/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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.apache.felix.configurator.impl;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

import org.apache.felix.configurator.impl.logger.SystemLogger;
import org.osgi.framework.BundleContext;
import org.osgi.framework.Constants;
import org.osgi.framework.InvalidSyntaxException;
import org.osgi.framework.ServiceEvent;
import org.osgi.framework.ServiceListener;
import org.osgi.framework.ServiceReference;
import org.osgi.service.cm.ConfigurationAdmin;
import org.osgi.util.tracker.ServiceTracker;
import org.osgi.util.tracker.ServiceTrackerCustomizer;

/**
 * The {@code ServicesListener} listens for the required services
 * and starts the configurator when all services are available.
 * It also handles optional services
 */
public class ServicesListener {

    /** The bundle context. */
    private final BundleContext bundleContext;

    /** The service tracker for configuration admin */
    private final ServiceTracker<ConfigurationAdmin, ServiceReference<ConfigurationAdmin>> caTracker;

    /** The listener for the coordinator. */
    private final Listener coordinatorListener;

    /** The current configurator. */
    private volatile Configurator configurator;

    private final List<ServiceReference<ConfigurationAdmin>> configAdminReferences;

    /**
     * Start listeners
     */
    public ServicesListener(final BundleContext bundleContext) {
        this.bundleContext = bundleContext;
        this.configAdminReferences = new ArrayList<>();
        this.caTracker = new ServiceTracker<>(bundleContext, ConfigurationAdmin.class,

                new ServiceTrackerCustomizer<ConfigurationAdmin, ServiceReference<ConfigurationAdmin>>() {

            @Override
            public ServiceReference<ConfigurationAdmin> addingService(final ServiceReference<ConfigurationAdmin> reference) {
                synchronized ( configAdminReferences ) {
                    configAdminReferences.add(reference);
                    Collections.sort(configAdminReferences);
                }
                notifyChange();
                return reference;
            }

            @Override
            public void modifiedService(final ServiceReference<ConfigurationAdmin> reference,
                    final ServiceReference<ConfigurationAdmin> service) {
                // nothing to do
            }

            @Override
            public void removedService(final ServiceReference<ConfigurationAdmin> reference,
                    final ServiceReference<ConfigurationAdmin> service) {
                synchronized ( configAdminReferences ) {
                    configAdminReferences.remove(reference);
                }
                notifyChange();
            }
        });
        this.coordinatorListener = new Listener("org.osgi.service.coordinator.Coordinator");
        this.caTracker.open();
        this.coordinatorListener.start();
        SystemLogger.debug("Started services listener for configurator.");
    }

    /**
     * Notify of service changes from the listeners.
     * If all services are available, start
     */
    public void notifyChange() {
        synchronized ( configAdminReferences ) {
            // check if there is at least a single configuration admin
            final boolean hasConfigAdmin = !this.configAdminReferences.isEmpty();
            final Object coordinator = this.coordinatorListener.getService();
            SystemLogger.debug("Services updated for configurator: " + configAdminReferences + " - " + coordinator);

            if ( hasConfigAdmin ) {
                boolean isNew = configurator == null;
                if ( isNew ) {
                    SystemLogger.debug("Starting new configurator");
                    configurator = new Configurator(this.bundleContext, this.configAdminReferences);
                }
                configurator.setCoordinator(coordinator);
                if ( isNew ) {
                    configurator.start();
                } else {
                    configurator.configAdminAdded();
                }
            } else {
                if ( configurator != null ) {
                    SystemLogger.debug("Stopping configurator");
                    configurator.shutdown();
                    configurator = null;
                }
            }
        }
    }

    /**
     * Deactivate this listener.
     */
    public void deactivate() {
        this.caTracker.close();
        this.coordinatorListener.deactivate();
        if ( configurator != null ) {
            configurator.shutdown();
            configurator = null;
        }
    }

    /**
     * Helper class listening for service events for a defined service.
     */
    protected final class Listener implements ServiceListener {

        /** The name of the service. */
        private final String serviceName;

        /** The service reference. */
        private volatile ServiceReference<?> reference;

        /** The service. */
        private volatile Object service;

        /**
         * Constructor
         */
        public Listener(final String serviceName) {
            this.serviceName = serviceName;
        }

        /**
         * Start the listener.
         * First register a service listener and then check for the service.
         */
        public void start() {
            try {
                bundleContext.addServiceListener(this, "("
                        + Constants.OBJECTCLASS + "=" + serviceName + ")");
            } catch (final InvalidSyntaxException ise) {
                // this should really never happen
                throw new RuntimeException("Unexpected exception occured.", ise);
            }
            this.retainService();
        }

        /**
         * Unregister the listener.
         */
        public void deactivate() {
            bundleContext.removeServiceListener(this);
        }

        /**
         * Return the service (if available)
         */
        public synchronized Object getService() {
            return this.service;
        }

        /**
         * Try to get the service and notify the change.
         */
        private synchronized void retainService() {
            if ( this.reference == null ) {
                this.reference = bundleContext.getServiceReference(this.serviceName);
                if ( this.reference != null ) {
                    this.service = bundleContext.getService(this.reference);
                    if ( this.service == null ) {
                        this.reference = null;
                    } else {
                        notifyChange();
                    }
                }
            }
        }

        /**
         * Try to release the service and notify the change.
         */
        private synchronized void releaseService(final ServiceReference<?> ref) {
            if ( this.reference != null && this.reference.compareTo(ref) == 0 ) {
                this.service = null;
                bundleContext.ungetService(this.reference);
                this.reference = null;
                notifyChange();
            }
        }

        /**
         * @see org.osgi.framework.ServiceListener#serviceChanged(org.osgi.framework.ServiceEvent)
         */
        @Override
        public void serviceChanged(ServiceEvent event) {
            if (event.getType() == ServiceEvent.REGISTERED) {
                this.retainService();
            } else if ( event.getType() == ServiceEvent.UNREGISTERING ) {
                this.releaseService(event.getServiceReference());
                this.retainService();
            }
        }
    }
}