/*
 * Copyright (c) OSGi Alliance (2011, 2012). 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.framework.hooks.resolver;

import java.util.Collection;
import org.osgi.framework.Bundle;
import org.osgi.framework.wiring.BundleRevision;
import org.osgi.framework.wiring.FrameworkWiring;

/**
 * OSGi Framework Resolver Hook Factory Service.
 * 
 * <p>
 * Bundles registering this service will be called by the framework during a
 * bundle resolver process to obtain a {@link ResolverHook resolver hook}
 * instance which will be used for the duration of a resolve process.
 * 
 * @ThreadSafe
 * @see ResolverHook
 * @version $Id: e0a2f3ad081c31bbb682fa366c15a3080bf6da2b $
 */
public interface ResolverHookFactory {
	/**
	 * This method is called by the framework each time a resolve process begins
	 * to obtain a {@link ResolverHook resolver hook} instance. This resolver
	 * hook instance will be used for the duration of the resolve process. At
	 * the end of the resolve process the method {@link ResolverHook#end()} must
	 * be called by the framework and the framework must not hold any references
	 * of the resolver hook instance.
	 * <p>
	 * The triggers represent the collection of bundles which triggered the
	 * resolve process. This collection may be empty if the triggers cannot be
	 * determined by the framework. In most cases the triggers can easily be
	 * determined. Calling certain methods on {@link Bundle bundle} when a
	 * bundle is in the {@link Bundle#INSTALLED INSTALLED} state will cause the
	 * framework to begin a resolve process in order to resolve the bundle. The
	 * following methods will start a resolve process in this case:
	 * <ul>
	 * <li>{@link Bundle#start() start}</li>
	 * <li>{@link Bundle#loadClass(String) loadClass}</li>
	 * <li>{@link Bundle#findEntries(String, String, boolean) findEntries}</li>
	 * <li>{@link Bundle#getResource(String) getResource}</li>
	 * <li>{@link Bundle#getResources(String) getResources}</li>
	 * </ul>
	 * In such cases the collection will contain the single bundle which the
	 * framework is trying to resolve. Other cases will cause multiple bundles
	 * to be included in the trigger bundles collection. When
	 * {@link FrameworkWiring#resolveBundles(Collection) resolveBundles} is
	 * called the collection of triggers must include all the current bundle
	 * revisions for bundles passed to resolveBundles which are in the
	 * {@link Bundle#INSTALLED INSTALLED} state.
	 * <p>
	 * When
	 * {@link FrameworkWiring#refreshBundles(Collection, org.osgi.framework.FrameworkListener...)}
	 * is called the collection of triggers is determined with the following
	 * steps:
	 * <ul>
	 * <li>If the collection of bundles passed is null then
	 * {@link FrameworkWiring#getRemovalPendingBundles()} is called to get the
	 * initial collection of bundles.</li>
	 * <li>The equivalent of calling
	 * {@link FrameworkWiring#getDependencyClosure(Collection)} is called with
	 * the initial collection of bundles to get the dependency closure
	 * collection of the bundles being refreshed.</li>
	 * <li>Remove any non-active bundles from the dependency closure collection.
	 * </li>
	 * <li>For each bundle remaining in the dependency closure collection get
	 * the current bundle revision and add it to the collection of triggers.</li>
	 * </ul>
	 * <p>
	 * As described above, a resolve process is typically initiated as a result
	 * of calling API that causes the framework to attempt to resolve one or
	 * more bundles. The framework is free to start a resolve process at any
	 * time for reasons other than calls to framework API. For example, a
	 * resolve process may be used by the framework for diagnostic purposes and
	 * result in no bundles actually becoming resolved at the end of the
	 * process. Resolver hook implementations must be prepared for resolve
	 * processes that are initiated for other reasons besides calls to framework
	 * API.
	 * 
	 * @param triggers an unmodifiable collection of bundles which triggered the
	 *        resolve process. This collection may be empty if the collection of
	 *        trigger bundles cannot be determined.
	 * @return a resolver hook instance to be used for the duration of the
	 *         resolve process. A {@code null} value may be returned which
	 *         indicates this resolver hook factory abstains from the resolve
	 *         process.
	 */
	ResolverHook begin(Collection<BundleRevision> triggers);
}
