001// Copyright 2006, 2007, 2008, 2010, 2011 The Apache Software Foundation
002//
003// Licensed under the Apache License, Version 2.0 (the "License");
004// you may not use this file except in compliance with the License.
005// You may obtain a copy of the License at
006//
007// http://www.apache.org/licenses/LICENSE-2.0
008//
009// Unless required by applicable law or agreed to in writing, software
010// distributed under the License is distributed on an "AS IS" BASIS,
011// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
012// See the License for the specific language governing permissions and
013// limitations under the License.
014
015package org.apache.tapestry5.services;
016
017import org.apache.tapestry5.ioc.ObjectLocator;
018
019/**
020 * Manages a set of {@link org.apache.tapestry5.services.UpdateListener}s. Periodically (say, every request during
021 * development, or every minute or so during production), request processing is locked down so that only a single thread
022 * is active, and the active thread invokes {@link #fireCheckForUpdates()}. Various services that are dependent on
023 * external resource files (such as classes or template files) can check to see if any file they've used has changed. If
024 * so, the service can invalidate its internal cache, or notify other services (typically via
025 * {@link org.apache.tapestry5.services.InvalidationListener} that they should do the same.
026 * <p>
027 * Note that this interface has moved from module tapestry-core to tapestry-ioc, but has kept the same package (for
028 * backwards compatibility reasons).
029 * <p>
030 * A <em>weak reference</em> to the listener is kept; this ensures that registering as a listener will not prevent a
031 * listener instance from being reclaimed by the garbage collector (this is useful as proxies created by
032 * {@link ObjectLocator#proxy(Class, Class)} may register as listeners, but still be ephemeral).
033 * <p>
034 * Starting in Tapestry 5.3, this services does <em>nothing</em> in production mode.
035 * 
036 * @since 5.1.0.0
037 */
038public interface UpdateListenerHub
039{
040    /**
041     * Adds a listener.
042     */
043    void addUpdateListener(UpdateListener listener);
044
045    /**
046     * Invoked periodically to allow services to check if underlying state has changed. For example, a template file may
047     * have changed. Listeners will typically notify applicable listeners of their own (they usually implement
048     * {@link org.apache.tapestry5.services.InvalidationEventHub}) when such a change occurs.
049     */
050    void fireCheckForUpdates();
051}