prevent_drag_drop.shtml
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<TITLE>Preventing inadvertant drag and drop</TITLE>
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<H3>
<FONT COLOR="#AOAO99">Preventing inadvertant drag and drop</FONT></H3></CENTER>
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<H3>
<HR></H3></CENTER>
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 here抯 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.
<BR>
<H4>
Step 1: Declare member variable</H4>
Add a member variable to hold the tick count when the user presses the
mouse button.
<BR>
<PRE><TT><FONT COLOR="#990000">protected:
DWORD m_dwDragStart;</FONT></TT></PRE>
<PRE><TT><FONT COLOR="#990000"></FONT></TT></PRE>
<H4>
Step 2: Define a constant to specify the delay</H4>
We define DRAG_DELAY with a value 80. You might want to use a different
value.
<PRE><TT><FONT COLOR="#990000">#define DRAG_DELAY 80</FONT></TT></PRE>
<PRE><TT><FONT COLOR="#990000"></FONT></TT></PRE>
<H4>
Step 3: Add handler for WM_LBUTTONDOWN</H4>
The only interesting thing we do here is initialize the m_dwDragStart variable.
GetTickCount() returns the number of milliseconds since Windows was started.
<BR>
<PRE><TT><FONT COLOR="#990000">void CTreeCtrlX::OnLButtonDown(UINT nFlags, CPoint point)
{
m_dwDragStart = GetTickCount();
CTreeCtrl::OnLButtonDown(nFlags, point);
}</FONT></TT></PRE>
<PRE><TT><FONT COLOR="#990000"></FONT></TT></PRE>
<H4>
Step 4: Check for sufficient delay in TVN_BEGINDRAG handler</H4>
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.
<BR>
<PRE><TT><FONT COLOR="#990000"> // This code is to prevent accidental drags.
if( (GetTickCount() - m_dwDragStart) < DRAG_DELAY )
return;</FONT></TT>
</PRE>
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<TD WIDTH="33%"><FONT SIZE=-1><A HREF="http://www.codeguru.com">Goto HomePage</A></FONT></TD>
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<CENTER><FONT SIZE=-2>© 1998 Zafir Anjum</FONT> </CENTER>
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<DIV ALIGN=right><FONT SIZE=-1>Contact me: <A HREF="mailto:zafir@home.com">zafir@home.com</A> </FONT></DIV>
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