=====Replacing the message pump=====

//by Richard Russell, January 2012//\\ \\  In Windows, every User Interface (UI) thread must have a [[http://en.wikipedia.org/wiki/Message_loop_in_Microsoft_Windows|message pump]] (or message loop), the function of which is to serialise and dispatch incoming messages to their destination windows. \\ \\  In //BBC BASIC for Windows// the message pump is provided by the Run Time Engine, and generally the programmer doesn't need to be aware of it. For anybody who may be interested, this is its C code:\\ 
<code c>
  while (GetMessage (&msg, NULL, 0, 0) != FALSE )
       {
         if (((hwndMDIC == NULL) ||
             !TranslateMDISysAccel (hwndMDIC, &msg))
          && ((haccel == NULL) ||
             !TranslateAccelerator (hwacc, haccel, &msg)))
           {
             TranslateMessage (&msg) ;
             DispatchMessage (&msg) ;
           }
       }
</code>
Very occasionally there may be a requirement to use a different message pump. For example, in order for **keyboard navigation** to work in a dialogue box, the message pump must include a call to the [[http://msdn.microsoft.com/en-us/library/windows/desktop/ms645498(v=vs.85).aspx|IsDialogMessage]] API.\\ \\  Until very recently I believed that it was impossible to //replace// the existing BB4W message pump, and that therefore the only way to provide a different one was to start a new thread (each thread has its own message pump). This is the principal reason why the **WINLIB2** library creates a new thread for every new dialogue box created.\\ \\  However I have discovered that in fact there is a way of replacing the 'main' message pump, and the procedure listed below does exactly that:

<code bb4w> 
        DEF PROCnewmessagepump
        LOCAL K%, L%, M%, N%, O%, P%, S%, U%, code%, pass%
        DIM code% 130, L% -1
        FOR pass% = 8 TO 10 STEP 2
          P% = code%
          [OPT pass%
          .K%
          add esp,28
          ret 16

          .N%
          sub esp,28
          push 0 : call "ReplyMessage"
          .U%
          push @hwnd% : call "IsWindow" : or eax,eax : jz K%
          mov eax,esp : push 0 : push 0 : push 0 : push eax
          call "GetMessage" : or eax,eax : jz K%
          push esp : push @haccel% : push @hwnd% : call "TranslateAccelerator"
          or eax,eax : jnz U%
          call "GetForegroundWindow"
          cmp eax,@hwnd% : jz S%
          push esp : push eax : call "IsDialogMessage"
          or eax,eax : jnz U%
          .S%
          push esp : call "TranslateMessage"
          push esp : call "DispatchMessage"
          jmps U%

          .M% cmp dword [esp+8],&502 : jz N%
          pop eax : push [^O%] : push eax : jmp "CallWindowProc"
          ]
        NEXT pass%
        SYS "GetWindowLong", @hwnd%, -4 TO O%
        SYS "SetWindowLong", @hwnd%, -4, M%
        SYS "SendMessage", @hwnd%, &502, 0, 0
        SYS "SetWindowLong", @hwnd%, -4, O%
        SYS "SendMessage", @hwnd%, 0, 0, 0
        ENDPROC
</code>