tapestry/tapestry-contrib/src/java/org/apache/tapestry/contrib/link/FormLinkRenderer.java

// Copyright 2004, 2005 The Apache Software Foundation
//
// 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 org.apache.tapestry.contrib.link;

import org.apache.hivemind.ApplicationRuntimeException;
import org.apache.tapestry.IMarkupWriter;
import org.apache.tapestry.IRequestCycle;
import org.apache.tapestry.PageRenderSupport;
import org.apache.tapestry.Tapestry;
import org.apache.tapestry.TapestryUtils;
import org.apache.tapestry.components.ILinkComponent;
import org.apache.tapestry.engine.ILink;
import org.apache.tapestry.link.DefaultLinkRenderer;
import org.apache.tapestry.link.ILinkRenderer;

/**
 * A link renderer that ensures that the generated link uses POST instead of GET
 * request and is therefore no longer limited in size.
 * <p>
 * Theoretically, browsers should support very long URLs, but in practice they
 * often start behaving strangely if the URLs are more than 256 characters. This
 * renderer uses JavaScript to generate forms containing the requested link
 * parameters and then "post" them when the link is selected. As a result, the
 * data is sent to the server using a POST request with a very short URL and
 * there is no longer a limitation in the size of the parameters.
 * <p>
 * In short, simply add the following parameter to your <code>DirectLink</code>,
 * <code>ExternalLink</code>, or other such link components:
 * 
 * <pre>
 * renderer = &quot;ognl: @org.apache.tapestry.contrib.link.FormLinkRenderer@RENDERER&quot;
 * </pre>
 * 
 * and they will automatically start using POST rather than GET requests. Their
 * parameters will no longer be limited in size.
 * @author mb
 * @since 4.0
 */
public class FormLinkRenderer extends DefaultLinkRenderer
{

    /**
     * A public singleton instance of the <code>FormLinkRenderer</code>.
     * <p>
     * Since the <code>FormLinkRenderer</code> is stateless, this instance can
     * serve all links within your application without interference.
     */
    public static final ILinkRenderer RENDERER = new FormLinkRenderer();

    public void renderLink(IMarkupWriter writer, IRequestCycle cycle,
            ILinkComponent linkComponent)
    {
        IMarkupWriter wrappedWriter = null;

        if (cycle.getAttribute(Tapestry.LINK_COMPONENT_ATTRIBUTE_NAME) != null)
            throw new ApplicationRuntimeException(Tapestry
                    .getMessage("AbstractLinkComponent.no-nesting"),
                    linkComponent, null, null);

        cycle.setAttribute(Tapestry.LINK_COMPONENT_ATTRIBUTE_NAME,
                linkComponent);
        
        String formName = cycle.getUniqueId("LinkForm");
        
        boolean hasBody = getHasBody();

        boolean disabled = linkComponent.isDisabled();

        if (!disabled && !cycle.isRewinding())
        {
            ILink l = linkComponent.getLink(cycle);
            String anchor = linkComponent.getAnchor();
            
            PageRenderSupport prs = TapestryUtils.getPageRenderSupport(cycle, linkComponent);
            
            String function = generateFormFunction(formName, l, anchor);
            prs.addBodyScript(linkComponent, function);
            
            if (hasBody)
                writer.begin(getElement());
            else 
                writer.beginEmpty(getElement());
            
            writer.attribute(getUrlAttribute(), "javascript: document."
                    + formName + ".submit();");
            
            beforeBodyRender(writer, cycle, linkComponent);
            
            // Allow the wrapped components a chance to render.
            // Along the way, they may interact with this component
            // and cause the name variable to get set.

            wrappedWriter = writer.getNestedWriter();
        }
        else wrappedWriter = writer;

        if (hasBody) linkComponent.renderBody(wrappedWriter, cycle);

        if (!disabled && !cycle.isRewinding())
        {
            afterBodyRender(writer, cycle, linkComponent);

            linkComponent.renderAdditionalAttributes(writer, cycle);

            if (hasBody)
            {
                wrappedWriter.close();

                // Close the <element> tag

                writer.end();
            }
            else writer.closeTag();
        }

        cycle.removeAttribute(Tapestry.LINK_COMPONENT_ATTRIBUTE_NAME);

    }

    private String generateFormFunction(String formName, ILink link,
            String anchor)
    {
        String[] parameterNames = link.getParameterNames();

        StringBuffer buf = new StringBuffer();
        buf.append("function prepare" + formName + "() {\n");

        buf.append("  var html = \"\";\n");
        buf.append("  html += \"<div style='position: absolute'>\";\n");

        String url = link.getURL(anchor, false);
        buf.append("  html += \"<form name='" + formName
                + "' method='post' action='" + url + "'>\";\n");

        for(int i = 0; i < parameterNames.length; i++)
        {
            String parameter = parameterNames[i];
            String[] values = link.getParameterValues(parameter);
            if (values != null) {               
                for (int j = 0; j < values.length; j++) {
                    String value = values[j];
                    buf.append("  html += \"<input type='hidden' name='" + parameter + "' value='" + value + "'/>\";\n");
                }
            }
        }
        buf.append("  html += \"<\" + \"/form>\";\n");
        buf.append("  html += \"<\" + \"/div>\";\n");
        buf.append("  document.write(html);\n");
        buf.append("}\n");

        buf.append("prepare" + formName + "();\n\n");

        return buf.toString();
    }

}