    001/*
    002 * Licensed to the Apache Software Foundation (ASF) under one or more
    003 * contributor license agreements.  See the NOTICE file distributed with
    004 * this work for additional information regarding copyright ownership.
    005 * The ASF licenses this file to You under the Apache License, Version 2.0
    006 * (the "License"); you may not use this file except in compliance with
    007 * the License.  You may obtain a copy of the License at
    008 *
    009 *     http://www.apache.org/licenses/LICENSE-2.0
    010 *
    011 * Unless required by applicable law or agreed to in writing, software
    012 * distributed under the License is distributed on an "AS IS" BASIS,
    013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    014 * See the License for the specific language governing permissions and
    015 * limitations under the License.
    016 */
    017package org.apache.commons.configuration2.event;
    018
    019import java.util.Collections;
    020import java.util.HashMap;
    021import java.util.Iterator;
    022import java.util.LinkedList;
    023import java.util.List;
    024import java.util.Map;
    025import java.util.NoSuchElementException;
    026import java.util.Set;
    027import java.util.concurrent.CopyOnWriteArrayList;
    028
    029/**
    030 * <p>
    031 * A class for managing event listeners for an event source.
    032 * </p>
    033 * <p>
    034 * This class allows registering an arbitrary number of event listeners for
    035 * specific event types. Event types are specified using the {@link EventType}
    036 * class. Due to the type parameters in method signatures, it is guaranteed that
    037 * registered listeners are compatible with the event types they are interested
    038 * in.
    039 * </p>
    040 * <p>
    041 * There are also methods for firing events. Here all registered listeners are
    042 * determined - based on the event type specified at registration time - which
    043 * should receive the event to be fired. So basically, the event type at
    044 * listener registration serves as a filter criterion. Because of the
    045 * hierarchical nature of event types it can be determined in a fine-grained way
    046 * which events are propagated to which listeners. It is also possible to
    047 * register a listener multiple times for different event types.
    048 * </p>
    049 * <p>
    050 * Implementation note: This class is thread-safe.
    051 * </p>
    052 *
    053 * @since 2.0
    054 */
    055public class EventListenerList
    056{
    057    /** A list with the listeners added to this object. */
    058    private final List<EventListenerRegistrationData<?>> listeners;
    059
    060    /**
    061     * Creates a new instance of {@code EventListenerList}.
    062     */
    063    public EventListenerList()
    064    {
    065        listeners =
    066                new CopyOnWriteArrayList<>();
    067    }
    068
    069    /**
    070     * Adds an event listener for the specified event type. This listener is
    071     * notified about events of this type and all its sub types.
    072     *
    073     * @param type the event type (must not be <b>null</b>)
    074     * @param listener the listener to be registered (must not be <b>null</b>)
    075     * @param <T> the type of events processed by this listener
    076     * @throws IllegalArgumentException if a required parameter is <b>null</b>
    077     */
    078    public <T extends Event> void addEventListener(final EventType<T> type,
    079            final EventListener<? super T> listener)
    080    {
    081        listeners.add(new EventListenerRegistrationData<>(type, listener));
    082    }
    083
    084    /**
    085     * Adds the specified listener registration data object to the internal list
    086     * of event listeners. This is an alternative registration method; the event
    087     * type and the listener are passed as a single data object.
    088     *
    089     * @param regData the registration data object (must not be <b>null</b>)
    090     * @param <T> the type of events processed by this listener
    091     * @throws IllegalArgumentException if the registration data object is
    092     *         <b>null</b>
    093     */
    094    public <T extends Event> void addEventListener(
    095            final EventListenerRegistrationData<T> regData)
    096    {
    097        if (regData == null)
    098        {
    099            throw new IllegalArgumentException(
    100                    "EventListenerRegistrationData must not be null!");
    101        }
    102        listeners.add(regData);
    103    }
    104
    105    /**
    106     * Removes the event listener registration for the given event type and
    107     * listener. An event listener instance may be registered multiple times for
    108     * different event types. Therefore, when removing a listener the event type
    109     * of the registration in question has to be specified. The return value
    110     * indicates whether a registration was removed. A value of <b>false</b>
    111     * means that no such combination of event type and listener was found.
    112     *
    113     * @param eventType the event type
    114     * @param listener the event listener to be removed
    115     * @param <T> the type of events processed by this listener
    116     * @return a flag whether a listener registration was removed
    117     */
    118    public <T extends Event> boolean removeEventListener(
    119            final EventType<T> eventType, final EventListener<? super T> listener)
    120    {
    121        return !(listener == null || eventType == null)
    122                && removeEventListener(new EventListenerRegistrationData<>(
    123                        eventType, listener));
    124    }
    125
    126    /**
    127     * Removes the event listener registration defined by the passed in data
    128     * object. This is an alternative method for removing a listener which
    129     * expects the event type and the listener in a single data object.
    130     *
    131     * @param regData the registration data object
    132     * @param <T> the type of events processed by this listener
    133     * @return a flag whether a listener registration was removed
    134     * @see #removeEventListener(EventType, EventListener)
    135     */
    136    public <T extends Event> boolean removeEventListener(
    137            final EventListenerRegistrationData<T> regData)
    138    {
    139        return listeners.remove(regData);
    140    }
    141
    142    /**
    143     * Fires an event to all registered listeners matching the event type.
    144     *
    145     * @param event the event to be fired (must not be <b>null</b>)
    146     * @throws IllegalArgumentException if the event is <b>null</b>
    147     */
    148    public void fire(final Event event)
    149    {
    150        if (event == null)
    151        {
    152            throw new IllegalArgumentException(
    153                    "Event to be fired must not be null!");
    154        }
    155
    156        for (final EventListenerIterator<? extends Event> iterator =
    157                getEventListenerIterator(event.getEventType()); iterator
    158                .hasNext();)
    159        {
    160            iterator.invokeNextListenerUnchecked(event);
    161        }
    162    }
    163
    164    /**
    165     * Returns an {@code Iterable} allowing access to all event listeners stored
    166     * in this list which are compatible with the specified event type.
    167     *
    168     * @param eventType the event type object
    169     * @param <T> the event type
    170     * @return an {@code Iterable} with the selected event listeners
    171     */
    172    public <T extends Event> Iterable<EventListener<? super T>> getEventListeners(
    173            final EventType<T> eventType)
    174    {
    175        return () -> getEventListenerIterator(eventType);
    176    }
    177
    178    /**
    179     * Returns a specialized iterator for obtaining all event listeners stored
    180     * in this list which are compatible with the specified event type.
    181     *
    182     * @param eventType the event type object
    183     * @param <T> the event type
    184     * @return an {@code Iterator} with the selected event listeners
    185     */
    186    public <T extends Event> EventListenerIterator<T> getEventListenerIterator(
    187            final EventType<T> eventType)
    188    {
    189        return new EventListenerIterator<>(listeners.iterator(), eventType);
    190    }
    191
    192    /**
    193     * Returns an (unmodifiable) list with registration information about all
    194     * event listeners registered at this object.
    195     *
    196     * @return a list with event listener registration information
    197     */
    198    public List<EventListenerRegistrationData<?>> getRegistrations()
    199    {
    200        return Collections.unmodifiableList(listeners);
    201    }
    202
    203    /**
    204     * Returns a list with {@code EventListenerRegistrationData} objects for all
    205     * event listener registrations of the specified event type or an event type
    206     * having this type as super type (directly or indirectly). Note that this
    207     * is the opposite direction than querying event types for firing events: in
    208     * this case event listener registrations are searched which are super event
    209     * types from a given type. This method in contrast returns event listener
    210     * registrations for listeners that extend a given super type.
    211     *
    212     * @param eventType the event type object
    213     * @param <T> the event type
    214     * @return a list with the matching event listener registration objects
    215     */
    216    public <T extends Event> List<EventListenerRegistrationData<? extends T>> getRegistrationsForSuperType(
    217            final EventType<T> eventType)
    218    {
    219        final Map<EventType<?>, Set<EventType<?>>> superTypes =
    220                new HashMap<>();
    221        final List<EventListenerRegistrationData<? extends T>> results =
    222                new LinkedList<>();
    223
    224        for (final EventListenerRegistrationData<?> reg : listeners)
    225        {
    226            Set<EventType<?>> base = superTypes.get(reg.getEventType());
    227            if (base == null)
    228            {
    229                base = EventType.fetchSuperEventTypes(reg.getEventType());
    230                superTypes.put(reg.getEventType(), base);
    231            }
    232            if (base.contains(eventType))
    233            {
    234                @SuppressWarnings("unchecked")
    235                final
    236                // This is safe because we just did a check
    237                EventListenerRegistrationData<? extends T> result =
    238                        (EventListenerRegistrationData<? extends T>) reg;
    239                results.add(result);
    240            }
    241        }
    242
    243        return results;
    244    }
    245
    246    /**
    247     * Removes all event listeners registered at this object.
    248     */
    249    public void clear()
    250    {
    251        listeners.clear();
    252    }
    253
    254    /**
    255     * Adds all event listener registrations stored in the specified
    256     * {@code EventListenerList} to this list.
    257     *
    258     * @param c the list to be copied (must not be <b>null</b>)
    259     * @throws IllegalArgumentException if the list to be copied is <b>null</b>
    260     */
    261    public void addAll(final EventListenerList c)
    262    {
    263        if (c == null)
    264        {
    265            throw new IllegalArgumentException(
    266                    "List to be copied must not be null!");
    267        }
    268
    269        for (final EventListenerRegistrationData<?> regData : c.getRegistrations())
    270        {
    271            addEventListener(regData);
    272        }
    273    }
    274
    275    /**
    276     * Helper method for calling an event listener with an event. We have to
    277     * operate on raw types to make this code compile. However, this is safe
    278     * because of the way the listeners have been registered and associated with
    279     * event types - so it is ensured that the event is compatible with the
    280     * listener.
    281     *
    282     * @param listener the event listener to be called
    283     * @param event the event to be fired
    284     */
    285    @SuppressWarnings("unchecked")
    286    private static void callListener(final EventListener<?> listener, final Event event)
    287    {
    288        @SuppressWarnings("rawtypes")
    289        final
    290        EventListener rowListener = listener;
    291        rowListener.onEvent(event);
    292    }
    293
    294    /**
    295     * A special {@code Iterator} implementation used by the
    296     * {@code getEventListenerIterator()} method. This iterator returns only
    297     * listeners compatible with a specified event type. It has a convenience
    298     * method for invoking the current listener in the iteration with an event.
    299     *
    300     * @param <T> the event type
    301     */
    302    public static final class EventListenerIterator<T extends Event> implements
    303            Iterator<EventListener<? super T>>
    304    {
    305        /** The underlying iterator. */
    306        private final Iterator<EventListenerRegistrationData<?>> underlyingIterator;
    307
    308        /** The base event type. */
    309        private final EventType<T> baseEventType;
    310
    311        /** The set with accepted event types. */
    312        private final Set<EventType<?>> acceptedTypes;
    313
    314        /** The next element in the iteration. */
    315        private EventListener<? super T> nextElement;
    316
    317        private EventListenerIterator(
    318                final Iterator<EventListenerRegistrationData<?>> it, final EventType<T> base)
    319        {
    320            underlyingIterator = it;
    321            baseEventType = base;
    322            acceptedTypes = EventType.fetchSuperEventTypes(base);
    323            initNextElement();
    324        }
    325
    326        @Override
    327        public boolean hasNext()
    328        {
    329            return nextElement != null;
    330        }
    331
    332        @Override
    333        public EventListener<? super T> next()
    334        {
    335            if (nextElement == null)
    336            {
    337                throw new NoSuchElementException("No more event listeners!");
    338            }
    339
    340            final EventListener<? super T> result = nextElement;
    341            initNextElement();
    342            return result;
    343        }
    344
    345        /**
    346         * Obtains the next event listener in this iteration and invokes it with
    347         * the given event object.
    348         *
    349         * @param event the event object
    350         * @throws NoSuchElementException if iteration is at its end
    351         */
    352        public void invokeNext(final Event event)
    353        {
    354            validateEvent(event);
    355            invokeNextListenerUnchecked(event);
    356        }
    357
    358        /**
    359         * {@inheritDoc} This implementation always throws an exception.
    360         * Removing elements is not supported.
    361         */
    362        @Override
    363        public void remove()
    364        {
    365            throw new UnsupportedOperationException(
    366                    "Removing elements is not supported!");
    367        }
    368
    369        /**
    370         * Determines the next element in the iteration.
    371         */
    372        private void initNextElement()
    373        {
    374            nextElement = null;
    375            while (underlyingIterator.hasNext() && nextElement == null)
    376            {
    377                final EventListenerRegistrationData<?> regData =
    378                        underlyingIterator.next();
    379                if (acceptedTypes.contains(regData.getEventType()))
    380                {
    381                    nextElement = castListener(regData);
    382                }
    383            }
    384        }
    385
    386        /**
    387         * Checks whether the specified event can be passed to an event listener
    388         * in this iteration. This check is done via the hierarchy of event
    389         * types.
    390         *
    391         * @param event the event object
    392         * @throws IllegalArgumentException if the event is invalid
    393         */
    394        private void validateEvent(final Event event)
    395        {
    396            if (event == null
    397                    || !EventType.fetchSuperEventTypes(event.getEventType()).contains(
    398                    baseEventType))
    399            {
    400                throw new IllegalArgumentException(
    401                        "Event incompatible with listener iteration: " + event);
    402            }
    403        }
    404
    405        /**
    406         * Invokes the next event listener in the iteration without doing a
    407         * validity check on the event. This method is called internally to
    408         * avoid duplicate event checks.
    409         *
    410         * @param event the event object
    411         */
    412        private void invokeNextListenerUnchecked(final Event event)
    413        {
    414            final EventListener<? super T> listener = next();
    415            callListener(listener, event);
    416        }
    417
    418        /**
    419         * Extracts the listener from the given data object and performs a cast
    420         * to the target type. This is safe because it has been checked before
    421         * that the type is compatible.
    422         *
    423         * @param regData the data object
    424         * @return the extracted listener
    425         */
    426        @SuppressWarnings("unchecked")
    427        private EventListener<? super T> castListener(
    428                final EventListenerRegistrationData<?> regData)
    429        {
    430            @SuppressWarnings("rawtypes")
    431            final
    432            EventListener listener = regData.getListener();
    433            return listener;
    434        }
    435    }
    436}