How can I add a custom Outline view for editing XML documents in the Text mode?

Let's say you have XML documents like

<doc startnumber="15">
    <sec counter="no">
        <info/>
        <title>Introduction</title>   
      </sec>
    <sec>
        <title>Section title</title>       
      <para>Content</para>
        <sec>
            <title>Section title</title> 
                <para>Content</para>
        </sec>
    </sec>
    <sec> 
            <title>Section title</title>
        <para>Content</para>   
      </sec>
</doc>

and you want to display the XML content in a simplified Outline view like:

doc "15"
sec Introduction
sec 15 Section title
sec 15.1 Section title
sec 16 Section title

Usually an Outline should have the following characteristics:

1. Double clicking in the Outline the corresponding XML content would get selected.
2. When the caret moves in the opened XML document the Outline would select the proper entry.
3. When modifications occur in the document, the Outline would refresh.

A simple implementation using a Workspace Access plugin type could be something like:

/**
 * Simple Outline for the Text mode based on executing XPaths over the text content.
 */
public class CustomWorkspaceAccessPluginExtension implements WorkspaceAccessPluginExtension {
  /**
   * The custom outline list.
   */
  private JList customOutlineList;
  
  /**
   * Maps outline nodes to ranges in document
   */
  private WSXMLTextNodeRange[] currentOutlineRanges; 
  
  /**
   * The current text page
   */
  private WSXMLTextEditorPage currentTextPage;
  
  /**
   * Disable caret listener when we select from the caret listener.
   */
  private boolean enableCaretListener = true;
  
  /**
   * @see ro.sync.exml.plugin.workspace.WorkspaceAccessPluginExtension#applicationStarted(ro.sync.exml.workspace.api.standalone.StandalonePluginWorkspace)
   */
  @Override
  public void applicationStarted(final StandalonePluginWorkspace pluginWorkspaceAccess) {
    pluginWorkspaceAccess.addViewComponentCustomizer(new ViewComponentCustomizer() {
      /**
       * @see ro.sync.exml.workspace.api.standalone.ViewComponentCustomizer#customizeView(ro.sync.exml.workspace.api.standalone.ViewInfo)
       */
      @Override
      public void customizeView(ViewInfo viewInfo) {
        if(
            //The view ID defined in the "plugin.xml"
            "SampleWorkspaceAccessID".equals(viewInfo.getViewID())) {
          customOutlineList = new JList();
          //Render the content in the Outline.
          customOutlineList.setCellRenderer(new DefaultListCellRenderer() {
            /**
             * @see javax.swing.DefaultListCellRenderer#getListCellRendererComponent(javax.swing.JList, java.lang.Object, int, boolean, boolean)
             */
            @Override
            public Component getListCellRendererComponent(JList<?> list, Object value, int index,
                boolean isSelected, boolean cellHasFocus) {
              JLabel label = (JLabel) super.getListCellRendererComponent(list, value, index, isSelected, cellHasFocus);
              String val = null;
              if(value instanceof Element) {
                Element element = ((Element)value);
                val = element.getNodeName();
                if(!"".equals(element.getAttribute("startnumber"))) {
                  val += " " + "'" + element.getAttribute("startnumber") + "'";
                }
                NodeList titles = element.getElementsByTagName("title");
                if(titles.getLength() > 0) {
                  val += " \"" + titles.item(0).getTextContent() + "\"";
                }
              }
              label.setText(val);
              return label;
            }
          });
          //When we click a node, select it in the text page.
          customOutlineList.addMouseListener(new MouseAdapter() {
            @Override
            public void mouseClicked(MouseEvent e) {
              if(SwingUtilities.isLeftMouseButton(e) && e.getClickCount() == 2) {
                int sel = customOutlineList.getSelectedIndex();
                enableCaretListener = false;
                try {
                  currentTextPage.select(currentTextPage.getOffsetOfLineStart(currentOutlineRanges[sel].getStartLine()) + currentOutlineRanges[sel].getStartColumn() - 1, 
                      currentTextPage.getOffsetOfLineStart(currentOutlineRanges[sel].getEndLine()) + currentOutlineRanges[sel].getEndColumn());
                } catch (BadLocationException e1) {
                  e1.printStackTrace();
                }
                enableCaretListener = true;
              }
            }
          });
          viewInfo.setComponent(new JScrollPane(customOutlineList));
          viewInfo.setTitle("Custom Outline");
        } 
      }
    }); 
    
    pluginWorkspaceAccess.addEditorChangeListener(new WSEditorChangeListener() {
      /**
       * @see ro.sync.exml.workspace.api.listeners.WSEditorChangeListener#editorOpened(java.net.URL)
       */
      @Override
      public void editorOpened(URL editorLocation) {
        //An editor was opened
        WSEditor editorAccess = pluginWorkspaceAccess.getEditorAccess(editorLocation, StandalonePluginWorkspace.MAIN_EDITING_AREA);
        if(editorAccess != null) {
          WSEditorPage currentPage = editorAccess.getCurrentPage();
          if(currentPage instanceof WSXMLTextEditorPage) {
            //User editing in Text mode an opened XML document.
            final WSXMLTextEditorPage xmlTP = (WSXMLTextEditorPage) currentPage;
            //Reconfigure outline on each change.
            xmlTP.getDocument().addDocumentListener(new DocumentListener() {
              @Override
              public void removeUpdate(DocumentEvent e) {
                reconfigureOutline(xmlTP);
              }
              @Override
              public void insertUpdate(DocumentEvent e) {
                reconfigureOutline(xmlTP);
              }
              @Override
              public void changedUpdate(DocumentEvent e) {
                reconfigureOutline(xmlTP);
              }
            });
            JTextArea textComponent = (JTextArea) xmlTP.getTextComponent();
            textComponent.addCaretListener(new CaretListener() {
              @Override
              public void caretUpdate(CaretEvent e) {
                if(currentOutlineRanges != null && currentTextPage != null && enableCaretListener) {
                  enableCaretListener = false;
                  //Find the node to select in the outline.
                  try {
                    int line = xmlTP.getLineOfOffset(e.getDot());
                    for (int i = currentOutlineRanges.length - 1; i >= 0; i--) {
                      if(line > currentOutlineRanges[i].getStartLine() && line < currentOutlineRanges[i].getEndLine()) {
                        customOutlineList.setSelectedIndex(i);
                        break;
                      }
                    }
                  } catch (BadLocationException e1) {
                    e1.printStackTrace();
                  }
                  enableCaretListener = true;
                }
              }
            });
          }
        }
      }
      /**
       * @see ro.sync.exml.workspace.api.listeners.WSEditorChangeListener#editorActivated(java.net.URL)
       */
      @Override
      public void editorActivated(URL editorLocation) {
        //An editor was selected, reconfigure the common outline
        WSEditor editorAccess = pluginWorkspaceAccess.getEditorAccess(editorLocation, StandalonePluginWorkspace.MAIN_EDITING_AREA);
        if(editorAccess != null) {
          WSEditorPage currentPage = editorAccess.getCurrentPage();
          if(currentPage instanceof WSXMLTextEditorPage) {
            //User editing in Text mode an opened XML document.
            WSXMLTextEditorPage xmlTP = (WSXMLTextEditorPage) currentPage;
            reconfigureOutline(xmlTP);
          }
        }
      }
    }, StandalonePluginWorkspace.MAIN_EDITING_AREA);
  }
  
  /**
   * Reconfigure the outline
   * 
   * @param xmlTP The XML Text page.
   */
  protected void reconfigureOutline(final WSXMLTextEditorPage xmlTP) {
    try {
      //These are DOM nodes.
      Object[] evaluateXPath = xmlTP.evaluateXPath("//doc | //sec");
      //These are the ranges each node takes in the document.
      currentOutlineRanges = xmlTP.findElementsByXPath("//doc | //sec");
      currentTextPage = xmlTP;
      DefaultListModel listModel = new DefaultListModel();
      if(evaluateXPath != null) {
        for (int i = 0; i < evaluateXPath.length; i++) { 
          listModel.addElement(evaluateXPath[i]);
        }
      }
      customOutlineList.setModel(listModel);
    } catch(XPathException ex) {
      ex.printStackTrace();
    }
  }

  /**
   * @see ro.sync.exml.plugin.workspace.WorkspaceAccessPluginExtension#applicationClosing()
   */
  @Override
  public boolean applicationClosing() {
    return true;
  }
}