user/src/com/google/gwt/place/shared/PlaceChangeRequestEvent.java

/*
 * 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.place.shared;

import com.google.gwt.event.shared.EventHandler;
import com.google.gwt.event.shared.GwtEvent;

/**
 * Event thrown when the user may go to a new place in the app, or tries to
 * leave it. Receivers can call {@link #setWarning(String)} request that the
 * user be prompted to confirm the change.
 */
public class PlaceChangeRequestEvent extends
    GwtEvent<PlaceChangeRequestEvent.Handler> {

  /**
   * Implemented by handlers of PlaceChangeRequestEvent.
   */
  public interface Handler extends EventHandler {
    /**
     * Called when a {@link PlaceChangeRequestEvent} is fired.
     *
     * @param event the {@link PlaceChangeRequestEvent}
     */
    void onPlaceChangeRequest(PlaceChangeRequestEvent event);
  }

  /**
   * A singleton instance of Type&lt;Handler&gt;.
   */
  public static final Type<Handler> TYPE = new Type<Handler>();

  private String warning;

  private final Place newPlace;

  /**
   * Constructs a PlaceChangeRequestEvent for the given {@link Place}.
   *
   * @param newPlace a {@link Place} instance
   */
  public PlaceChangeRequestEvent(Place newPlace) {
    this.newPlace = newPlace;
  }

  @Override
  public Type<Handler> getAssociatedType() {
    return TYPE;
  }

  /**
   * Returns the place we may navigate to, or null on window close.
   *
   * @return a {@link Place} instance
   */
  public Place getNewPlace() {
    return newPlace;
  }

  /**
   * Returns the warning message to show the user before allowing the place
   * change, or null if none has been set.
   *
   * @return the warning message as a String
   * @see #setWarning(String)
   */
  public String getWarning() {
    return warning;
  }

  /**
   * Set a message to warn the user that it might be unwise to navigate away
   * from the current place, e.g. due to unsaved changes. If the user clicks
   * okay to that message, navigation will be canceled.
   * <p>
   * Calling with a null warning is the same as not calling the method at all --
   * the user will not be prompted.
   * <p>
   * Only the first non-null call to setWarning has any effect. That is, once
   * the warning message has been set it cannot be cleared.
   *
   * @param warning the warning message as a String
   * @see #getWarning()
   */
  public void setWarning(String warning) {
    if (this.warning == null) {
      this.warning = warning;
    }
  }

  @Override
  protected void dispatch(Handler handler) {
    handler.onPlaceChangeRequest(this);
  }
}