AbstractObserver.java

/*******************************************************************************
 * Copyright (c) 2007, 2014 compeople AG and others.
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 *
 * Contributors:
 *    compeople AG - initial API and implementation
 *******************************************************************************/
package org.eclipse.riena.internal.ui.ridgets.swt;

import org.eclipse.core.runtime.Assert;
import org.eclipse.swt.events.SelectionAdapter;
import org.eclipse.swt.events.SelectionEvent;

import org.eclipse.riena.core.util.ListenerList;
import org.eclipse.riena.ui.ridgets.IRidget;

/**
 * This class notifies a collection of listeners of the type T when a widget is
 * selected.
 */
abstract class AbstractObserver<T> extends SelectionAdapter {

	private final IRidget source;

	private ListenerList<T> listeners;

	/**
	 * Create a new instance.
	 * 
	 * @param source
	 *            the ridget where the selection occured; never null
	 */
	public AbstractObserver(final IRidget source) {
		Assert.isNotNull(source);
		this.source = source;
	}

	@Override
	public final void widgetSelected(final SelectionEvent evt) {
		fireAction(evt);
	}

	/**
	 * Adds a listener to the collection of listeners which are notified. Adding
	 * the same listener twice has no effect.
	 * 
	 * @param listener
	 *            a listener instance of type T (non-null)
	 * @throws RuntimeException
	 *             if IActionListener is null
	 */
	public final void addListener(final T listener) {
		Assert.isNotNull(listener, "listener is null"); //$NON-NLS-1$
		if (listeners == null) {
			listeners = createList();
		}
		listeners.add(listener);
	}

	/**
	 * Removes a listener from the collection of listeners which are notified.
	 * 
	 * @param listener
	 *            a listener instance of type T
	 */
	public final void removeListener(final T listener) {
		if (listeners != null) {
			listeners.remove(listener);
		}
	}

	// protected methods
	////////////////////

	/**
	 * Creates an instance of ListenerList<T>. This will be invoked when
	 * the first listener is added.
	 * 
	 * @return a ListenerList; never null
	 */
	protected abstract ListenerList<T> createList();

	/**
	 * This method forwards the given SelectionEvent to the collection of
	 * listeners.
	 * <p>
	 * Must be implemented by subclasses. Subclasses are free to create an
	 * entirely new event and forward that instead of the original one, if
	 * necessary.
	 */
	protected abstract void fireAction(SelectionEvent evt);

	/**
	 * Get the ridget where the selection occurred.
	 * 
	 * @return an IRidget instance; never null
	 */
	protected final IRidget getSource() {
		return source;
	}

	/**
	 * Get the collection of listeners.
	 * 
	 * @return A ListenerList or <b>null</b>, if no listeners have been added.
	 *         May be empty.
	 */
	protected final ListenerList<T> getListeners() {
		return listeners;
	}
}