[[creating-a-component-extension]]
Creating a component extension
------------------------------

In this tutorial we create a simple extension that can be attached to a
`PasswordField`, displaying a floating notification if the user's Caps
Lock seems to be enabled. We assume the reader is already familiar with
the link:CreatingAUIExtension.asciidoc[Creating a UI extension]
tutorial.

This extension has almost no server-side functionality; the whole Extension
class is as follows:

[source,java]
....
public class CapsLockWarning extends AbstractExtension {
  protected CapsLockWarning(PasswordField field) {
    // Non-public constructor to discourage direct instantiation
    extend(field);
  }

  public static CapsLockWarning warnFor(PasswordField field) {
    return new CapsLockWarning(field);
  }
}
....

When there's nothing to configure for the extension, users just want to
enable it for some component and be done with it. By defining a static
factory method, the user only needs to do something like
`CapsLockWarning.warnFor(myPasswordField);` to make `myPasswordField`
get the new functionality.

The client side is not overly complicated, either. We override the
`extend` method, called by the framework when the client-side extension
connector is attached to its target the client-side counterpart of the
connector to which the server-side extension instance is attached in
this case, `PasswordFieldConnector`.

We add a key press handler to the password widget, checking if the input
looks like Caps Lock might be enabled. The Caps Lock state cannot be
directly queried in GWT/JavaScript, so we use a trick: check if either

* the shift key was not held but the entered character was uppercase, or
* the shift key _was_ held but the entered character was lowercase.

If this is the case, we show a warning in the form of a floating widget
(`VOverlay`). This demonstrates how an extension may make use of UI
elements even though it is not a part of the layout hierarchy. A
frequent use case for extensions is showing different types of floating
overlay elements that are temporary in character.

[source,java]
....

@Connect(CapsLockWarning.class)
public class CapsLockWarningConnector extends AbstractExtensionConnector {
  @Override
  protected void extend(ServerConnector target) {
    final Widget passwordWidget = ((ComponentConnector) target).getWidget();

    final VOverlay warning = new VOverlay();
    warning.setOwner(passwordWidget);
    warning.add(new HTML("Caps Lock is enabled!"));

    passwordWidget.addDomHandler(new KeyPressHandler() {
      @Override
      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());
  }
}
....

To use the Caps Lock warning, compile your widgetset and extend a
PasswordField with something like this

[source,java]
....
PasswordField field = new PasswordField("Enter your password");
CapsLockWarning.warnFor(field);
addComponent(field);
....
