Before you begin coding, you should understand the general steps involved in the creation and function of a communication object. The steps are as follows:

• IPF encounters a :docprof. or :acviewport. tag.

• Based on the content of the tag, IPF loads a DLL and calls a function that corresponds to a communication object.

• IPF passes 2 items to the communication object:

  a.

  A pointer to an ACVP data structure. This data structure includes the following elements:

  ULONG  cb;         /* length of data structure              */
  HAB    hAB;        /* anchor block handle                   */
  HMQ    hmq;        /* handle to message queue               */
  ULONG  ObjectID;   /* ObjectID attribute as specified in    */
                     /* an acviewport tag                     */
  HWND   hWndParent; /* handle to acviewport parent window    */
  HWND   hWndOwner;  /* handle to acviewport owner window     */
  HWND   hWndACVP;   /* handle to acviewport                  */
  

  IPF supplies all but the last piece of information in this data structure for the communication object. If the communication object creates an application-controlled window, it must place the handle to that window in the last element of the data structure before returning to IPF. IPF uses the handle to size and position the window.

  If the communication object is global (loaded by means of a :docprof. tag), the last 3 items are not needed and are set to null by IPF.

  b.
  A string identified in the :docprof. or :acviewport. tag as objectinfo. This is simply a generic parameter passed to the communication object. Your communication object may or may not use this parameter, but it should process it.
  4.
  The communication object begins execution. It must complete the following three steps, sequentially:

  a.

  Register a window class.
  b.
  Create a window.
  c.
  Insert the window in the communication chain (see The Communication Chain).
  5.
  The next step depends on whether or not the communication object DLL was loaded through a :docprof. tag or an :acviewport. tag.

  • If the communication object DLL was loaded through a :docprof. tag, it simply returns.

  • If the communication object DLL was loaded through an :acviewport. tag, the communication object passes the window handle of the window it created to IPF.
  6.
  The communication object begins message processing:

  • If the communication object DLL was loaded through :docprof. tag, it must process HM_ messages and the WM_CLOSE message.

  • If the communication object DLL was loaded through an :acviewport. tag, it must process:

    • HM_ messages
      
    • Various WM_ messages
      
    • Optionally, other messages, dependent on the particular design of the communication object.

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