    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    /**
    054     * A special {@code Iterator} implementation used by the {@code getEventListenerIterator()} method. This iterator
    055     * returns only listeners compatible with a specified event type. It has a convenience method for invoking the current
    056     * listener in the iteration with an event.
    057     *
    058     * @param <T> the event type
    059     */
    060    public static final class EventListenerIterator<T extends Event> implements Iterator<EventListener<? super T>> {
    061
    062        /** The underlying iterator. */
    063        private final Iterator<EventListenerRegistrationData<?>> underlyingIterator;
    064
    065        /** The base event type. */
    066        private final EventType<T> baseEventType;
    067
    068        /** The set with accepted event types. */
    069        private final Set<EventType<?>> acceptedTypes;
    070
    071        /** The next element in the iteration. */
    072        private EventListener<? super T> nextElement;
    073
    074        private EventListenerIterator(final Iterator<EventListenerRegistrationData<?>> it, final EventType<T> base) {
    075            underlyingIterator = it;
    076            baseEventType = base;
    077            acceptedTypes = EventType.fetchSuperEventTypes(base);
    078            initNextElement();
    079        }
    080
    081        /**
    082         * Extracts the listener from the given data object and performs a cast to the target type. This is safe because it has
    083         * been checked before that the type is compatible.
    084         *
    085         * @param regData the data object
    086         * @return the extracted listener
    087         */
    088        @SuppressWarnings("unchecked")
    089        private EventListener<? super T> castListener(final EventListenerRegistrationData<?> regData) {
    090            @SuppressWarnings("rawtypes")
    091            final EventListener listener = regData.getListener();
    092            return listener;
    093        }
    094
    095        @Override
    096        public boolean hasNext() {
    097            return nextElement != null;
    098        }
    099
    100        /**
    101         * Determines the next element in the iteration.
    102         */
    103        private void initNextElement() {
    104            nextElement = null;
    105            while (underlyingIterator.hasNext() && nextElement == null) {
    106                final EventListenerRegistrationData<?> regData = underlyingIterator.next();
    107                if (acceptedTypes.contains(regData.getEventType())) {
    108                    nextElement = castListener(regData);
    109                }
    110            }
    111        }
    112
    113        /**
    114         * Obtains the next event listener in this iteration and invokes it with the given event object.
    115         *
    116         * @param event the event object
    117         * @throws NoSuchElementException if iteration is at its end
    118         */
    119        public void invokeNext(final Event event) {
    120            validateEvent(event);
    121            invokeNextListenerUnchecked(event);
    122        }
    123
    124        /**
    125         * Invokes the next event listener in the iteration without doing a validity check on the event. This method is called
    126         * internally to avoid duplicate event checks.
    127         *
    128         * @param event the event object
    129         */
    130        private void invokeNextListenerUnchecked(final Event event) {
    131            callListener(next(), event);
    132        }
    133
    134        @Override
    135        public EventListener<? super T> next() {
    136            if (nextElement == null) {
    137                throw new NoSuchElementException("No more event listeners.");
    138            }
    139
    140            final EventListener<? super T> result = nextElement;
    141            initNextElement();
    142            return result;
    143        }
    144
    145        /**
    146         * {@inheritDoc} This implementation always throws an exception. Removing elements is not supported.
    147         */
    148        @Override
    149        public void remove() {
    150            throw new UnsupportedOperationException("Removing elements is not supported.");
    151        }
    152
    153        /**
    154         * Checks whether the specified event can be passed to an event listener in this iteration. This check is done via the
    155         * hierarchy of event types.
    156         *
    157         * @param event the event object
    158         * @throws IllegalArgumentException if the event is invalid
    159         */
    160        private void validateEvent(final Event event) {
    161            if (event == null || !EventType.fetchSuperEventTypes(event.getEventType()).contains(baseEventType)) {
    162                throw new IllegalArgumentException("Event incompatible with listener iteration: " + event);
    163            }
    164        }
    165    }
    166
    167    /**
    168     * Helper method for calling an event listener with an event. We have to operate on raw types to make this code compile.
    169     * However, this is safe because of the way the listeners have been registered and associated with event types - so it
    170     * is ensured that the event is compatible with the listener.
    171     *
    172     * @param listener the event listener to be called
    173     * @param event the event to be fired
    174     */
    175    @SuppressWarnings("unchecked")
    176    private static void callListener(final EventListener<?> listener, final Event event) {
    177        @SuppressWarnings("rawtypes")
    178        final EventListener rowListener = listener;
    179        rowListener.onEvent(event);
    180    }
    181
    182    /** A list with the listeners added to this object. */
    183    private final List<EventListenerRegistrationData<?>> listeners;
    184
    185    /**
    186     * Creates a new instance of {@code EventListenerList}.
    187     */
    188    public EventListenerList() {
    189        listeners = new CopyOnWriteArrayList<>();
    190    }
    191
    192    /**
    193     * Adds all event listener registrations stored in the specified {@code EventListenerList} to this list.
    194     *
    195     * @param c the list to be copied (must not be <strong>null</strong>)
    196     * @throws IllegalArgumentException if the list to be copied is <strong>null</strong>
    197     */
    198    public void addAll(final EventListenerList c) {
    199        if (c == null) {
    200            throw new IllegalArgumentException("List to be copied must not be null.");
    201        }
    202        c.getRegistrations().forEach(this::addEventListener);
    203    }
    204
    205    /**
    206     * Adds the specified listener registration data object to the internal list of event listeners. This is an alternative
    207     * registration method; the event type and the listener are passed as a single data object.
    208     *
    209     * @param regData the registration data object (must not be <strong>null</strong>)
    210     * @param <T> the type of events processed by this listener
    211     * @throws IllegalArgumentException if the registration data object is <strong>null</strong>
    212     */
    213    public <T extends Event> void addEventListener(final EventListenerRegistrationData<T> regData) {
    214        if (regData == null) {
    215            throw new IllegalArgumentException("EventListenerRegistrationData must not be null.");
    216        }
    217        listeners.add(regData);
    218    }
    219
    220    /**
    221     * Adds an event listener for the specified event type. This listener is notified about events of this type and all its
    222     * sub types.
    223     *
    224     * @param type the event type (must not be <strong>null</strong>)
    225     * @param listener the listener to be registered (must not be <strong>null</strong>)
    226     * @param <T> the type of events processed by this listener
    227     * @throws IllegalArgumentException if a required parameter is <strong>null</strong>
    228     */
    229    public <T extends Event> void addEventListener(final EventType<T> type, final EventListener<? super T> listener) {
    230        listeners.add(new EventListenerRegistrationData<>(type, listener));
    231    }
    232
    233    /**
    234     * Removes all event listeners registered at this object.
    235     */
    236    public void clear() {
    237        listeners.clear();
    238    }
    239
    240    /**
    241     * Fires an event to all registered listeners matching the event type.
    242     *
    243     * @param event the event to be fired (must not be <strong>null</strong>)
    244     * @throws IllegalArgumentException if the event is <strong>null</strong>
    245     */
    246    public void fire(final Event event) {
    247        if (event == null) {
    248            throw new IllegalArgumentException("Event to be fired must not be null.");
    249        }
    250
    251        for (final EventListenerIterator<? extends Event> iterator = getEventListenerIterator(event.getEventType()); iterator.hasNext();) {
    252            iterator.invokeNextListenerUnchecked(event);
    253        }
    254    }
    255
    256    /**
    257     * Gets a specialized iterator for obtaining all event listeners stored in this list which are compatible with the
    258     * specified event type.
    259     *
    260     * @param eventType the event type object
    261     * @param <T> the event type
    262     * @return an {@code Iterator} with the selected event listeners
    263     */
    264    public <T extends Event> EventListenerIterator<T> getEventListenerIterator(final EventType<T> eventType) {
    265        return new EventListenerIterator<>(listeners.iterator(), eventType);
    266    }
    267
    268    /**
    269     * Gets an {@code Iterable} allowing access to all event listeners stored in this list which are compatible with the
    270     * specified event type.
    271     *
    272     * @param eventType the event type object
    273     * @param <T> the event type
    274     * @return an {@code Iterable} with the selected event listeners
    275     */
    276    public <T extends Event> Iterable<EventListener<? super T>> getEventListeners(final EventType<T> eventType) {
    277        return () -> getEventListenerIterator(eventType);
    278    }
    279
    280    /**
    281     * Gets an (unmodifiable) list with registration information about all event listeners registered at this object.
    282     *
    283     * @return a list with event listener registration information
    284     */
    285    public List<EventListenerRegistrationData<?>> getRegistrations() {
    286        return Collections.unmodifiableList(listeners);
    287    }
    288
    289    /**
    290     * Gets a list with {@code EventListenerRegistrationData} objects for all event listener registrations of the
    291     * specified event type or an event type having this type as super type (directly or indirectly). Note that this is the
    292     * opposite direction than querying event types for firing events: in this case event listener registrations are
    293     * searched which are super event types from a given type. This method in contrast returns event listener registrations
    294     * for listeners that extend a given super type.
    295     *
    296     * @param eventType the event type object
    297     * @param <T> the event type
    298     * @return a list with the matching event listener registration objects
    299     */
    300    public <T extends Event> List<EventListenerRegistrationData<? extends T>> getRegistrationsForSuperType(final EventType<T> eventType) {
    301        final Map<EventType<?>, Set<EventType<?>>> superTypes = new HashMap<>();
    302        final List<EventListenerRegistrationData<? extends T>> results = new LinkedList<>();
    303
    304        listeners.forEach(reg -> {
    305            final Set<EventType<?>> base = superTypes.computeIfAbsent(reg.getEventType(), EventType::fetchSuperEventTypes);
    306            if (base.contains(eventType)) {
    307                @SuppressWarnings("unchecked")
    308                final
    309                // This is safe because we just did a check
    310                EventListenerRegistrationData<? extends T> result = (EventListenerRegistrationData<? extends T>) reg;
    311                results.add(result);
    312            }
    313        });
    314
    315        return results;
    316    }
    317
    318    /**
    319     * Removes the event listener registration defined by the passed in data object. This is an alternative method for
    320     * removing a listener which expects the event type and the listener in a single data object.
    321     *
    322     * @param regData the registration data object
    323     * @param <T> the type of events processed by this listener
    324     * @return a flag whether a listener registration was removed
    325     * @see #removeEventListener(EventType, EventListener)
    326     */
    327    public <T extends Event> boolean removeEventListener(final EventListenerRegistrationData<T> regData) {
    328        return listeners.remove(regData);
    329    }
    330
    331    /**
    332     * Removes the event listener registration for the given event type and listener. An event listener instance may be
    333     * registered multiple times for different event types. Therefore, when removing a listener the event type of the
    334     * registration in question has to be specified. The return value indicates whether a registration was removed. A value
    335     * of <strong>false</strong> means that no such combination of event type and listener was found.
    336     *
    337     * @param eventType the event type
    338     * @param listener the event listener to be removed
    339     * @param <T> the type of events processed by this listener
    340     * @return a flag whether a listener registration was removed
    341     */
    342    public <T extends Event> boolean removeEventListener(final EventType<T> eventType, final EventListener<? super T> listener) {
    343        return !(listener == null || eventType == null) && removeEventListener(new EventListenerRegistrationData<>(eventType, listener));
    344    }
    345}