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An implementation of conversation-within-page concept for Tapestry 5
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package org.trailsframework.conversations.services;

import static org.apache.tapestry5.ioc.internal.util.CollectionFactory.newList;
import static org.apache.tapestry5.ioc.internal.util.Defense.notBlank;

import java.util.Collection;
import java.util.Collections;
import java.util.List;

import org.apache.tapestry5.internal.services.PersistentFieldChangeImpl;
import org.apache.tapestry5.services.PersistentFieldChange;
import org.apache.tapestry5.services.PersistentFieldStrategy;
import org.apache.tapestry5.services.Request;
import org.apache.tapestry5.services.Session;

public class ConversationalPersistentFieldStrategy implements PersistentFieldStrategy {
	/**
	 * Prefix used to identify keys stored in the session that are being used to store persistent field data.
	 */
	static final String PREFIX = "state:";

	private final ConversationManager conversationManager;

	public ConversationalPersistentFieldStrategy(Request request, ConversationManager conversationManager) {
		this.prefix = PREFIX;
		this.request = request;
		conversationManager.setPagePersistentFieldStrategy(this);
		this.conversationManager = conversationManager;
	}

	private final String prefix;

	private final Request request;

	private String buildPrefix(String pageName) {
		// System.out.println("Conversation id is: " + conversationManager.getActiveConversation());
		return prefix + pageName + "_" + "conversation" + conversationManager.getActiveConversation() + ":";
	}

	public final Collection gatherFieldChanges(String pageName) {
		Session session = request.getSession(false);

		if (session == null) return Collections.emptyList();

		List result = newList();

		// if conversation is not active don't gather any changes
		if (conversationManager.getActiveConversation() == null) return result;

		String fullPrefix = buildPrefix(pageName);

		for (String name : session.getAttributeNames(fullPrefix)) {
			Object persistedValue = session.getAttribute(name);

			Object applicationValue = persistedValue == null ? null : convertPersistedToApplicationValue(persistedValue);

			PersistentFieldChange change = buildChange(name, applicationValue);

			result.add(change);

			didReadChange(session, name);
		}

		return result;
	}

	public void discardChanges(String pageName) {
		Session session = request.getSession(false);

		if (session == null || conversationManager.getActiveConversation() == null) return;

		String fullPrefix = buildPrefix(pageName);

		for (String name : session.getAttributeNames(fullPrefix)) {
			session.setAttribute(name, null);
		}
	}

	/**
	 * Called after each key is read by {@link #gatherFieldChanges(String)}. This implementation does nothing, subclasses
	 * may override.
	 * 
	 * @param session
	 *          the session from which a value was just read
	 * @param attributeName
	 *          the name of the attribute used to read a value
	 */
	protected void didReadChange(Session session, String attributeName) {
	}

	private PersistentFieldChange buildChange(String name, Object newValue) {
		String[] chunks = name.split(":");

		// Will be empty string for the root component
		String componentId = chunks[2];
		String fieldName = chunks[3];

		return new PersistentFieldChangeImpl(componentId, fieldName, newValue);
	}

	public final void postChange(String pageName, String componentId, String fieldName, Object newValue) {
		notBlank(pageName, "pageName");
		notBlank(fieldName, "fieldName");

		// If no active conversation, no changes to post
		if (conversationManager.getActiveConversation() == null) return;

		Object persistedValue = newValue == null ? null : convertApplicationValueToPersisted(newValue);

		StringBuilder builder = new StringBuilder(buildPrefix(pageName));

		if (componentId != null) builder.append(componentId);

		builder.append(':');
		builder.append(fieldName);

		Session session = request.getSession(persistedValue != null);

		// TAPESTRY-2308: The session will be false when newValue is null and the session
		// does not already exist.

		if (session != null) {
			session.setAttribute(builder.toString(), persistedValue);
		}
	}

	/**
	 * Hook that allows a value to be converted as it is written to the session. Passed the new value provided by the
	 * application, returns the object to be stored in the session. This implementation simply returns the provided value.
	 * 
	 * @param newValue
	 *          non-null value
	 * @return persisted value
	 * @see #convertPersistedToApplicationValue(Object)
	 */
	protected Object convertApplicationValueToPersisted(Object newValue) {
		return newValue;
	}

	/**
	 * Converts a persisted value stored in the session back into an application value. This implementation returns the
	 * persisted value as is.
	 * 
	 * @param persistedValue
	 *          non-null persisted value
	 * @return application value
	 * @see #convertPersistedToApplicationValue(Object)
	 */
	protected Object convertPersistedToApplicationValue(Object persistedValue) {
		return persistedValue;
	}
}