How to load a tree view with a large XML file

Introduction

Recently I was required to display a very large XML file in a tree view, I tried to implement like "virtual list view" which displays whatever is visible in a view (when the user scrolls). After brainstorm and some web surfing I remembered windows explorer. I'm not sure how they implement it exactly but I guess they do same method as mine which expand any tree node on the fly i.e. don't populate the whole tree at start-up. If you don't do this, populate the whole tree for a 10 MB XML file size takes up to 45 minutes on a fast machine! doing my way will takes only approximately 3-5 seconds! The trick really is how to find out the relationship and info of any tree node and associated DOM node any where any time. To incorporate this into your app, either derive your class from CXmlTreeView or add appropriate message handlers and all helper methods to your own class.

Note:

• It'd be easier if you know how to incorporate XML parser into your app (this article uses Microsoft XML parser because of its available!), however, it's not a requirement.
• You can apply the same method to any application-specific value instead of DOM node to populate a tree control
• You can easily change this class to use as a tree control instead.

The first you need to add a notify message handler for TVN_ITEMEXPANDING which notifies a tree view control's parent window that a parent item's list of child items is about to expand or collapse.

void CXmlTreeView::OnItemexpanding(NMHDR*  pNMHDR, LRESULT* pResult) 
{
    NM_TREEVIEW* pNMTreeView = (NM_TREEVIEW*)pNMHDR;
    HTREEITEM hItem = pNMTreeView->itemNew.hItem;
    MSXML::IXMLDOMElement* node = (MSXML::IXMLDOMElement*)GetTreeCtrl().GetItemData(hItem);
    HRESULT hr;
  
    *pResult = 0;
 
    // If m_bOptimizeMemory is false then we optimize by not adding and deleting 
    // already added items. The trade-off is memory exhausted!
  
    CWaitCursor waitCursor;     // This could take a while!
    GetTreeCtrl().LockWindowUpdate();
    if (pNMTreeView->action == TVE_EXPAND) 
    {
        if (m_bOptimizeMemory == FALSE) 
        {
            HTREEITEM hChildItem;
            if ((hChildItem = GetTreeCtrl().GetChildItem(hItem)) != NULL) 
            {
               MSXML::IXMLDOMElement* childNode = 
                          (MSXML::IXMLDOMElement*) GetTreeCtrl().GetItemData(hChildItem);
                 
               if (childNode == NULL)
               {
                  GetTreeCtrl().DeleteItem(hChildItem);
                  MSXML::IXMLDOMNode* firstChild = NULL;
                  hr = node->get_firstChild(&firstChild);
                  if (SUCCEEDED(hr) && firstChild != NULL)
                  {
                      if (populateNode((MSXML::IXMLDOMElement*)firstChild, hItem) == FALSE) 
                      {
                          *pResult = 1; 
                      }                
                  }
               }
            }
         } 
         else
         {
             deleteFirstChild(hItem);
             MSXML::IXMLDOMNode* firstChild = NULL;
             hr = node->get_firstChild(&firstChild);
             if (SUCCEEDED(hr) && firstChild != NULL) 
             {
                 if (populateNode((MSXML::IXMLDOMElement*)firstChild, hItem) == FALSE) 
                 {
                     *pResult = 1; 
                 }
             }
          }
      }
      else
      {
          // pNMTreeView->action == TVE_COLLAPSE
          if (m_bOptimizeMemory == TRUE) 
          {
              deleteAllChildren(hItem);
              // Set button state.
              if (node->hasChildNodes()) 
              {
                   int nImage, nSelectedImage;
                   nImage = nSelectedImage = getIconIndex(node);
                   HTREEITEM hChildItem = GetTreeCtrl().InsertItem(_T(""), nImage, 
                       nSelectedImage, hItem);
                   GetTreeCtrl().SetItemData(hChildItem, (DWORD)NULL);
              }
          }
      }
      GetTreeCtrl().UnlockWindowUpdate();
  }

The key in this function is the calls to GetItemData(..) and populateNode(..).

• GetItemData(..) will retrieve the DOM node value associated with the specified item so we can use this DOM node to figure out the relationship with the rest.
• populateNode(..) will only populate all siblings of the [in] node which is the child node of the current node.
• m_bOptimizeMemory is used to optimize memory by delete all children when collapse or not. This value is set to false by default when you can loadXML(..).

// This function populates all siblings of the
  [in] node.
  //
  BOOL CXmlTreeView::populateNode(MSXML::IXMLDOMElement *node, const HTREEITEM &hParent)
  {
      HRESULT hr = S_OK;
      BSTR nodeType, nodeName;
      HTREEITEM hItem;
  
      node->get_nodeTypeString(&nodeType);
      if (!wcscmp(nodeType, L"element")) {
          node->get_nodeName(&nodeName);
          hItem = insertItem(node, CString(nodeName), ILI_ELEMENT, ILI_ELEMENT, hParent);
          populateAttributes(node, hItem);
      } else if(!wcscmp(nodeType, L"text")) {
          node->get_text(&nodeName);
          hItem = insertItem(node, CString(nodeName), ILI_TEXT, ILI_TEXT, hParent); 
      } else if(!wcscmp(nodeType, L"comment")) {
          node->get_nodeName(&nodeName);
          hItem = insertItem(node, CString(nodeName), ILI_COMMENT, ILI_COMMENT, hParent); 
      } else {    // Handle more data types here if needed.
          node->get_nodeName(&nodeName);
          hItem = insertItem(node, CString(nodeName), ILI_OTHER, ILI_OTHER, hParent); 
      }
  
      MSXML::IXMLDOMNode* nextSibling = NULL;
      hr = node->get_nextSibling(&nextSibling);
      if (SUCCEEDED(hr) && nextSibling != NULL) {
          populateNode((MSXML::IXMLDOMElement*)nextSibling, hParent);
      }
  
      return TRUE;
  }

The key in this function is the call to helper function insertItem(..) which set DOM node value associated with the specified tree item.

HTREEITEM CXmlTreeView::insertItem(MSXML::IXMLDOMElement *node, 
      const CString &nodeName, int nImage, int nSelectedImage, 
      HTREEITEM hParent /*= TVI_ROOT*/, HTREEITEM hInsertAfter /*= TVI_LAST*/)
  {
      HTREEITEM hItem = GetTreeCtrl().InsertItem(nodeName, nImage, 
          nSelectedImage, hParent, hInsertAfter); 
      GetTreeCtrl().SetItemData(hItem, (DWORD)node);
  
      // Set button state.
      if (node->hasChildNodes()) {
          HTREEITEM hChildItem = GetTreeCtrl().InsertItem(_T(""), nImage, 
               nSelectedImage, hItem);
          GetTreeCtrl().SetItemData(hChildItem, (DWORD)NULL);
      }
  
      return hItem;
  }

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