hlship-20080520/tapestry-core/src/main/java/org/apache/tapestry/internal/services/UpdateListenerHub.java

// Copyright 2006, 2007 The Apache Software Foundation
//
// 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 org.apache.tapestry.internal.services;

import org.apache.tapestry.internal.events.UpdateListener;

/**
 * Manages a set of {@link org.apache.tapestry.internal.events.UpdateListener}s. Periodically (say, every request during
 * development, or every minute or so during production), request processing is locked down so that only a single thread
 * is active, and the active thread invokes {@link #fireUpdateEvent()}. Various services that are dependent on external
 * resource files (such as classes or template files) can check to see if any file they've used has changed. If so, the
 * service can invalidate its internal cache, or notify other services (typically via {@link
 * org.apache.tapestry.internal.events.InvalidationListener} that they should do the same.
 *
 * @see org.apache.tapestry.internal.util.URLChangeTracker
 */
public interface UpdateListenerHub
{
    void addUpdateListener(UpdateListener listener);

    /**
     * Invoked periodically to allow services to check if underlying state has changed. For example, a template file may
     * have changed. Listeners will typically notify applicable listeners of their own (they usually implement {@link
     * org.apache.tapestry.internal.event.InvalidationEventHub}) when such a change occurs.
     */
    void fireUpdateEvent();
}