Copyright (c) 2004, 2016 IBM Corporation and others.
This program and the accompanying materials
are made available under the terms of the Eclipse Public License 2.0
which accompanies this distribution, and is available at
https://www.eclipse.org/legal/epl-2.0/
SPDX-License-Identifier: EPL-2.0
Contributors:
    IBM Corporation - initial API and implementation
    Mikael Barbero (Eclipse Foundation) - Bug 509234
/*******************************************************************************
 * Copyright (c) 2004, 2016 IBM Corporation and others.
 *
 * This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License 2.0
 * which accompanies this distribution, and is available at
 * https://www.eclipse.org/legal/epl-2.0/
 *
 * SPDX-License-Identifier: EPL-2.0
 * 
 * Contributors:
 *     IBM Corporation - initial API and implementation
 *     Mikael Barbero (Eclipse Foundation) - Bug 509234
 *******************************************************************************/
package org.eclipse.core.runtime;

import java.util.*;
import java.util.stream.Stream;
import java.util.stream.StreamSupport;

This class is a thread safe list that is designed for storing lists of listeners.
The implementation is optimized for minimal memory footprint, frequent reads 
and infrequent writes.  Modification of the list is synchronized and relatively
expensive, while accessing the listeners is very fast.  For legacy code, readers are given access 
to the underlying array data structure for reading, with the trust that they will 
not modify the underlying array.

A listener list handles the same listener being added 
multiple times, and tolerates removal of listeners that are the same as other
listeners in the list.  For this purpose, listeners can be compared with each other 
using either equality or identity, as specified in the list constructor.


Use an enhanced 'for' loop to notify listeners. The recommended
code sequence for notifying all registered listeners of say,
FooListener#eventHappened(Event), is:

ListenerList<FooListener> fooListeners = new ListenerList<>();
//...
for (FooListener listener : fooListeners) {
listener.eventHappened(event);
}

 Legacy code may still call getListeners() and then use a 'for' loop to iterate the Object[]. This might be insignificantly faster, but it lacks type-safety and risks inadvertent modifications to the array. 

This class can be used without OSGi running.

Type parameters:
<E> – the type of listeners in this list
Since:
org.eclipse.equinox.common 3.2/**
 * This class is a thread safe list that is designed for storing lists of listeners.
 * The implementation is optimized for minimal memory footprint, frequent reads 
 * and infrequent writes.  Modification of the list is synchronized and relatively
 * expensive, while accessing the listeners is very fast.  For legacy code, readers are given access 
 * to the underlying array data structure for reading, with the trust that they will 
 * not modify the underlying array.
 * <p>
 * A listener list handles the <i>same</i> listener being added 
 * multiple times, and tolerates removal of listeners that are the same as other
 * listeners in the list.  For this purpose, listeners can be compared with each other 
 * using either equality or identity, as specified in the list constructor.
 * </p>
 * <p>
 * Use an enhanced 'for' loop to notify listeners. The recommended
 * code sequence for notifying all registered listeners of say,
 * <code>FooListener#eventHappened(Event)</code>, is:
 * </p>
 * <pre>
ListenerList&lt;FooListener&gt; fooListeners = new ListenerList&lt;&gt;();
//...
for (FooListener listener : fooListeners) {
	listener.eventHappened(event);
}
 * </pre>
 * <p>
 * Legacy code may still call {@link #getListeners()} and then use a 'for' loop
 * to iterate the {@code Object[]}. This might be insignificantly faster, but
 * it lacks type-safety and risks inadvertent modifications to the array.
 * </p>
 * <p>
 * This class can be used without OSGi running.
 * </p>
 * 
 * @param <E> the type of listeners in this list
 * @since org.eclipse.equinox.common 3.2
 */
public class ListenerList<E> implements Iterable<E> {

	
The empty array singleton instance.
/**
	 * The empty array singleton instance.
	 */
	private static final Object[] EmptyArray = new Object[0];

	
Mode constant (value 0) indicating that listeners should be considered
the same if they are equal.
/**
	 * Mode constant (value 0) indicating that listeners should be considered
	 * the <a href="ListenerList.html#same">same</a> if they are equal.
	 */
	public static final int EQUALITY = 0;

	
Mode constant (value 1) indicating that listeners should be considered
the same if they are identical.
/**
	 * Mode constant (value 1) indicating that listeners should be considered
	 * the <a href="ListenerList.html#same">same</a> if they are identical.
	 */
	public static final int IDENTITY = 1;

	
Indicates the comparison mode used to determine if two
listeners are equivalent
/**
	 * Indicates the comparison mode used to determine if two
	 * listeners are equivalent
	 */
	private final boolean identity;

	
The list of listeners.  Initially empty but initialized
to an array of size capacity the first time a listener is added.
Maintains invariant: listeners != null
/**
	 * The list of listeners.  Initially empty but initialized
	 * to an array of size capacity the first time a listener is added.
	 * Maintains invariant: listeners != null
	 */
	private volatile Object[] listeners = EmptyArray;

	
Creates a listener list in which listeners are compared using equality.
/**
	 * Creates a listener list in which listeners are compared using equality.
	 */
	public ListenerList() {
		this(EQUALITY);
	}

	
Creates a listener list using the provided comparison mode.
Params:
mode – The mode used to determine if listeners are the same./**
	 * Creates a listener list using the provided comparison mode.
	 * 
	 * @param mode The mode used to determine if listeners are the <a href="ListenerList.html#same">same</a>.
	 */
	public ListenerList(int mode) {
		if (mode != EQUALITY && mode != IDENTITY)
			throw new IllegalArgumentException();
		this.identity = mode == IDENTITY;
	}

	
Adds a listener to this list. This method has no effect if the same
listener is already registered.
Params:
listener – the non-null listener to add/**
	 * Adds a listener to this list. This method has no effect if the <a href="ListenerList.html#same">same</a>
	 * listener is already registered.
	 * 
	 * @param listener the non-<code>null</code> listener to add
	 */
	public synchronized void add(E listener) {
		// This method is synchronized to protect against multiple threads adding 
		// or removing listeners concurrently. This does not block concurrent readers.
		if (listener == null)
			throw new IllegalArgumentException();
		// check for duplicates 
		final int oldSize = listeners.length;
		for (int i = 0; i < oldSize; ++i) {
			Object listener2 = listeners[i];
			if (identity ? listener == listener2 : listener.equals(listener2))
				return;
		}
		// Thread safety: create new array to avoid affecting concurrent readers
		Object[] newListeners = new Object[oldSize + 1];
		System.arraycopy(listeners, 0, newListeners, 0, oldSize);
		newListeners[oldSize] = listener;
		//atomic assignment
		this.listeners = newListeners;
	}

	
Returns an array containing all the registered listeners.
The resulting array is unaffected by subsequent adds or removes.
If there are no listeners registered, the result is an empty array.
Use this method when notifying listeners, so that any modifications
to the listener list during the notification will have no effect on 
the notification itself.

Note: Callers of this method must not modify the returned array.

 Note: The recommended and type-safe way to iterate this list is to use an enhanced 'for' statement, see ListenerList. This method is deprecated for new code. 
Returns:
the list of registered listeners/**
	 * Returns an array containing all the registered listeners.
	 * The resulting array is unaffected by subsequent adds or removes.
	 * If there are no listeners registered, the result is an empty array.
	 * Use this method when notifying listeners, so that any modifications
	 * to the listener list during the notification will have no effect on 
	 * the notification itself.
	 * <p>
	 * Note: Callers of this method <b>must not</b> modify the returned array.
	 * </p>
	 * <p>
	 * Note: The recommended and type-safe way to iterate this list is to use
	 * an enhanced 'for' statement, see {@link ListenerList}.
	 * This method is deprecated for new code.
	 * </p>
	 *
	 * @return the list of registered listeners
	 */
	public Object[] getListeners() {
		return listeners;
	}

	
Returns an iterator over all the registered listeners.
The resulting iterator is unaffected by subsequent adds or removes.
Use this method when notifying listeners, so that any modifications
to the listener list during the notification will have no effect on 
the notification itself.
Returns:
an iterator
Since:
org.eclipse.equinox.common 3.8/**
	 * Returns an iterator over all the registered listeners.
	 * The resulting iterator is unaffected by subsequent adds or removes.
	 * Use this method when notifying listeners, so that any modifications
	 * to the listener list during the notification will have no effect on 
	 * the notification itself.
	 * 
	 * @return an iterator
	 * @since org.eclipse.equinox.common 3.8
	 */
	@Override
	public Iterator<E> iterator() {
		return new ListenerListIterator<>(listeners);
	}

	private static class ListenerListIterator<E> implements Iterator<E> {
		private Object[] listeners;
		private int i;

		public ListenerListIterator(Object[] listeners) {
			this.listeners = listeners;
		}

		@Override
		public boolean hasNext() {
			return i < listeners.length;
		}

		@Override
		public E next() {
			if (i >= listeners.length) {
				throw new NoSuchElementException();
			}
			@SuppressWarnings("unchecked") // (E) is safe, because #add(E) only accepts Es
			E next = (E) listeners[i++];
			return next;
		}

		@Override
		public void remove() {
			throw new UnsupportedOperationException();
		}
	}

	
Returns whether this listener list is empty.
Returns:
true if there are no registered listeners, and
  false otherwise/**
	 * Returns whether this listener list is empty.
	 *
	 * @return <code>true</code> if there are no registered listeners, and
	 *   <code>false</code> otherwise
	 */
	public boolean isEmpty() {
		return listeners.length == 0;
	}

	
Removes a listener from this list. Has no effect if the same 
listener was not already registered.
Params:
listener – the non-null listener to remove/**
	 * Removes a listener from this list. Has no effect if the <a href="ListenerList.html#same">same</a> 
	 * listener was not already registered.
	 *
	 * @param listener the non-<code>null</code> listener to remove
	 */
	public synchronized void remove(Object listener) {
		// This method is synchronized to protect against multiple threads adding 
		// or removing listeners concurrently. This does not block concurrent readers.
		if (listener == null)
			throw new IllegalArgumentException();
		int oldSize = listeners.length;
		for (int i = 0; i < oldSize; ++i) {
			Object listener2 = listeners[i];
			if (identity ? listener == listener2 : listener.equals(listener2)) {
				if (oldSize == 1) {
					listeners = EmptyArray;
				} else {
					// Thread safety: create new array to avoid affecting concurrent readers
					Object[] newListeners = new Object[oldSize - 1];
					System.arraycopy(listeners, 0, newListeners, 0, i);
					System.arraycopy(listeners, i + 1, newListeners, i, oldSize - i - 1);
					//atomic assignment to field
					this.listeners = newListeners;
				}
				return;
			}
		}
	}

	
Returns the number of registered listeners.
Returns:
the number of registered listeners/**
	 * Returns the number of registered listeners.
	 *
	 * @return the number of registered listeners
	 */
	public int size() {
		return listeners.length;
	}

	
Removes all listeners from this list.
/**
	 * Removes all listeners from this list.
	 */
	public synchronized void clear() {
		listeners = EmptyArray;
	}

	
Returns a Spliterator covering the registered listeners.

The spliterator reports Spliterator.SIZED, Spliterator.SUBSIZED, Spliterator.ORDERED, and Spliterator.IMMUTABLE
Returns:
a spliterator for listeners
Since:
org.eclipse.equinox.common 3.9/**
	 * Returns a Spliterator covering the registered listeners.
	 * <p>
	 * The spliterator reports Spliterator.SIZED, Spliterator.SUBSIZED, Spliterator.ORDERED, and Spliterator.IMMUTABLE
	 * 
	 * @return a spliterator for listeners
	 * @since org.eclipse.equinox.common 3.9
	 */
	@Override
	@SuppressWarnings("unchecked")
	public Spliterator<E> spliterator() {
		return (Spliterator<E>) Arrays.spliterator(listeners);
	}

	
Returns a sequential Stream over the registered listeners. 
Returns:
a sequential Stream over the registered listeners.
Since:
org.eclipse.equinox.common 3.9/**
	 * Returns a sequential {@code Stream} over the registered listeners.
	 * 
	 * @return a sequential {@code Stream} over the registered listeners.
	 * @since org.eclipse.equinox.common 3.9
	 */
	public Stream<E> stream() {
		return StreamSupport.stream(spliterator(), false);
	}

	
Returns a parallel Stream over the registered listeners. 
Returns:
a parallel Stream over the registered listeners.
Since:
org.eclipse.equinox.common 3.9/**
	 * Returns a parallel {@code Stream} over the registered listeners.
	 * 
	 * @return a parallel {@code Stream} over the registered listeners.
	 * @since org.eclipse.equinox.common 3.9
	 */
	public Stream<E> parallelStream() {
		return StreamSupport.stream(spliterator(), true);
	}

	@Override
	public String toString() {
		return Arrays.toString(listeners);
	}
}