/*
 * Copyright (c) OSGi Alliance (2007, 2010). 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.Bundle;
import org.osgi.framework.BundleEvent;

/**
 * The {@code BundleTrackerCustomizer} interface allows a
 * {@code BundleTracker} to customize the {@code Bundle}s that are
 * tracked. A {@code BundleTrackerCustomizer} is called when a bundle is
 * being added to a {@code BundleTracker}. The
 * {@code BundleTrackerCustomizer} can then return an object for the
 * tracked bundle. A {@code BundleTrackerCustomizer} is also called when a
 * tracked bundle is modified or has been removed from a
 * {@code BundleTracker}.
 * 
 * <p>
 * The methods in this interface may be called as the result of a
 * {@code BundleEvent} being received by a {@code BundleTracker}.
 * Since {@code BundleEvent}s are received synchronously by the
 * {@code BundleTracker}, it is highly recommended that implementations of
 * these methods do not alter bundle states while being synchronized on any
 * object.
 * 
 * <p>
 * The {@code BundleTracker} class is thread-safe. It does not call a
 * {@code BundleTrackerCustomizer} while holding any locks.
 * {@code BundleTrackerCustomizer} implementations must also be
 * thread-safe.
 * 
 * @param <T> The type of the tracked object.
 * @ThreadSafe
 * @version $Id: 0e80f2555530b217faef57726a5938f0087a45c5 $
 * @since 1.4
 */
public interface BundleTrackerCustomizer<T> {
	/**
	 * A bundle is being added to the {@code BundleTracker}.
	 * 
	 * <p>
	 * This method is called before a bundle which matched the search parameters
	 * of the {@code BundleTracker} is added to the
	 * {@code BundleTracker}. This method should return the object to be
	 * tracked for the specified {@code Bundle}. The returned object is
	 * stored in the {@code BundleTracker} and is available from the
	 * {@link BundleTracker#getObject(Bundle) getObject} method.
	 * 
	 * @param bundle The {@code Bundle} being added to the
	 *        {@code BundleTracker}.
	 * @param event The bundle event which caused this customizer method to be
	 *        called or {@code null} if there is no bundle event associated
	 *        with the call to this method.
	 * @return The object to be tracked for the specified {@code Bundle}
	 *         object or {@code null} if the specified {@code Bundle}
	 *         object should not be tracked.
	 */
	public T addingBundle(Bundle bundle, BundleEvent event);

	/**
	 * A bundle tracked by the {@code BundleTracker} has been modified.
	 * 
	 * <p>
	 * This method is called when a bundle being tracked by the
	 * {@code BundleTracker} has had its state modified.
	 * 
	 * @param bundle The {@code Bundle} whose state has been modified.
	 * @param event The bundle event which caused this customizer method to be
	 *        called or {@code null} if there is no bundle event associated
	 *        with the call to this method.
	 * @param object The tracked object for the specified bundle.
	 */
	public void modifiedBundle(Bundle bundle, BundleEvent event,
			T object);

	/**
	 * A bundle tracked by the {@code BundleTracker} has been removed.
	 * 
	 * <p>
	 * This method is called after a bundle is no longer being tracked by the
	 * {@code BundleTracker}.
	 * 
	 * @param bundle The {@code Bundle} that has been removed.
	 * @param event The bundle event which caused this customizer method to be
	 *        called or {@code null} if there is no bundle event associated
	 *        with the call to this method.
	 * @param object The tracked object for the specified bundle.
	 */
	public void removedBundle(Bundle bundle, BundleEvent event,
			T object);
}
