Implemented by objects that control a piece of user interface, with a life cycle managed by an {@link ActivityManager}, in response to {@link com.google.gwt.place.shared.PlaceChangeEvent} events as the user navigates through the app.

abstract class Activity {
 /**
  * Called when the user is trying to navigate away from this activity.
  *
  * @return A message to display to the user, e.g. to warn of unsaved work, or
  *         null to say nothing
  */
 String mayStop();

 /**
  * Called when {@link #start} has not yet replied to its callback, but the
  * user has lost interest.
  */
 void onCancel();

 /**
  * Called when the Activity's widget has been removed from view. All event
  * handlers it registered will have been removed before this method is called.
  */
 void onStop();

 /**
  * Called when the Activity should ready its widget for the user. When the
  * widget is ready (typically after an RPC response has been received),
  * receiver should present it by calling
  * {@link AcceptsOneWidget#setWidget} on the given panel.
  * <p>
  * Any handlers attached to the provided event bus will be de-registered when
  * the activity is stopped, so activities will rarely need to hold on to the
  * {@link com.google.gwt.event.shared.HandlerRegistration HandlerRegistration}
  * instances returned by {@link EventBus#addHandler}.
  *
  * @param panel the panel to display this activity's widget when it is ready
  * @param eventBus the event bus
  */
 void start(AcceptsOneWidget panel, EventBus eventBus);
}