    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 *     https://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 specific event types. Event types are
    035 * specified using the {@link EventType} class. Due to the type parameters in method signatures, it is guaranteed that
    036 * registered listeners are compatible with the event types they are interested in.
    037 * </p>
    038 * <p>
    039 * There are also methods for firing events. Here all registered listeners are determined - based on the event type
    040 * specified at registration time - which should receive the event to be fired. So basically, the event type at listener
    041 * registration serves as a filter criterion. Because of the hierarchical nature of event types it can be determined in
    042 * a fine-grained way which events are propagated to which listeners. It is also possible to register a listener
    043 * multiple times for different event types.
    044 * </p>
    045 * <p>
    046 * Implementation note: This class is thread-safe.
    047 * </p>
    048 *
    049 * @since 2.0
    050 */
    051public class EventListenerList {
    052    /**
    053     * A special {@code Iterator} implementation used by the {@code getEventListenerIterator()} method. This iterator
    054     * returns only listeners compatible with a specified event type. It has a convenience method for invoking the current
    055     * listener in the iteration with an event.
    056     *
    057     * @param <T> the event type
    058     */
    059    public static final class EventListenerIterator<T extends Event> implements Iterator<EventListener<? super T>> {
    060        /** The underlying iterator. */
    061        private final Iterator<EventListenerRegistrationData<?>> underlyingIterator;
    062
    063        /** The base event type. */
    064        private final EventType<T> baseEventType;
    065
    066        /** The set with accepted event types. */
    067        private final Set<EventType<?>> acceptedTypes;
    068
    069        /** The next element in the iteration. */
    070        private EventListener<? super T> nextElement;
    071
    072        private EventListenerIterator(final Iterator<EventListenerRegistrationData<?>> it, final EventType<T> base) {
    073            underlyingIterator = it;
    074            baseEventType = base;
    075            acceptedTypes = EventType.fetchSuperEventTypes(base);
    076            initNextElement();
    077        }
    078
    079        /**
    080         * Extracts the listener from the given data object and performs a cast to the target type. This is safe because it has
    081         * been checked before that the type is compatible.
    082         *
    083         * @param regData the data object
    084         * @return the extracted listener
    085         */
    086        @SuppressWarnings("unchecked")
    087        private EventListener<? super T> castListener(final EventListenerRegistrationData<?> regData) {
    088            @SuppressWarnings("rawtypes")
    089            final EventListener listener = regData.getListener();
    090            return listener;
    091        }
    092
    093        @Override
    094        public boolean hasNext() {
    095            return nextElement != null;
    096        }
    097
    098        /**
    099         * Determines the next element in the iteration.
    100         */
    101        private void initNextElement() {
    102            nextElement = null;
    103            while (underlyingIterator.hasNext() && nextElement == null) {
    104                final EventListenerRegistrationData<?> regData = underlyingIterator.next();
    105                if (acceptedTypes.contains(regData.getEventType())) {
    106                    nextElement = castListener(regData);
    107                }
    108            }
    109        }
    110
    111        /**
    112         * Obtains the next event listener in this iteration and invokes it with the given event object.
    113         *
    114         * @param event the event object
    115         * @throws NoSuchElementException if iteration is at its end
    116         */
    117        public void invokeNext(final Event event) {
    118            validateEvent(event);
    119            invokeNextListenerUnchecked(event);
    120        }
    121
    122        /**
    123         * Invokes the next event listener in the iteration without doing a validity check on the event. This method is called
    124         * internally to avoid duplicate event checks.
    125         *
    126         * @param event the event object
    127         */
    128        private void invokeNextListenerUnchecked(final Event event) {
    129            callListener(next(), event);
    130        }
    131
    132        @Override
    133        public EventListener<? super T> next() {
    134            if (nextElement == null) {
    135                throw new NoSuchElementException("No more event listeners!");
    136            }
    137
    138            final EventListener<? super T> result = nextElement;
    139            initNextElement();
    140            return result;
    141        }
    142
    143        /**
    144         * {@inheritDoc} This implementation always throws an exception. Removing elements is not supported.
    145         */
    146        @Override
    147        public void remove() {
    148            throw new UnsupportedOperationException("Removing elements is not supported!");
    149        }
    150
    151        /**
    152         * Checks whether the specified event can be passed to an event listener in this iteration. This check is done via the
    153         * hierarchy of event types.
    154         *
    155         * @param event the event object
    156         * @throws IllegalArgumentException if the event is invalid
    157         */
    158        private void validateEvent(final Event event) {
    159            if (event == null || !EventType.fetchSuperEventTypes(event.getEventType()).contains(baseEventType)) {
    160                throw new IllegalArgumentException("Event incompatible with listener iteration: " + event);
    161            }
    162        }
    163    }
    164
    165    /**
    166     * Helper method for calling an event listener with an event. We have to operate on raw types to make this code compile.
    167     * However, this is safe because of the way the listeners have been registered and associated with event types - so it
    168     * is ensured that the event is compatible with the listener.
    169     *
    170     * @param listener the event listener to be called
    171     * @param event the event to be fired
    172     */
    173    @SuppressWarnings("unchecked")
    174    private static void callListener(final EventListener<?> listener, final Event event) {
    175        @SuppressWarnings("rawtypes")
    176        final EventListener rowListener = listener;
    177        rowListener.onEvent(event);
    178    }
    179
    180    /** A list with the listeners added to this object. */
    181    private final List<EventListenerRegistrationData<?>> listeners;
    182
    183    /**
    184     * Creates a new instance of {@code EventListenerList}.
    185     */
    186    public EventListenerList() {
    187        listeners = new CopyOnWriteArrayList<>();
    188    }
    189
    190    /**
    191     * Adds all event listener registrations stored in the specified {@code EventListenerList} to this list.
    192     *
    193     * @param c the list to be copied (must not be <strong>null</strong>)
    194     * @throws IllegalArgumentException if the list to be copied is <strong>null</strong>
    195     */
    196    public void addAll(final EventListenerList c) {
    197        if (c == null) {
    198            throw new IllegalArgumentException("List to be copied must not be null!");
    199        }
    200        c.getRegistrations().forEach(this::addEventListener);
    201    }
    202
    203    /**
    204     * Adds the specified listener registration data object to the internal list of event listeners. This is an alternative
    205     * registration method; the event type and the listener are passed as a single data object.
    206     *
    207     * @param regData the registration data object (must not be <strong>null</strong>)
    208     * @param <T> the type of events processed by this listener
    209     * @throws IllegalArgumentException if the registration data object is <strong>null</strong>
    210     */
    211    public <T extends Event> void addEventListener(final EventListenerRegistrationData<T> regData) {
    212        if (regData == null) {
    213            throw new IllegalArgumentException("EventListenerRegistrationData must not be null!");
    214        }
    215        listeners.add(regData);
    216    }
    217
    218    /**
    219     * Adds an event listener for the specified event type. This listener is notified about events of this type and all its
    220     * sub types.
    221     *
    222     * @param type the event type (must not be <strong>null</strong>)
    223     * @param listener the listener to be registered (must not be <strong>null</strong>)
    224     * @param <T> the type of events processed by this listener
    225     * @throws IllegalArgumentException if a required parameter is <strong>null</strong>
    226     */
    227    public <T extends Event> void addEventListener(final EventType<T> type, final EventListener<? super T> listener) {
    228        listeners.add(new EventListenerRegistrationData<>(type, listener));
    229    }
    230
    231    /**
    232     * Removes all event listeners registered at this object.
    233     */
    234    public void clear() {
    235        listeners.clear();
    236    }
    237
    238    /**
    239     * Fires an event to all registered listeners matching the event type.
    240     *
    241     * @param event the event to be fired (must not be <strong>null</strong>)
    242     * @throws IllegalArgumentException if the event is <strong>null</strong>
    243     */
    244    public void fire(final Event event) {
    245        if (event == null) {
    246            throw new IllegalArgumentException("Event to be fired must not be null!");
    247        }
    248
    249        for (final EventListenerIterator<? extends Event> iterator = getEventListenerIterator(event.getEventType()); iterator.hasNext();) {
    250            iterator.invokeNextListenerUnchecked(event);
    251        }
    252    }
    253
    254    /**
    255     * Gets a specialized iterator for obtaining all event listeners stored in this list which are compatible with the
    256     * specified event type.
    257     *
    258     * @param eventType the event type object
    259     * @param <T> the event type
    260     * @return an {@code Iterator} with the selected event listeners
    261     */
    262    public <T extends Event> EventListenerIterator<T> getEventListenerIterator(final EventType<T> eventType) {
    263        return new EventListenerIterator<>(listeners.iterator(), eventType);
    264    }
    265
    266    /**
    267     * Gets an {@code Iterable} allowing access to all event listeners stored in this list which are compatible with the
    268     * specified event type.
    269     *
    270     * @param eventType the event type object
    271     * @param <T> the event type
    272     * @return an {@code Iterable} with the selected event listeners
    273     */
    274    public <T extends Event> Iterable<EventListener<? super T>> getEventListeners(final EventType<T> eventType) {
    275        return () -> getEventListenerIterator(eventType);
    276    }
    277
    278    /**
    279     * Gets an (unmodifiable) list with registration information about all event listeners registered at this object.
    280     *
    281     * @return a list with event listener registration information
    282     */
    283    public List<EventListenerRegistrationData<?>> getRegistrations() {
    284        return Collections.unmodifiableList(listeners);
    285    }
    286
    287    /**
    288     * Gets a list with {@code EventListenerRegistrationData} objects for all event listener registrations of the
    289     * specified event type or an event type having this type as super type (directly or indirectly). Note that this is the
    290     * opposite direction than querying event types for firing events: in this case event listener registrations are
    291     * searched which are super event types from a given type. This method in contrast returns event listener registrations
    292     * for listeners that extend a given super type.
    293     *
    294     * @param eventType the event type object
    295     * @param <T> the event type
    296     * @return a list with the matching event listener registration objects
    297     */
    298    public <T extends Event> List<EventListenerRegistrationData<? extends T>> getRegistrationsForSuperType(final EventType<T> eventType) {
    299        final Map<EventType<?>, Set<EventType<?>>> superTypes = new HashMap<>();
    300        final List<EventListenerRegistrationData<? extends T>> results = new LinkedList<>();
    301
    302        listeners.forEach(reg -> {
    303            final Set<EventType<?>> base = superTypes.computeIfAbsent(reg.getEventType(), EventType::fetchSuperEventTypes);
    304            if (base.contains(eventType)) {
    305                @SuppressWarnings("unchecked")
    306                final
    307                // This is safe because we just did a check
    308                EventListenerRegistrationData<? extends T> result = (EventListenerRegistrationData<? extends T>) reg;
    309                results.add(result);
    310            }
    311        });
    312
    313        return results;
    314    }
    315
    316    /**
    317     * Removes the event listener registration defined by the passed in data object. This is an alternative method for
    318     * removing a listener which expects the event type and the listener in a single data object.
    319     *
    320     * @param regData the registration data object
    321     * @param <T> the type of events processed by this listener
    322     * @return a flag whether a listener registration was removed
    323     * @see #removeEventListener(EventType, EventListener)
    324     */
    325    public <T extends Event> boolean removeEventListener(final EventListenerRegistrationData<T> regData) {
    326        return listeners.remove(regData);
    327    }
    328
    329    /**
    330     * Removes the event listener registration for the given event type and listener. An event listener instance may be
    331     * registered multiple times for different event types. Therefore, when removing a listener the event type of the
    332     * registration in question has to be specified. The return value indicates whether a registration was removed. A value
    333     * of <strong>false</strong> means that no such combination of event type and listener was found.
    334     *
    335     * @param eventType the event type
    336     * @param listener the event listener to be removed
    337     * @param <T> the type of events processed by this listener
    338     * @return a flag whether a listener registration was removed
    339     */
    340    public <T extends Event> boolean removeEventListener(final EventType<T> eventType, final EventListener<? super T> listener) {
    341        return !(listener == null || eventType == null) && removeEventListener(new EventListenerRegistrationData<>(eventType, listener));
    342    }
    343}