Subclassing a Window

To subclass a window, an application calls the WinSubclassWindow function, specifying the handle of the window to subclass and a pointer to the new window procedure. The WinSubclassWindow function returns a pointer to the original window procedure; the application can use this pointer to pass unprocessed messages to the original procedure. The following code fragment subclasses a push button control window. The new window procedure generates a beep whenever the user clicks the push button.

    PFNWP pfnPushBtn;
    CHAR szCancel[] = "Cancel";
    HWND hwndClient;
    HWND hwndPushBtn;
        .
        .
        .

    /* Create a push button control.                  */
    hwndPushBtn = WinCreateWindow(
        hwndClient,     /* Parent-window handle       */
        WC_BUTTON,      /* Window class               */
        szCancel,       /* Window text                */
        WS_VISIBLE   |  /* Window style               */
        WS_SYNCPAINT |  /* Window style               */
        BS_PUSHBUTTON,  /* Button style               */
        50, 50,         /* Physical position          */
        70, 30,         /* Width and height           */
        hwndClient,     /* Owner-window handle        */
        HWND_TOP,       /* Z-order position           */
        1,              /* Window identifier          */
        NULL,           /* No control data            */
        NULL);          /* No presentation parameters */

    /* Subclass the push button control.              */
    pfnPushBtn = WinSubclassWindow(hwndPushBtn,
        SubclassPushBtnProc);
        .
        .
        .
}
    /* This procedure subclasses the push button.     */
    MRESULT EXPENTRY SubclassPushBtnProc(HWND hwnd,ULONG msg,MPARAM mp1, MPARAM mp2)
    {
        switch (msg) {

    /* Beep when the user clicks the push button.     */
            case WM_BUTTON1DOWN:
                DosBeep(1000, 250);
                break;

            default:
                break;
        }

    /* Pass all messages to the original window procedure. */
        return (MRESULT) pfnPushBtn(hwnd, msg, mp1, mp2);
    }