BUG: WM_MOUSEMOVE Messages Returned Out of Sequence

• Microsoft Visual C++, Macintosh Cross-Development Edition, versions 2.0, 4.0

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