/* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* ***** BEGIN LICENSE BLOCK *****
 * Version: MPL 1.1/GPL 2.0/LGPL 2.1
 *
 * The contents of this file are subject to the Mozilla Public License Version
 * 1.1 (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.mozilla.org/MPL/
 *
 * Software distributed under the License is distributed on an "AS IS" basis,
 * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
 * for the specific language governing rights and limitations under the
 * License.
 *
 * The Original Code is Mozilla Communicator client code.
 *
 * The Initial Developer of the Original Code is
 * Netscape Communications Corporation.
 * Portions created by the Initial Developer are Copyright (C) 1998
 * the Initial Developer. All Rights Reserved.
 *
 * Contributor(s):
 *   Alec Flett <alecf@netscape.com>
 *   Brian Nesse <bnesse@netscape.com>
 *   Benjamin Smedberg <benjamin@smedbergs.us>
 *
 * Alternatively, the contents of this file may be used under the terms of
 * either the GNU General Public License Version 2 or later (the "GPL"), or
 * the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
 * in which case the provisions of the GPL or the LGPL are applicable instead
 * of those above. If you wish to allow use of your version of this file only
 * under the terms of either the GPL or the LGPL, and not to allow others to
 * use your version of this file under the terms of the MPL, indicate your
 * decision by deleting the provisions above and replace them with the notice
 * and other provisions required by the GPL or the LGPL. If you do not delete
 * the provisions above, a recipient may use your version of this file under
 * the terms of any one of the MPL, the GPL or the LGPL.
 *
 * ***** END LICENSE BLOCK ***** */

#include "nsIPrefBranch.idl"

interface nsIObserver;

/**
 * nsIPrefBranch2 allows clients to observe changes to pref values.
 *
 * @see nsIPrefBranch
 */
[scriptable, uuid(784de8e2-e72f-441a-ae74-9d5fdfe13be3)]
interface nsIPrefBranch2 : nsIPrefBranch
{
  /**
   * Add a preference change observer. On preference changes, the following
   * arguments will be passed to the nsIObserver.observe() method:
   *   aSubject - The nsIPrefBranch object (this)
   *   aTopic   - The string defined by NS_PREFBRANCH_PREFCHANGE_TOPIC_ID
   *   aData    - The name of the preference which has changed, relative to
   *              the |root| of the aSubject branch.
   *
   * aSubject.get*Pref(aData) will get the new value of the modified
   * preference. For example, if your observer is registered with
   * addObserver("bar.", ...) on a branch with root "foo.", modifying
   * the preference "foo.bar.baz" will trigger the observer, and aData
   * parameter will be "bar.baz".
   *
   * @param aDomain   The preference on which to listen for changes. This can
   *                  be the name of an entire branch to observe.
   *                  e.g. Holding the "root" prefbranch and calling
   *                  addObserver("foo.bar.", ...) will observe changes to
   *                  foo.bar.baz and foo.bar.bzip
   * @param aObserver The object to be notified if the preference changes.
   * @param aHoldWeak true  Hold a weak reference to |aObserver|. The object
   *                        must implement the nsISupportsWeakReference
   *                        interface or this will fail.
   *                  false Hold a strong reference to |aObserver|.
   *
   * @note
   * Registering as a preference observer can open an object to potential
   * cyclical references which will cause memory leaks. These cycles generally
   * occur because an object both registers itself as an observer (causing the
   * branch to hold a reference to the observer) and holds a reference to the
   * branch object for the purpose of getting/setting preference values. There
   * are 3 approaches which have been implemented in an attempt to avoid these
   * situations.
   * 1) The nsPrefBranch object supports nsISupportsWeakReference. Any consumer
   *    may hold a weak reference to it instead of a strong one.
   * 2) The nsPrefBranch object listens for xpcom-shutdown and frees all of the
   *    objects currently in its observer list. This ensures that long lived
   *    objects (services for example) will be freed correctly.
   * 3) The observer can request to be held as a weak reference when it is
   *    registered. This insures that shorter lived objects (say one tied to an
   *    open window) will not fall into the cyclical reference trap.
   *
   * @note
   * The list of registered observers may be changed during the dispatch of
   * nsPref:changed notification. However, the observers are not guaranteed
   * to be notified in any particular order, so you can't be sure whether the
   * added/removed observer will be called during the notification when it
   * is added/removed.
   *
   * @note
   * It is possible to change preferences during the notification.
   *
   * @note
   * It is not safe to change observers during this callback in Gecko 
   * releases before 1.9. If you want a safe way to remove a pref observer,
   * please use an nsITimer.
   *
   * @see nsIObserver
   * @see removeObserver
   */
  void addObserver(in string aDomain, in nsIObserver aObserver,
                   in boolean aHoldWeak);

  /**
   * Remove a preference change observer.
   *
   * @param aDomain   The preference which is being observed for changes.
   * @param aObserver An observer previously registered with addObserver().
   *
   * @note
   * Note that you must call removeObserver() on the same nsIPrefBranch2
   * instance on which you called addObserver() in order to remove aObserver;
   * otherwise, the observer will not be removed.
   *
   * @see nsIObserver
   * @see addObserver
   */
  void removeObserver(in string aDomain, in nsIObserver aObserver);
};

%{C++

/**
 * Notification sent when a preference changes.
 */
#define NS_PREFBRANCH_PREFCHANGE_TOPIC_ID "nsPref:changed"

%}
