Preventing inadvertant drag and drop




Have you ever had the experience that you clicked on the Explorer to bring
it to the foreground and the directory got moved. This happens if the mouse
moves before the mouse button is released. Many of the users of my program
had this experience and heres what I did to prevent inadvertant drag –
drop. I basically imposed the restriction that after pressing the mouse
button the cursor should remain on the item for a few milliseconds before
dragging can be initiated.

 


Step 1: Declare member variable


Add a member variable to hold the tick count when the user presses the
mouse button.

 

protected:
        DWORD   m_dwDragStart;


Step 2: Define a constant to specify the delay


We define DRAG_DELAY with a value 80. You might want to use a different
value.

#define DRAG_DELAY 80


Step 3: Add handler for WM_LBUTTONDOWN


The only interesting thing we do here is initialize the m_dwDragStart variable.
GetTickCount() returns the number of milliseconds since Windows was started.

 

void CTreeCtrlX::OnLButtonDown(UINT nFlags, CPoint point) 
{
        m_dwDragStart = GetTickCount();
        CTreeCtrl::OnLButtonDown(nFlags, point);
}


Step 4: Check for sufficient delay in TVN_BEGINDRAG handler


Insert the following code right at the beginning of the TVN_BEGINDRAG handler.
In a previous section we have used the name OnBeginDrag() for the handler
function. If the delay since the user pressed the left mouse button is
not sufficient, we not initiate the drag and drop process.

 

        // This code is to prevent accidental drags.
        if( (GetTickCount() - m_dwDragStart) < DRAG_DELAY )
                return;

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