Keeping the indentation level with Drag '& Drop

I can suggest an extension to the Drag and Drop of your TreeView control. The standard drag and drop as in your control will add the drag branch as a child of the drop item. Quite often during drag and drop operations parent items should stay parent items, child items should stay child items, grandchildren should stay grandchildren, etc, etc.

The following allows just that - with a few logical rules of course:

  1. Dragging an item onto another item on the same indentation level (no matter in which branch), will add the dragitem as a sibling of the dropitem, directly following the dropitem.
  2. Dragging an item onto an item of the next higher (parent-level) indentation, will add the dragitem as the last child of the dropitem.

Therefore the indentation level of the dragitem will stay the same after dropping it. Dragging onto lower (child-level) indentation level, and dragging onto more than 1 higher (parent) indentations are not allowed.

Add a member variable:
   BOOL m_bKeepIndentLevel;

In the constructor, initialize this variable:

m_bKeepIndentLevel = FALSE;

When this member is FALSE, the drag and drop operation will work as it currently is. To keep the indentation level, you can change this value to TRUE inside your program, either by directly accessing the variable, or by Get/Set functions.

In the OnLButtonUp() function, add/modify the lines indicated by '->' :

void CTreeCtrlX::OnLButtonUp(UINT nFlags, CPoint point) 

      while ((htiParent = GetParentItem(htiParent)) != NULL)
         if (htiParent == m_hDragItem) return;

->    // check for same indentation level

->    HTREEITEM htiPosition = TVI_LAST;

->    if (m_bKeepIndentLevel)
->    {
->       int nDragIndent = GetIndentLevel(m_hDragItem);
->       int nDropIndent = GetIndentLevel(m_hDropItem);
->       if (nDragIndent == nDropIndent)
->       {
->          // add after the drop item, as child of drop item's parent
->          htiPosition = m_hDropItem;
->          m_hDropItem = GetParentItem(m_hDropItem);
->       }
->       else if (nDragIndent == (nDropIndent + 1))
->       {
->          // if dropped on 'upper' indentation, add as last child of drop item
->          htiPosition = TVI_LAST;
->       }
->       else
->       {
->          // invalid drop target
->          return;
->       }
->    }

      Expand (m_hDropItem,TVE_EXPAND);

->    HTREEITEM htiNew = CopyBranch(m_hDragItem,m_hDropItem,htiPosition);

The implementation of GetIndentLevel() is as follows:

int CRTreeCtrl::GetIndentLevel(HTREEITEM hItem)
   int iIndent = 0;
   while ((hItem = GetParentItem(hItem)) != NULL)
   return iIndent;


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