package org.apache.tapestry5.corelib.internal;

 import org.apache.tapestry5.MarkupWriter;

 import org.apache.tapestry5.MarkupWriterListener;

 import org.apache.tapestry5.dom.Element;

 import org.apache.tapestry5.ioc.internal.util.OneShotLock;

 import org.apache.tapestry5.services.HiddenFieldLocationRules;

 /**

  * Used to position a hidden field (as part of a form-related component).  Hidden fields are not allowed to go just

  * anywhere, there are rules, dicatated by the (X)HTML schema, about where they are allowed. We use the {@link

  * org.apache.tapestry5.MarkupWriterListener} interface to monitor elements as they are started and ended to find a

  * place to put content.

  */

 public class HiddenFieldPositioner

 {

     /**

      * The type of element to create.

      */

     private static final String ELEMENT = "input";

     private final MarkupWriter writer;

     private final HiddenFieldLocationRules rules;

     private final OneShotLock lock = new OneShotLock();

     private Element hiddenFieldElement;

     private final MarkupWriterListener listener = new MarkupWriterListener()

     {

         public void elementDidStart(Element element)

         {

             if (rules.placeHiddenFieldInside(element))

             {

                 hiddenFieldElement = element.element(ELEMENT);

                 writer.removeListener(this);

             }

         }

         public void elementDidEnd(Element element)

         {

             if (rules.placeHiddenFieldAfter(element))

             {

                 hiddenFieldElement = element.getParent().element(ELEMENT);

                 writer.removeListener(this);

             }

         }

     };

     public HiddenFieldPositioner(MarkupWriter writer, HiddenFieldLocationRules rules)

     {

         this.writer = writer;

         this.rules = rules;

         this.writer.addListener(listener);

     }

     /**

      * Returns the hidden field element, which can have its attributes filled in.

      *

      * @return the element

      * @throws IllegalStateException if the element was not placed.

      */

     public Element getElement()

     {

         lock.lock();

         // Remove the listener if it hasn't been removed already.

         writer.removeListener(listener);

         if (hiddenFieldElement == null)

             throw new IllegalStateException(

                     "The rendered content did not include any elements that allow for the positioning of the hidden form field's element.");

         return hiddenFieldElement;

     }

 }