/*
 * 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.wicket.request.cycle;

import org.apache.wicket.MetaDataKey;
import org.apache.wicket.core.request.handler.IPageRequestHandler;
import org.apache.wicket.request.IRequestHandler;
import org.apache.wicket.request.IRequestHandlerDelegate;

/**
 * Registers and retrieves first and last executed {@link IPageRequestHandler} in a request cycle.
 * Can be used to find out what is the requested page and what is the actual response page.
 * <p>
 * To use it an application needs to register it with:
 *     <pre><code>
 *          application.getRequestCycleListeners().add(new PageRequestHandlerTracker());
 *     </code></pre>
 * <p>
 * The result can then be accessed at the end of each {@link RequestCycle} with:
 *     <pre><code>
 *          IPageRequestHandler first = PageRequestHandlerTracker.getFirstHandler(RequestCycle.get());
 *          IPageRequestHandler last = PageRequestHandlerTracker.getLastHandler(RequestCycle.get());
 *     </code></pre>
 *
 * @since 1.5.8
 */
public class PageRequestHandlerTracker implements IRequestCycleListener
{
	/**
	 * The key for the first handler
	 */
	public static final  MetaDataKey<IPageRequestHandler> FIRST_HANDLER_KEY = new MetaDataKey<IPageRequestHandler>() {};

	/**
	 * The key for the last handler
	 */
	public static final MetaDataKey<IPageRequestHandler> LAST_HANDLER_KEY = new MetaDataKey<IPageRequestHandler>() {};

	@Override
	public void onRequestHandlerResolved(RequestCycle cycle, IRequestHandler handler)
	{
		registerFirstHandler(cycle,handler);
		registerLastHandler(cycle,handler);
	}

	/**
	 * Registers pagerequesthandler when it's resolved ,keeps up with the most recent handler resolved
	 *
	 * @param cycle
	 *      the current request cycle
	 * @param handler
	 *      the request handler to register
	 */
	private void registerLastHandler(RequestCycle cycle, IRequestHandler handler)
	{
		final IPageRequestHandler pageRequestHandler = findPageRequestHandler(handler);
		if (pageRequestHandler != null)
		{
			cycle.setMetaData(LAST_HANDLER_KEY, pageRequestHandler);
		}
	}

	/**
	 * Registers firsthandler if it's not already registered
	 *
	 * @param cycle
	 *      the current request cycle
	 * @param handler
	 *      the request handler to register
	 */
	private void registerFirstHandler(RequestCycle cycle, IRequestHandler handler)
	{
		if (getFirstHandler(cycle) == null)
		{
			final IPageRequestHandler pageRequestHandler = findPageRequestHandler(handler);
			if (pageRequestHandler != null)
			{
				cycle.setMetaData(FIRST_HANDLER_KEY, pageRequestHandler);
			}
		}
	}

	/**
	 * Looking for IPageRequestHandler 
	 * 
	 * @param handler
	 * @return IPageRequestHandler if exist otherwise null
	 */
	private IPageRequestHandler findPageRequestHandler(IRequestHandler handler)
	{
		if (handler instanceof IPageRequestHandler)
		{
			return (IPageRequestHandler)handler;
		}
	    if (handler instanceof IRequestHandlerDelegate)
	    {
	    	return findPageRequestHandler(((IRequestHandlerDelegate)handler).getDelegateHandler());
	    }
	    return null;
	}

   /**
	* retrieves last handler from request cycle
	*
	* @param cycle
	* @return last handler
	*/
	public static IPageRequestHandler getLastHandler(RequestCycle cycle)
	{
		return cycle.getMetaData(LAST_HANDLER_KEY);
	}

	/**
	 * retrieves first handler from the request cycle
	 *
	 * @param cycle
	 * @return first handler
	 */
	public static IPageRequestHandler getFirstHandler(RequestCycle cycle)
	{
		return cycle.getMetaData(FIRST_HANDLER_KEY);
	}
}