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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    Posted by Legacy on 04/09/2002 12:00am

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