Responding to the Removal of Another Object in the Chain

If a communication object that was added to the chain before your communication object removes itself from the chain, your communication object must respond to keep the chain intact.

This involves several steps:

• Your communication object must process the HM_UPDATE_OBJCOM_WINDOW_CHAIN help manager message. This message provides your communication object with:

  a.

  The handle of the communication object that is removing itself from the chain.
  b.
  The window handle for the object to serve as a replacement in the chain.
  Your communication object does not need to respond unless the first parameter, the handle of the communication object removing itself, is identical to the handle returned to your communication object when it initially sent a HM_SET_OBJCOM_WINDOW message to add itself to the chain (see Adding Your Communication Object to the Communication Chain). Your communication object should have stored this handle.
  2.
  If the second parameter returned by HM_UPDATE_OBJCOM_WINDOW_CHAIN does not match the handle previously returned by HM_SET_OBJCOM_WINDOW, your communication object is not affected by the modification to the chain and does not need to respond other than to pass on the message to the next communication object in the chain.

  However, if these handles are identical, your communication object has a new "neighbor" in the chain, and should begin passing all HM_ help manager messages to the object identified by the handle returned as the first parameter of HM_UPDATE_OBJCOM_WINDOW_CHAIN.

  The following code fragment illustrates this process.

  #define INCL_WIN#define INCL_WINHELP
  #include <os2.h>
  
  #define HM_MSG_MAX (HM_MSG_BASE+0x0024)
  
  USHORT   IPFClassRegistered = 0;         /* IPF class registered flag        */
  
  /* Window procedure */
  MRESULT EXPENTRY IPFWinProc (HWND hwnd, USHORT msg, MPARAM mp1, MPARAM mp2);
  
  
  MRESULT EXPENTRY IPFWinProc (HWND hwnd, USHORT msg, MPARAM mp1, MPARAM mp2)
  /* The window procedure will handle deleting ourselves from the chain
     as well as forwarding all standard IPF message on the the next window in
     the chain */
  {
     HWND hwndPrevious, hwndLatest;
  
     /* Get previous comm obj */
     hwndPrevious = (HWND) WinQueryWindowULong (hwnd, QWL_USER);
     /* Handle the messages you want here, but do not return unless you
        now the message will not need to be handled by another comm obj.
        Let the the message be forwarded on to the next comm obj.  The
        only exception to this is if you change the cover page size.  You
        need to return TRUE to prevent the coverpage from being resized. */
     switch (msg)
     {
        case HM_UPDATE_OBJCOM_WINDOW_CHAIN:
           /* If another window is being inserted, replace previous with it */
           if (hwndPrevious == (HWND)mp1) {
              hwndPrevious = (HWND)mp2;
              if (!WinSetWindowULong (hwnd, QWL_USER, (ULONG) hwndPrevious)) {
                 /* Put up error message */
                 WinMessageBox (HWND_DESKTOP,
                                HWND_DESKTOP,
                                (PCH)"Can not save handle into reserved memory",
                                (PCH)"IPF Sample Error Message",
                                1,
                                MB_OK | MB_APPLMODAL |
                                MB_MOVEABLE | MB_ICONASTERISK);
                 break;
              }
  
           } else {
              /* Otherwise simply forward the message on */
              if (hwndPrevious != 0L) {
                 WinSendMsg (hwndPrevious, HM_UPDATE_BJCOM_WINDOW_CHAIN,
                             (MPARAM) mp1, (MPARAM) mp2);
              }
           }
           return (MPARAM) NULL;
           break;
        case WM_CLOSE:
           WinDestroyWindow (WinQueryWindow (hwnd, QW_PARENT));
           return (MPARAM) NULL;
           break;
        case WM_DESTROY:
           /* Take ourselves out of the chain */
           hwndLatest = (HWND)WinSendMsg (hwnd, HM_QUERY,
                                          MPFROM2SHORT (0, HMQW_OBJCOM_WINDOW),
                                          (MPARAM)NULL);
           WinSendMsg (hwndLatest, HM_UPDATE_OBJCOM_WINDOW_CHAIN,
                       (MPARAM) WinQueryWindow (hwnd, QW_PARENT),
                       (MPARAM) hwndPrevious );
           return (MPARAM) NULL;
           break;
     }
     if ((msg > HM_MSG_BASE) && (msg <= HM_MSG_MAX)) {
        return WinSendMsg( hwndPrevious, msg, mp1, mp2);
     } else {
        return WinDefWindowProc (hwnd, msg, mp1, mp2);
     } /* endif */
  }
  
  

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