/*
 * Copyright 2010 Google Inc.
 * 
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not
 * use this file except in compliance with the License. You may obtain a copy of
 * the License at
 * 
 * http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations under
 * the License.
 */
package com.google.gwt.activity.shared;

import com.google.gwt.event.shared.EventBus;
import com.google.gwt.user.client.ui.AcceptsOneWidget;

/**
 * Object that controls a piece of user interface, with a life cycle managed by
 * an {@link ActivityManager}.
 * <p>
 * Ideally this would be an interface rather than an abstract class, but we
 * expect its api will need to evolve (slightly) in the near term. When it
 * settles down, an interface may be introduced. To this end, future versions of
 * this class should not introduce non-trivial behavior.
 * <p>
 * For composition, see {@link IsActivity}.
 */
public 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
   */
  public abstract String mayStop();

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