/*
 * $Header: /cvshome/build/org.osgi.util.tracker/src/org/osgi/util/tracker/ServiceTrackerCustomizer.java,v 1.13 2007/02/19 19:04:33 hargrave Exp $
 * 
 * Copyright (c) OSGi Alliance (2000, 2007). All Rights Reserved.
 * 
 * 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.osgi.util.tracker;

import org.osgi.framework.ServiceReference;

/**
 * The <code>ServiceTrackerCustomizer</code> interface allows a
 * <code>ServiceTracker</code> object to customize the service objects that
 * are tracked. The <code>ServiceTrackerCustomizer</code> object is called
 * when a service is being added to the <code>ServiceTracker</code> object.
 * The <code>ServiceTrackerCustomizer</code> can then return an object for the
 * tracked service. The <code>ServiceTrackerCustomizer</code> object is also
 * called when a tracked service is modified or has been removed from the
 * <code>ServiceTracker</code> object.
 * 
 * <p>
 * The methods in this interface may be called as the result of a
 * <code>ServiceEvent</code> being received by a <code>ServiceTracker</code>
 * object. Since <code>ServiceEvent</code> s are synchronously delivered by
 * the Framework, it is highly recommended that implementations of these methods
 * do not register (<code>BundleContext.registerService</code>), modify (
 * <code>ServiceRegistration.setProperties</code>) or unregister (
 * <code>ServiceRegistration.unregister</code>) a service while being
 * synchronized on any object.
 * 
 * <p>
 * The <code>ServiceTracker</code> class is thread-safe. It does not call a
 * <code>ServiceTrackerCustomizer</code> object while holding any locks.
 * <code>ServiceTrackerCustomizer</code> implementations must also be
 * thread-safe.
 * 
 * @ThreadSafe
 * @version $Revision: 1.13 $
 */
public interface ServiceTrackerCustomizer {
	/**
	 * A service is being added to the <code>ServiceTracker</code> object.
	 * 
	 * <p>
	 * This method is called before a service which matched the search
	 * parameters of the <code>ServiceTracker</code> object is added to it.
	 * This method should return the service object to be tracked for this
	 * <code>ServiceReference</code> object. The returned service object is
	 * stored in the <code>ServiceTracker</code> object and is available from
	 * the <code>getService</code> and <code>getServices</code> methods.
	 * 
	 * @param reference Reference to service being added to the
	 *        <code>ServiceTracker</code> object.
	 * @return The service object to be tracked for the
	 *         <code>ServiceReference</code> object or <code>null</code> if
	 *         the <code>ServiceReference</code> object should not be tracked.
	 */
	public Object addingService(ServiceReference reference);

	/**
	 * A service tracked by the <code>ServiceTracker</code> object has been
	 * modified.
	 * 
	 * <p>
	 * This method is called when a service being tracked by the
	 * <code>ServiceTracker</code> object has had it properties modified.
	 * 
	 * @param reference Reference to service that has been modified.
	 * @param service The service object for the modified service.
	 */
	public void modifiedService(ServiceReference reference, Object service);

	/**
	 * A service tracked by the <code>ServiceTracker</code> object has been
	 * removed.
	 * 
	 * <p>
	 * This method is called after a service is no longer being tracked by the
	 * <code>ServiceTracker</code> object.
	 * 
	 * @param reference Reference to service that has been removed.
	 * @param service The service object for the removed service.
	 */
	public void removedService(ServiceReference reference, Object service);
}