the constructor
-new CapsLockWarning(password);
-
-// ... or with the extend() method
-new CapsLockWarning().extend(password);
-
-// ... or with the static helper
-CapsLockWarning.addTo(password);
-
-layout.addComponent(password);
-----
-
-Adding a feature in such a "reverse" way is a bit unusual in the Vaadin API, but
-allows type safety for extensions, as the method can limit the target type to
-which the extension can be applied, and whether it is a regular component or a
-UI.
-
-
-[[gwt.extension.connector]]
-== Extension Connectors
-
-An extension does not have a corresponding widget on the client-side, but only
-an extension connector that extends the [classname]#AbstractExtensionConnector#
-class. The server-side extension class is specified with a
-[literal]#++@Connect++# annotation, just like in component connectors.
-
-An extension connector needs to implement the [methodname]#extend()# method,
-which allows hooking to the extended component. The normal extension mechanism
-is to modify the extended component as needed and add event handlers to it to
-handle user interaction. An extension connector can share a state with the
-server-side extension as well as make RPC calls, just like with components.
-
-In the following example, we implement a "Caps Lock warning" extension. It
-listens for changes in Caps Lock state and displays a floating warning element
-over the extended component if the Caps Lock is on.
-
-
-----
-@Connect(CapsLockWarning.class)
-public class CapsLockWarningConnector
-        extends AbstractExtensionConnector {
-
-    @Override
-    protected void extend(ServerConnector target) {
-        // Get the extended widget
-        final Widget pw =
-                ((ComponentConnector) target).getWidget();
-
-        // Preparations for the added feature
-        final VOverlay warning = new VOverlay();
-        warning.setOwner(pw);
-        warning.add(new HTML("Caps Lock is enabled!"));
-
-        // Add an event handler
-        pw.addDomHandler(new KeyPressHandler() {
-            public void onKeyPress(KeyPressEvent event) {
-                if (isEnabled() && isCapsLockOn(event)) {
-                    warning.showRelativeTo(passwordWidget);
-                } else {
-                    warning.hide();
-                }
-            }
-        }, KeyPressEvent.getType());
-    }
-
-    private boolean isCapsLockOn(KeyPressEvent e) {
-        return e.isShiftKeyDown() ^
-               Character.isUpperCase(e.getCharCode());
-    }
-}
-----
-
-The [methodname]#extend()# method gets the connector of the extended component
-as the parameter, in the above example a [classname]#PasswordFieldConnector#. It
-can access the widget with the [methodname]#getWidget()#.
-
-An extension connector needs to be included in a widget set. The class must
-therefore be defined under the [filename]#client# package of a widget set, just
-like with component connectors.
-
-
-
-