Drag and Drop Enhancement - Auto expand while hovering


I used the code from CodeGuru to implement the Drag and Drop processing in my tree control. I wanted to modify this slightly so that a node would automaticall expand when I dragged a node over a drop target and hovered over it. This is similar to what occurs in Windows Explorer.

The following code snippet could be used in conjunction with the Drag and Drop code offered previously in CodeGuru.

The first step is to add two new variables to your class definition, a Hover Timer Id, and a Hover Point. The Timer Id identifies the timer that is started when you are dragging a tree node and you hover over a drop node for a certain period of time. The Hover Point stores the current point of the hover. This value needs to be stored as it is not available in the OnTimer event handler.

Two events must be defined, if not defined already. They are OnTimer and OnMouseMove. I used the Class Wizard to create them. The following code chunk is from the header file for the tree control. The only items that I included are those that need to be added.


class CMyTree : public CTreeCtrl
{
		.
		. // Definition
		.
private:
	UINT			m_nHoverTimerID;
	POINT			m_HoverPoint;
		.
		.	// Other definition
		.
protected:
	// Generated message map function
	//{{AFX_MSG(CMyTree
	afx_msg void OnTimer(UINT nIDEvent);
	afx_msg void OnMouseMove(UINT nFlags, CPoint point);
	//}}AFX_MS
	DECLARE_MESSAGE_MAP()
}


In the .cpp file, code needs to be added to two functions: OnMouseMove and OnTimer. In OnMouseMove, the hover timer is constantly checked for existence, and if it exists, it is deleted. You only want the OnTimer event called when the mouse pointer is still for a certain period of time. The SetTimer will create a timer, and after the expiration of the selected time period, will send a message triggering the OnTimer event. In the OnTimer event handler, it is a simple matter to kill the timer, get the item currently being hovered over, and issuing an expand node command.

BEGIN_MESSAGE_MAP(CMyTree, CTreeCtrl)
	//{{AFX_MSG_MAP(CMyTree
	ON_WM_TIMER()
	ON_WM_MOUSEMOVE()
	//}}AFX_MSG_MA
	END_MESSAGE_MAP()

void CMyTree::OnMouseMove(UINT nFlags, CPoint point) 
{
	HTREEITEM		hitem;
	UINT			flags;

	if ( m_nHoverTimerID )
	{
		KillTimer( m_nHoverTimerID );
		m_nHoverTimerID = 0;
	}

	if (m_bDragging)
	{
		ASSERT(m_pImageList != NULL);
		m_pImageList->DragMove(point);

		m_nHoverTimerID = SetTimer(2, 750, NULL);
		m_HoverPoint = point;

		if ((hitem = HitTest(point, &flags)) != NULL)
		{
			m_pImageList->DragLeave(this);
			SelectDropTarget(hitem);
			m_hitemDrop = hitem;
			m_pImageList->DragEnter(this, point);
		}

		m_pImageList->DragShowNolock(TRUE);
	}
	
	CTreeCtrl::OnMouseMove(nFlags, point);
}

void CMyTree::OnTimer(UINT nIDEvent) 
{
	if ( nIDEvent == m_nHoverTimerID )
	{
		KillTimer( m_nHoverTimerID );
		m_nHoverTimerID = 0;

		HTREEITEM	trItem	= 0;
		uint		uFlag	= 0;

		trItem = HitTest(m_HoverPoint, &uFlag);

		if ( trItem )
		{
			SelectItem( trItem );
			Expand(trItem,TVE_EXPAND);	
		}
	}
	else
	{
		CTreeCtrl::OnTimer(nIDEvent);
	}
}