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