Q141357: BUG: WM_MOUSEMOVE Messages Returned Out of Sequence

Article: Q141357
Product(s): Microsoft C Compiler
Version(s): MACINTOSH:2.0,4.0
Operating System(s): 
Keyword(s): kbHWMAC kbMFC kbMouse KbUIDesign kbVC200bug kbVC400bug kbWndwMsg kbGrpDSMFCATL
Last Modified: 07-MAY-2001

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The information in this article applies to:

- Microsoft Visual C++, Macintosh Cross-Development Addon, versions 2.0, 4.0 
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SYMPTOMS
========

When you move the mouse pointer over a window, some of the WM_MOUSEMOVE messages
arrive out of order. For example, when you draw using the Scribble sample
program, an old point is occasionally received causing a zig-zag effect.

CAUSE
=====

There is a bug in the WLM implementation of ::PeekMessage that causes some of
the mouse move messages to be retrieved later.


RESOLUTION
==========

Check the time field of the MSG record of the current message when processing a
WM_MOUSEMOVE message. If the time is older than that of the previous
WM_MOUSEMOVE message, discard the message. See the Sample Code section of this
article for an example.

An alternative solution is to use Mac APIs (such as StillDown() and GetMouse())
directly to track the mouse until the button is released. This method
dramatically increases mouse tracking and is a common method that native
Macintosh applications use. Keep in mind that this will keep the system locked
until the mouse is released.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article. We are researching this problem and will post new
information here in the Microsoft Knowledge Base as it becomes available.

MORE INFORMATION
================

The WLM message system was designed by imitating Window's message system using
the Macintosh API. The bug is more precisely related to the WLM message system
used by ::PeekMessage.

This problem is very apparent in the released version of the target Macintosh
executable using Visual C++ 2.0 (try the Scribble sample program). It occurs
much less frequently on Visual C++ 4.0 but it still exists.

Sample Code
-----------

     // Method one
     // Check by timestamp of messages

     void CMyView::OnMouseMove(UINT, CPoint point)
     {
         ...
         static DWORD lastTime = 0;
         if (lastTime > GetCurrentMessage()->time)
             return; // discard message
         else
             lastTime = GetCurrentMessage()->time;
         // process message
     }

     // Method two
     // Use native MacOS calls to track mouse movements

     #ifdef _MAC
     #include <macname1.h>
     #include <Types.h>
     #include <Events.h>
     #include <macname2.h>
     #endif

     void CMyView::OnLButtonDown(UINT nFlags, CPoint point)
     {
     #ifdef _MAC
        Point macPt;    // Mac specific data type
        CPoint winPt;   // temp Windows Point var

        while(StillDown()) {
           GetMouse(&macPt);       // Get GLOBAL mouse coordinates
           winPt.x = macPt.h;      // Convert to windows struct
           winPt.y = macPt.v;
           ScreenToClient(&winPt); // Convert to Client Coordinates
           TRACE("Where? (%d,%d)\n", winPt.x, winPt.y);
        }
     #else
        CView::OnLButtonDown(nFlags, point);
     #endif
     }

REFERENCES
==========

Books Online - WM_MOUSEMOVE, ::PeekMessage, GetCurrentMessage
68k Porting Reference - PeekMessage
Inside Macintosh - Macintosh Toolbox Essentials

Additional query words:

======================================================================
Keywords          : kbHWMAC kbMFC kbMouse KbUIDesign kbVC200bug kbVC400bug kbWndwMsg kbGrpDSMFCATL 
Technology        : kbVCsearch kbHWMAC kbOSMAC kbAudDeveloper kbVCXDev200Mac kbVCXDev400Mac
Version           : MACINTOSH:2.0,4.0
Issue type        : kbbug
Solution Type     : kbfix

=============================================================================

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