VB Forum / Win API / September 2008

I thought that I was being SPECIFIC, but perhaps not enough FOR YOU so let me
CLARIFY.

1st of all, I am concerned about DoEvents in Visual Basic 6.0 (your
newsgroup grouping structure simply uses "Visual Basic" and "Windows API").  
The pitfalls of DoEvents in VB 6 is a relatively worn topic:

http://blogs.msdn.com/tmiller/archive/2003/11/07/57524.aspx

http://blogs.msdn.com/jfoscoding/archive/2005/08/06/448560.aspx

http://discuss.fogcreek.com/joelonsoftware3/default.asp?cmd=show&ixPost=92327&ix
Replies=20

http://discuss.joelonsoftware.com/default.asp?dotnet.12.481760.7

http://www.codinghorror.com/blog/archives/000159.html

I have read the VB 6 documentation and its explanation of DoEvents.  The
warning to watch for re-entrancy problems is understated at best.  In
multi-component, multi-tier applications of any complexity it is nearly
impossible to avoid re-entrancy when a call to DoEvents is made inside of
some component (i.e. one team's right hand doesn't know what the left hand of
the other team is doing).

To my understanding DoEvents is essentially a wrapper for a Do loop that
utilizes PeekMessage(), TranslateMessage(), and DispatchMessage().  Many non
C programmers today are unaware of message pumps etc... but DoEvents is
essentially a nested message pump.

I mentioned .NET because it has the IMessageFilter interface which allows a
programmer to place a filter on the underlying message pump and even on the
nested message pump that Application.DoEvents() uses.  VB6 does not have this
feature so I wanted to determine the best way to implement similar
functionality in VB 6.

Again, DoEvents, if I understand it correctly, represents a non-filtered
nested message loop.  I want to created a filtered nested message loop which
enables me to retrieve and dispatch a set of messages by message type or
target (i.e. hwnd).
MY QUESTION TO YOU IS:  what are the potential pitfalls of creating such a
filtering system using PeekMessage?

HERE IS AN EXAMPLE:

Public Function Pump(ByVal bRenderUIs As Boolean, ByVal colWinHandles As
MTKList, ByVal colMessagesToDispatch As MTKList) As Boolean
On Error Resume Next
   
   Dim tMSG As MSG
   Dim lRet As Long
   Dim lNum1 As Long
   Dim lNum2 As Long
   Dim lVal1 As Long
   Dim lVal2 As Long

   'OBTAIN AND DISPATCH SPECIFIC MESSAGES FOR SPECIFIC WINDOWS
   If Not colWinHandles Is Nothing And Not colMessagesToDispatch Is Nothing
Then
       For lNum1 = 1 To colWinHandles.Count
           lVal1 = colWinHandles.Item(lNum1)
           If IsValidWinHandle(lVal1) = True Then
               For lNum2 = 1 To colMessagesToDispatch.Count
                   lVal2 = colMessagesToDispatch.Item(lNum2)
                   If IsValidMessage(lVal2) = True Then
                       lRet = PeekMessage(tMSG, lVal1, lVal2, lVal2,
PM_REMOVE)
                       If lRet = 0 Then
                           TranslateMessage tMSG
                           DispatchMessage tMSG                            
                       End If
                   End If
               Next lNum2
           End If
       Next lNum1
   End If
   
   If bRenderUIs = True Then
       PumpWMPaintForAllWindowHandles
   End If
   
   PumpMessagesForWindowHandles = True

End Function

"WHY WOULD I LIKE TO CREATE SUCH A FUNCTION?" (gee... I'm glad that you are
so courteous and helpful and asked...):

In some rare situations a developer may be working with a 3rd party control
(i.e. closed source, black box).  At a specific point in execution the
program may invoke a method (say .CellFocus(x,y)) on that control in order to
place the control in a specified state (lets say at line 100).  THE PROBLEM
is that at line 101 the control is not actually in that state even though,
according to the documentation, it should be.
Lines 102 to 105 of the program expect the control to be in that state.  The
programmer discovers that if he places DoEvents at line 102 that the control
then enters the proper state (presumably because a particular MESSAGE was
received by that control).  However, re-entrancy occurrs in some scenarios
because DoEvents was utilized and because the large program written by 15
developers over 8 years has places where re-entrancy is possible.  

EXAMPLE:

   wbBrowser.Navigate2 (msDisplayFileName)
       DoEvents

SOLUTION:

So, it seems like that it might be useful to create a version of DoEvents
that filters out certain Windows Messages and dispatches others (thus
avoiding re-entrancy via the pump).

AGAIN, TO CLARIFY, MY QUESTION:  

What are the potential pitfalls of creating a filtered nested message pump
in a VB6 application vai the PeekMessage() Win32 API function?

Guten Tag.  Thank you for your reply.