Some Tricks with Peek Message

Introduction

I started off in VC++ after programming for sometime in VB (a few months at home). One function that I missed from VB was DoEvents. But I have found an equivalent one in VC++ using the function PeekMessage. Most of you might know these tricks, but this is mainly for those who don't know how to do this.

Why I needed a DoEvents function

Well, I am in the midst of developing an interesting personal project, the idea is a secret ;). So what happened was that I had a tight loop kind of a situation, and I didn't want to use a thread because it was un-necessary. That's when I started thinking about a DoEvents equivalent in VC++.

DoEvents code

Now enough of preaching :). Let's get to the code that will help us in doing this.

static void DoEvents( HWND hWnd_i )
{
   MSG stMsg = { 0 };
   while( PeekMessage( &stMsg, hWnd_i, 0, 0, PM_REMOVE ))
   {
       TranslateMessage( &stMsg );
       DispatchMessage( &stMsg );
    }
}

The loop in which this is called is as follows...

for( UINT uFileIndex = 0; uFileIndex < uFileCount; ++uFileIndex )
{
   UpdateProgressBar( uFileIndex + 1 );
   if( !( uFileIndex % LOOP_THRESH_HOLD_FOR_DO_EVENTS ))
   {
      ResUtils::DoEvents( this->GetSafeHwnd() );
   }
}

I defined a LOOP_THRESH_HOLD_FOR_DO_EVENTS for calling this function. This will be useful if your loop is a low priority one. Now my GUI does not hang, painting takes place properly too.

Flushing the keyboard and mouse input buffer

How about flushing the keyboard input buffer and the mouse input buffer selectively...

static void FlushKeyboardInputBuffer( HWND hWnd_i )
{
   MSG stMsg = { 0 };
   while( PeekMessage( &stMsg, hWnd_i , WM_KEYFIRST, WM_KEYLAST, PM_REMOVE ))
      ;
}

static void FlushMouseInputBuffer( HWND hWnd_i )
{
   MSG stMsg = { 0 };
   while( PeekMessage( &stMsg, hWnd_i, WM_MOUSEFIRST, WM_MOUSELAST, PM_REMOVE ))
      ;
}

Conclusion

These are a few of the ideas that I got while programming. Hope this helps. There are some other flags that you can pass to PeekMessage, e.g.:

PM_QS_INPUT	Windows 98/Me, Windows 2000/XP: Process mouse and keyboard messages.
PM_QS_PAINT	Windows 98/Me, Windows 2000/XP: Process paint messages.
PM_QS_POSTMESSAGE	Windows 98/Me, Windows 2000/XP: Process all posted messages, including timers and hotkeys.
PM_QS_SENDMESSAGE	Windows 98/Me, Windows 2000/XP: Process all sent messages.

For more information on how to do idle time processing using PeekMessage, search MSDN with the OnIdle keyword, and you will bump into pretty interesting topics.