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    
015    package org.apache.tapestry5.services;
016    
017    import 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     */
038    public 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    }