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<FONT COLOR="#AOAO99">Enhanced Drag & Drop</FONT></H3></CENTER>

<CENTER><H3><HR></H3></CENTER>

This article was contributed by <A HREF="mailto:mgrinberg@impsat1.com.ar">Miguel Grinberg</A>



<p>I modified the code for using drag & drop on tree controls. The idea 
was to add three features found on the ClassView tree control of the 
VC++ ide. With the following code you can disable the drag & drop 
interface for certain items on the tree (the root item in ClassView, 
for example), and you can decide whether an item is a valid drop 
target or not, and in case not, provide an alternative item as a drop 
target (behavior observed in ClassView when you drag classes between 
folders; only folders are drop targets). Also, I update the cursor 
shape to reflect the status of the drag & drop operation.

<p>The member variables needed are:

<PRE><TT><FONT COLOR="#990000">
protected:
	CImageList*     m_pDragImage;
	BOOL            m_bLDragging;
	HTREEITEM       m_hitemDrag,m_hitemDrop;
	HCURSOR         m_dropCursor,m_noDropCursor;
</FONT></TT></PRE>

<p>Member variables m_dropCursor and m_noDropCursor must be initialized to
the desired cursors.

<p>Here is my version of OnBeginDrag:

<PRE><TT><FONT COLOR="#990000">
void CTreeCtrlX::OnBeginDrag(NMHDR* pNMHDR, LRESULT* pResult) 
{
	NM_TREEVIEW* pNMTreeView = (NM_TREEVIEW*)pNMHDR;

	// TODO: Add your control notification handler code here
	*pResult = 0;

	m_hitemDrag = pNMTreeView->itemNew.hItem;
	m_hitemDrop = NULL;
	SelectItem( m_hitemDrag );
	if (!IsDropSource(m_hitemDrag))
		return;

	// get the image list for dragging 
	m_pDragImage = CreateDragImage(m_hitemDrag);  
	// CreateDragImage() returns NULL if no image list 
	// associated with the tree view control 
	if( !m_pDragImage )
		return;

	m_bLDragging = TRUE;
	m_pDragImage->BeginDrag(0, CPoint(15, 15));
	POINT pt = pNMTreeView->ptDrag;
	ClientToScreen( &pt );
	m_pDragImage->DragEnter(NULL, pt);
	SetCapture();
}
</FONT></TT></PRE>

<p>This member function calls IsDropSource() virtual function to decide 
if the item can be a source of a drag & drop operation. If 
IsDropSource() returns FALSE, the drag is cancelled. Here is the 
default IsDropSource() member function:

<PRE><TT><FONT COLOR="#990000">
/* virtual */ BOOL CTreeCtrlX::IsDropSource(HTREEITEM hItem)
{
	return TRUE;  // all items are valid sources
}
</FONT></TT></PRE>

<p>I also modified the OnMouseMove() handler:

<PRE><TT><FONT COLOR="#990000">
void CTreeCtrlX::OnMouseMove(UINT nFlags, CPoint point) 
{
	HTREEITEM hitem;
	UINT flags;

	if (m_bLDragging)
	{
		POINT pt = point;
		ClientToScreen( &pt );
		CImageList::DragMove(pt);
		if ((hitem = HitTest(point, &flags)) != NULL)
		{
			CImageList::DragShowNolock(FALSE);
			m_hitemDrop = GetDropTarget(hitem);
			SelectDropTarget(m_hitemDrop);
			CImageList::DragShowNolock(TRUE);
		}
		else
			m_hitemDrop = NULL;
		if (m_hitemDrop)
			SetCursor(m_dropCursor);
		else
			SetCursor(m_noDropCursor);
	}

	CTreeCtrl::OnMouseMove(nFlags, point);
}
</FONT></TT></PRE>

<p>This version, calls GetDropTarget() virtual function to decide if the 
item behind the cursor is a valid drop target. This function should 
return NULL if the item is not a valid target for a drop. If the item 
can accept a drop, it should return its HTREEITEM handle. As an 
alternative, a different HTREEITEM can be returned to specify a 
different drop target. This can be useful, for example, if you don't 
accept drops on tree leafs, but want to direct the drop to the 
parent. Also I change the cursor to reflect the status of the 
operation (OnSetCursor is not called while mouse is captured, so the 
cursor can be safely changed here).

<p>Here is the default implemmentation of the GetDropTarget() virtual 
function:

<PRE><TT><FONT COLOR="#990000">
/* virtual */ HTREEITEM CTreeCtrlX::GetDropTarget(HTREEITEM hItem)
{
	// inhibit drop on the drop source or its parent
	if (hItem == m_hitemDrag || hItem == GetParentItem(m_hitemDrag))
		return NULL;
	return hItem;
}
</FONT></TT></PRE>

<p>Finally, here is the OnLButtonUp() handler:

<PRE><TT><FONT COLOR="#990000">
void CTreeCtrlX::OnLButtonUp(UINT nFlags, CPoint point) 
{
	CTreeCtrl::OnLButtonUp(nFlags, point);

	if (m_bLDragging)
	{
		m_bLDragging = FALSE;
		CImageList::DragLeave(this);
		CImageList::EndDrag();
		ReleaseCapture();

		delete m_pDragImage;

		// Remove drop target highlighting
		SelectDropTarget(NULL);

		if( m_hitemDrag == m_hitemDrop || m_hitemDrop == NULL)
			return;

		// If Drag item is an ancestor of Drop item then return
		HTREEITEM htiParent = m_hitemDrop;
		while( (htiParent = GetParentItem( htiParent )) != NULL ) 
		{
			if( htiParent == m_hitemDrag ) return;
		}

		Expand( m_hitemDrop, TVE_EXPAND ) ;

		HTREEITEM htiNew = CopyBranch(m_hitemDrag, m_hitemDrop, TVI_LAST ); 
 		DeleteItem(m_hitemDrag);
		SelectItem( htiNew );
	}
}
</FONT></TT></PRE>


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