    001// Copyright 2008, 2009, 2010, 2011, 2015, 2020, 2023 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.ioc.services;
    016
    017import org.apache.tapestry5.commons.ObjectLocator;
    018
    019/**
    020 * Manages a set of {@link org.apache.tapestry5.ioc.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.commons.services.InvalidationListener} that they should do the same.
    026 *
    027 * Note that this interface has moved from module tapestry-core to tapestry-ioc. It was, however, not possible to keep
    028 * the same package (for backwards compatibility reasons) without causing a split package (in terms of Java 9 Modules).
    029 *
    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 *
    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.commons.services.InvalidationEventHub}) when such a change occurs.
    049     */
    050    void fireCheckForUpdates();
    051}