A widget that implements this interface contains Widget and can enumerate them.

abstract class HasWidgets {

 /**
  * Adds a child widget.
  *
  * @param w the widget to be added
  * @throws UnsupportedOperationException if this method is not supported (most
  *           often this means that a specific overload must be called)
  */
 void add(Widget w);

 /**
  * Removes all child widgets.
  */
 void clear();

 /**
  * Removes a child widget.
  *
  * @param w the widget to be removed
  * @return <code>true</code> if the widget was present
  */
 bool remove(Widget w);

 /**
  * Returns an [Iterator] that iterates over this [Iterable] object.
  */
 Iterator<Widget> iterator();
}