NAME
        BeginRefresh -- Sets up a window for optimized refreshing.

    SYNOPSIS
        BeginRefresh( window )
                      A0

        VOID BeginRefresh( struct window * );

    FUNCTION
        This routine sets up your window for optimized refreshing.

        Its role is to provide Intuition integrated access to the Layers
        library function beginupdate().  its additional contribution is
        to be sure that locking protocols for layers are followed, by
        locking both layers of a WFLG_GIMMEZEROZERO window only after the
        parent layer_info has been locked.  also, the wflg_windowrefresh
        flag is set in your window, for your information.

        The purpose of beginupdate(), and hence beginrefresh(), is to
        restrict rendering in a window (layer) to the region that needs
        refreshing after an operation such as window sizing or uncovering.
        This restriction to the "damage region" persists until you call
        endrefresh().

        For instance, if you have a WFLG_SIMPLE_REFRESH window which is
        partially concealed and the user brings it to the front, you can
        receive an IDCMP_REFRESHWINDOW message asking you to refresh your
        display.  If you call BeginRefresh() before doing any of the
        rendering, then the layer that underlies your window will be arranged
        so that the only rendering that will actually take place will be that
        which goes to the newly-revealed areas.  This is very performance-
        efficient, and visually attractive.

        After you have performed your refresh of the display, you should call
        endrefresh() to reset the state of the layer and the window.  then you
        may proceed with rendering to the entire window as usual.

        You learn that your window needs refreshing by receiving either a
        message of class IDCMP_REFRESHWINDOW through the IDCMP, or an input
        event of class IECLASS_REFRESHWINDOW through the Console device.
        Whenever you are told that your window needs refreshing, you should
        call BeginRefresh() and endrefresh() to clear the refresh-needed
        state, even if you don't plan on doing any rendering.  You may relieve
        yourself of even this burden by setting the WFLG_NOCAREREFRESH flag
        when opening your window.

    NOTES
        WARNING: You should only perform simple graphics refreshing
        operations between BeginRefresh() and endrefresh().  these include
        any graphics.library drawing functions or the simple Intuition
        rendering functions (DrawImage(), printitext(), drawborder(), and
        so on).  In particular, do not call refreshgadgets(), refreshglist(),
        refreshwindowframe(), etc., since any of the complex intuition
        functions run the risk of creating a deadlock.  Also avoid any
        other high-level calls in Intuition or ones that might invoke
        Intuition.  This means no calling setpointer(), no calling
        lockibase(), no dos.library functions (since an easyrequest()
        might result), etc.  Keep it simple.

        By the time the application receives its IDCMP_REFRESHWINDWOW
        message, Intuition has already repaired any damaged regions
        of the window border or the gadgets in the window, so attempts
        to redraw gadgets or borders are as unnecessary as they are
        dangerous.

        ANOTHER WARNING: The concept of multiple refresh passes using
        EndRefresh( w, FALSE ) is not completely sound without further
        protection.  The reason is that between two sessions, more
        damage can occur to your window.  Your final EndRefresh( w, TRUE )
        will dispose of all damage, including the new, and your
        initial refreshing pass will never get the chance to refresh
        the new damage.

        To avoid this, you must protect your session using locklayerinfo()
        which will prevent Intuition from performing window operations
        or anything else which might cause further damage from occurring.
        Again, while holding the LayerInfo lock make no Intuition
        function calls dealing with gadgets; just render.

        You can, however, call installclipregion() for the different
        refresh passes, if you have two clip regions.

        SIMILAR WARNING: Your program and Intuition "share" your window
        layer's DamageList.  BeginRefresh() helps arbitrate this
        sharing, but the lower-level function layers.library/beginupdate()
        does not.  It isn't really supported to use beginupdate() on
        a window's layer, but if you do--for whatever reason--it is
        critical that you first acquire the LayerInfo lock as in
        the above example: even if you only have one pass of refresh
        rendering to do.  Otherwise, the refreshing of your window's
        borders and gadgets can be incomplete, and the problem might
        occur only under certain conditions of task priority and
        system load.

    EXAMPLE
        Code fragment for "two pass" window refreshing, in response
        to an IDCMP_REFRESHWINDOW message:
        switch ( imsg->Class )
        {
        ...
        case IDCMP_REFRESHWINDOW:
            window = imsg->IDCMPWindow;

            /* this lock only needed for "two-pass" refreshing */
            LockLayerInfo( &window->WScreen->LayerInfo );

            /* refresh pass for region 1 */
            origclip = InstallClipRegion( window->WLayer, region1 );
            BeginRefresh( window );
            myRefreshRegion1( window );
            EndRefresh( window, FALSE );

            /* refresh pass for region 2 */
            InstallClipRegion( window->WLayer, region2 );
            BeginRefresh( window );
            myRefreshRegion2( window );
            EndRefresh( window, TRUE );         /* and dispose damage list */

            /* restore and unlock */
            InstallClipRegion( window->WLayer, origclip );
            UnlockLayerInfo( &window->WScreen->LayerInfo );
            break;
        ...
        }


    INPUTS
        window = pointer to the window structure which needs refreshing

    RESULT
        None

    BUGS
        This function should check the return code of
        layers.library/beginupdate(), and abort if that function fails.

    SEE ALSO
       endrefresh(), layers.library/beginupdate(), openwindow()
        layer.library/installclipregion(), layers.library/locklayerinfo()
        The "Windows" chapter of the Intuition Reference Manual