/*
 * Copyright 2002-2012 the original author or authors.
 *
 * Licensed 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 com.erudika.scoold.velocity;

import java.io.StringWriter;
import java.util.Locale;
import javax.servlet.http.HttpServletResponse;

import org.apache.velocity.Template;
import org.apache.velocity.context.Context;
import org.apache.velocity.exception.ResourceNotFoundException;

import org.springframework.core.NestedIOException;

/**
 * VelocityLayoutView emulates the functionality offered by Velocity's VelocityLayoutServlet to ease page composition
 * from different templates.
 *
 * <p>
 * The {@code url} property should be set to the content template for the view, and the layout template location should
 * be specified as {@code layoutUrl} property. A view can override the configured layout template location by setting
 * the appropriate key (the default is "layout") in the content template.
 *
 * <p>
 * When the view is rendered, the VelocityContext is first merged with the content template (specified by the
 * {@code url} property) and then merged with the layout template to produce the final output.
 *
 * <p>
 * The layout template can include the screen content through a VelocityContext variable (the default is
 * "screen_content"). At runtime, this variable will contain the rendered content template.
 *
 * @author Darren Davison
 * @author Juergen Hoeller
 * @since 1.2
 * @see #setLayoutUrl
 * @see #setLayoutKey
 * @see #setScreenContentKey
 */
public class VelocityLayoutView extends VelocityView {

	/**
	 * The default {@link #setLayoutUrl(String) layout url}.
	 */
	public static final String DEFAULT_LAYOUT_URL = "layout.vm";

	/**
	 * The default {@link #setLayoutKey(String) layout key}.
	 */
	public static final String DEFAULT_LAYOUT_KEY = "layout";

	/**
	 * The default {@link #setScreenContentKey(String) screen content key}.
	 */
	public static final String DEFAULT_SCREEN_CONTENT_KEY = "screen_content";

	private String layoutUrl = DEFAULT_LAYOUT_URL;

	private String layoutKey = DEFAULT_LAYOUT_KEY;

	private String screenContentKey = DEFAULT_SCREEN_CONTENT_KEY;

	/**
	 * Set the layout template to use. Default is {@link #DEFAULT_LAYOUT_URL "layout.vm"}.
	 *
	 * @param layoutUrl the template location (relative to the template root directory)
	 */
	public void setLayoutUrl(String layoutUrl) {
		this.layoutUrl = layoutUrl;
	}

	/**
	 * Set the context key used to specify an alternate layout to be used instead of the default layout. Screen content
	 * templates can override the layout template that they wish to be wrapped with by setting this value in the
	 * template, for example:<br> {@code #set($layout = "MyLayout.vm" )}
	 * <p>
	 * Default key is {@link #DEFAULT_LAYOUT_KEY "layout"}, as illustrated above.
	 *
	 * @param layoutKey the name of the key you wish to use in your screen content templates to override the layout
	 * template
	 */
	public void setLayoutKey(String layoutKey) {
		this.layoutKey = layoutKey;
	}

	/**
	 * Set the name of the context key that will hold the content of the screen within the layout template. This key
	 * must be present in the layout template for the current screen to be rendered.
	 * <p>
	 * Default is {@link #DEFAULT_SCREEN_CONTENT_KEY "screen_content"}: accessed in VTL as {@code $screen_content}.
	 *
	 * @param screenContentKey the name of the screen content key to use
	 */
	public void setScreenContentKey(String screenContentKey) {
		this.screenContentKey = screenContentKey;
	}

	/**
	 * Overrides {@code VelocityView.checkTemplate()} to additionally check that both the layout template and the screen
	 * content template can be loaded. Note that during rendering of the screen content, the layout template can be
	 * changed which may invalidate any early checking done here.
	 */
	@Override
	public boolean checkResource(Locale locale) throws Exception {
		if (!super.checkResource(locale)) {
			return false;
		}

		try {
			// Check that we can get the template, even if we might subsequently get it again.
			getTemplate(this.layoutUrl);
			return true;
		} catch (ResourceNotFoundException ex) {
			throw new NestedIOException("Cannot find Velocity template for URL [" + this.layoutUrl
					+ "]: Did you specify the correct resource loader path?", ex);
		} catch (Exception ex) {
			throw new NestedIOException(
					"Could not load Velocity template for URL [" + this.layoutUrl + "]", ex);
		}
	}

	/**
	 * Overrides the normal rendering process in order to pre-process the Context, merging it with the screen template
	 * into a single value (identified by the value of screenContentKey). The layout template is then merged with the
	 * modified Context in the super class.
	 */
	@Override
	protected void doRender(Context context, HttpServletResponse response) throws Exception {
		renderScreenContent(context);

		// Velocity context now includes any mappings that were defined
		// (via #set) in screen content template.
		// The screen template can overrule the layout by doing
		// #set( $layout = "MyLayout.vm" )
		String layoutUrlToUse = (String) context.get(this.layoutKey);
		if (layoutUrlToUse != null) {
			if (logger.isDebugEnabled()) {
				logger.debug("Screen content template has requested layout [" + layoutUrlToUse + "]");
			}
		} else {
			// No explicit layout URL given -> use default layout of this view.
			layoutUrlToUse = this.layoutUrl;
		}

		mergeTemplate(getTemplate(layoutUrlToUse), context, response);
	}

	/**
	 * The resulting context contains any mappings from render, plus screen content.
	 */
	private void renderScreenContent(Context velocityContext) throws Exception {
		if (logger.isDebugEnabled()) {
			logger.debug("Rendering screen content template [" + getUrl() + "]");
		}

		StringWriter sw = new StringWriter();
		Template screenContentTemplate = getTemplate(getUrl());
		screenContentTemplate.merge(velocityContext, sw);

		// Put rendered content into Velocity context.
		velocityContext.put(this.screenContentKey, sw.toString());
	}

}