EventListenerList.java
/*
* 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.commons.configuration2.event;
import java.util.Collections;
import java.util.HashMap;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;
import java.util.concurrent.CopyOnWriteArrayList;
/**
* <p>
* A class for managing event listeners for an event source.
* </p>
* <p>
* This class allows registering an arbitrary number of event listeners for specific event types. Event types are
* specified using the {@link EventType} class. Due to the type parameters in method signatures, it is guaranteed that
* registered listeners are compatible with the event types they are interested in.
* </p>
* <p>
* There are also methods for firing events. Here all registered listeners are determined - based on the event type
* specified at registration time - which should receive the event to be fired. So basically, the event type at listener
* registration serves as a filter criterion. Because of the hierarchical nature of event types it can be determined in
* a fine-grained way which events are propagated to which listeners. It is also possible to register a listener
* multiple times for different event types.
* </p>
* <p>
* Implementation note: This class is thread-safe.
* </p>
*
* @since 2.0
*/
public class EventListenerList {
/**
* A special {@code Iterator} implementation used by the {@code getEventListenerIterator()} method. This iterator
* returns only listeners compatible with a specified event type. It has a convenience method for invoking the current
* listener in the iteration with an event.
*
* @param <T> the event type
*/
public static final class EventListenerIterator<T extends Event> implements Iterator<EventListener<? super T>> {
/** The underlying iterator. */
private final Iterator<EventListenerRegistrationData<?>> underlyingIterator;
/** The base event type. */
private final EventType<T> baseEventType;
/** The set with accepted event types. */
private final Set<EventType<?>> acceptedTypes;
/** The next element in the iteration. */
private EventListener<? super T> nextElement;
private EventListenerIterator(final Iterator<EventListenerRegistrationData<?>> it, final EventType<T> base) {
underlyingIterator = it;
baseEventType = base;
acceptedTypes = EventType.fetchSuperEventTypes(base);
initNextElement();
}
/**
* Extracts the listener from the given data object and performs a cast to the target type. This is safe because it has
* been checked before that the type is compatible.
*
* @param regData the data object
* @return the extracted listener
*/
@SuppressWarnings("unchecked")
private EventListener<? super T> castListener(final EventListenerRegistrationData<?> regData) {
@SuppressWarnings("rawtypes")
final EventListener listener = regData.getListener();
return listener;
}
@Override
public boolean hasNext() {
return nextElement != null;
}
/**
* Determines the next element in the iteration.
*/
private void initNextElement() {
nextElement = null;
while (underlyingIterator.hasNext() && nextElement == null) {
final EventListenerRegistrationData<?> regData = underlyingIterator.next();
if (acceptedTypes.contains(regData.getEventType())) {
nextElement = castListener(regData);
}
}
}
/**
* Obtains the next event listener in this iteration and invokes it with the given event object.
*
* @param event the event object
* @throws NoSuchElementException if iteration is at its end
*/
public void invokeNext(final Event event) {
validateEvent(event);
invokeNextListenerUnchecked(event);
}
/**
* Invokes the next event listener in the iteration without doing a validity check on the event. This method is called
* internally to avoid duplicate event checks.
*
* @param event the event object
*/
private void invokeNextListenerUnchecked(final Event event) {
callListener(next(), event);
}
@Override
public EventListener<? super T> next() {
if (nextElement == null) {
throw new NoSuchElementException("No more event listeners!");
}
final EventListener<? super T> result = nextElement;
initNextElement();
return result;
}
/**
* {@inheritDoc} This implementation always throws an exception. Removing elements is not supported.
*/
@Override
public void remove() {
throw new UnsupportedOperationException("Removing elements is not supported!");
}
/**
* Checks whether the specified event can be passed to an event listener in this iteration. This check is done via the
* hierarchy of event types.
*
* @param event the event object
* @throws IllegalArgumentException if the event is invalid
*/
private void validateEvent(final Event event) {
if (event == null || !EventType.fetchSuperEventTypes(event.getEventType()).contains(baseEventType)) {
throw new IllegalArgumentException("Event incompatible with listener iteration: " + event);
}
}
}
/**
* Helper method for calling an event listener with an event. We have to operate on raw types to make this code compile.
* However, this is safe because of the way the listeners have been registered and associated with event types - so it
* is ensured that the event is compatible with the listener.
*
* @param listener the event listener to be called
* @param event the event to be fired
*/
@SuppressWarnings("unchecked")
private static void callListener(final EventListener<?> listener, final Event event) {
@SuppressWarnings("rawtypes")
final EventListener rowListener = listener;
rowListener.onEvent(event);
}
/** A list with the listeners added to this object. */
private final List<EventListenerRegistrationData<?>> listeners;
/**
* Creates a new instance of {@code EventListenerList}.
*/
public EventListenerList() {
listeners = new CopyOnWriteArrayList<>();
}
/**
* Adds all event listener registrations stored in the specified {@code EventListenerList} to this list.
*
* @param c the list to be copied (must not be <b>null</b>)
* @throws IllegalArgumentException if the list to be copied is <b>null</b>
*/
public void addAll(final EventListenerList c) {
if (c == null) {
throw new IllegalArgumentException("List to be copied must not be null!");
}
c.getRegistrations().forEach(this::addEventListener);
}
/**
* Adds the specified listener registration data object to the internal list of event listeners. This is an alternative
* registration method; the event type and the listener are passed as a single data object.
*
* @param regData the registration data object (must not be <b>null</b>)
* @param <T> the type of events processed by this listener
* @throws IllegalArgumentException if the registration data object is <b>null</b>
*/
public <T extends Event> void addEventListener(final EventListenerRegistrationData<T> regData) {
if (regData == null) {
throw new IllegalArgumentException("EventListenerRegistrationData must not be null!");
}
listeners.add(regData);
}
/**
* Adds an event listener for the specified event type. This listener is notified about events of this type and all its
* sub types.
*
* @param type the event type (must not be <b>null</b>)
* @param listener the listener to be registered (must not be <b>null</b>)
* @param <T> the type of events processed by this listener
* @throws IllegalArgumentException if a required parameter is <b>null</b>
*/
public <T extends Event> void addEventListener(final EventType<T> type, final EventListener<? super T> listener) {
listeners.add(new EventListenerRegistrationData<>(type, listener));
}
/**
* Removes all event listeners registered at this object.
*/
public void clear() {
listeners.clear();
}
/**
* Fires an event to all registered listeners matching the event type.
*
* @param event the event to be fired (must not be <b>null</b>)
* @throws IllegalArgumentException if the event is <b>null</b>
*/
public void fire(final Event event) {
if (event == null) {
throw new IllegalArgumentException("Event to be fired must not be null!");
}
for (final EventListenerIterator<? extends Event> iterator = getEventListenerIterator(event.getEventType()); iterator.hasNext();) {
iterator.invokeNextListenerUnchecked(event);
}
}
/**
* Gets a specialized iterator for obtaining all event listeners stored in this list which are compatible with the
* specified event type.
*
* @param eventType the event type object
* @param <T> the event type
* @return an {@code Iterator} with the selected event listeners
*/
public <T extends Event> EventListenerIterator<T> getEventListenerIterator(final EventType<T> eventType) {
return new EventListenerIterator<>(listeners.iterator(), eventType);
}
/**
* Gets an {@code Iterable} allowing access to all event listeners stored in this list which are compatible with the
* specified event type.
*
* @param eventType the event type object
* @param <T> the event type
* @return an {@code Iterable} with the selected event listeners
*/
public <T extends Event> Iterable<EventListener<? super T>> getEventListeners(final EventType<T> eventType) {
return () -> getEventListenerIterator(eventType);
}
/**
* Gets an (unmodifiable) list with registration information about all event listeners registered at this object.
*
* @return a list with event listener registration information
*/
public List<EventListenerRegistrationData<?>> getRegistrations() {
return Collections.unmodifiableList(listeners);
}
/**
* Gets a list with {@code EventListenerRegistrationData} objects for all event listener registrations of the
* specified event type or an event type having this type as super type (directly or indirectly). Note that this is the
* opposite direction than querying event types for firing events: in this case event listener registrations are
* searched which are super event types from a given type. This method in contrast returns event listener registrations
* for listeners that extend a given super type.
*
* @param eventType the event type object
* @param <T> the event type
* @return a list with the matching event listener registration objects
*/
public <T extends Event> List<EventListenerRegistrationData<? extends T>> getRegistrationsForSuperType(final EventType<T> eventType) {
final Map<EventType<?>, Set<EventType<?>>> superTypes = new HashMap<>();
final List<EventListenerRegistrationData<? extends T>> results = new LinkedList<>();
listeners.forEach(reg -> {
final Set<EventType<?>> base = superTypes.computeIfAbsent(reg.getEventType(), EventType::fetchSuperEventTypes);
if (base.contains(eventType)) {
@SuppressWarnings("unchecked")
final
// This is safe because we just did a check
EventListenerRegistrationData<? extends T> result = (EventListenerRegistrationData<? extends T>) reg;
results.add(result);
}
});
return results;
}
/**
* Removes the event listener registration defined by the passed in data object. This is an alternative method for
* removing a listener which expects the event type and the listener in a single data object.
*
* @param regData the registration data object
* @param <T> the type of events processed by this listener
* @return a flag whether a listener registration was removed
* @see #removeEventListener(EventType, EventListener)
*/
public <T extends Event> boolean removeEventListener(final EventListenerRegistrationData<T> regData) {
return listeners.remove(regData);
}
/**
* Removes the event listener registration for the given event type and listener. An event listener instance may be
* registered multiple times for different event types. Therefore, when removing a listener the event type of the
* registration in question has to be specified. The return value indicates whether a registration was removed. A value
* of <b>false</b> means that no such combination of event type and listener was found.
*
* @param eventType the event type
* @param listener the event listener to be removed
* @param <T> the type of events processed by this listener
* @return a flag whether a listener registration was removed
*/
public <T extends Event> boolean removeEventListener(final EventType<T> eventType, final EventListener<? super T> listener) {
return !(listener == null || eventType == null) && removeEventListener(new EventListenerRegistrationData<>(eventType, listener));
}
}