bundles/org.eclipse.core.databinding.observable/src/org/eclipse/core/databinding/observable/ListenerList.java

package org.eclipse.core.databinding.observable;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

/**
 * 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. 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 name="same">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 the <code>getListeners</code> method when notifying listeners. The
 * recommended code sequence for notifying all registered listeners of say,
 * <code>FooListener.eventHappened</code>, is:
 * 
 * <pre>
 * Object[] listeners = myListenerList.getListeners();
 * for (int i = 0; i &lt; listeners.length; ++i) {
 * 	((FooListener) listeners[i]).eventHappened(event);
 * }
 * </pre>
 * 
 * </p>
 * <p>
 * This class can be used without OSGi running.
 * </p>
 * 
 * @param <L>
 * @since 1.5
 */
public class ListenerList<L extends IObservablesListener<L>> {

	/**
	 * 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 <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
	 */
	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
	 */
	private volatile List<L> listeners = Collections.emptyList();

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

	/**
	 * 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 <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(L 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.size();
		for (L listener2 : listeners) {
			if (identity ? listener == listener2 : listener.equals(listener2))
				return;
		}
		// Thread safety: create new array to avoid affecting concurrent readers
		List<L> newListeners = new ArrayList<L>(oldSize + 1);
		newListeners.addAll(listeners);
		newListeners.add(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.
	 * <p>
	 * Note: Callers of this method <b>must not</b> modify the returned array.
	 * 
	 * @return the list of registered listeners
	 */
	public List<L> getListeners() {
		return listeners;
	}

	/**
	 * @param event
	 */
	public void fireEvent(ObservableEvent<?, L> event) {
		for (L listener : listeners) {
			event.dispatch(listener);
		}
	}

	/**
	 * 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.isEmpty();
	}

	/**
	 * 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(L 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.size();
		if (oldSize == 0) {
			return;
		} else if (oldSize == 1) {
			L listener2 = listeners.get(0);
			if (identity ? listener == listener2 : listener.equals(listener2)) {
				listeners = Collections.emptyList();
			}
		} else {
			// Thread safety: create new array to avoid affecting
			// concurrent readers
			List<L> newListeners = new ArrayList<L>(oldSize - 1);

			boolean match = false;
			for (L listener2 : listeners) {
				if (identity ? listener == listener2 : listener
						.equals(listener2)) {
					match = true;
				} else {
					newListeners.add(listener2);
				}
			}

			if (match) {
				// atomic assignment
				this.listeners = newListeners;
			}
		}
	}

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

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

	/**
	 * @return true if listeners have been added, false if there are no
	 *         listeners
	 */
	public boolean hasListeners() {
		return !isEmpty();
	}

	/**
	 * This method is typically called within a synchronization block. The event
	 * can then be fired using the immutable list when outside the
	 * synchronization block. By doing this, the calling code can ensure that
	 * all the listeners and only the listeners active at the time the change
	 * was applied will receive the event.
	 * 
	 * @return an immutable listener list
	 */
	public ListenerListCopy<L> getReadOnlyCopy() {
		return new ListenerListCopy<L>(listeners);
	}

	/**
	 * This method is intended to be called from ListenerListCopy only.
	 * <P>
	 * Indicates if the copy of the listener list is still the current listener
	 * list. Note that if the list was changed and then changed back, this
	 * method will return <code>false</code>.
	 * 
	 * @param copy
	 *            the copy taken at the time getReadOnlyCopy was called
	 * @return true if <code>copy</code> is still the current list
	 */
	boolean isListStillCurrent(List<L> copy) {
		return listeners == copy;
	}
}