scope of GRAPHIC ATTACH

Please could GW experts give me a sanity check on this before I bother PowerBasic Inc with it?

Quoth the F1 help for GRAPHIC ATTACH:

This statement chooses a graphic target. All further graphic operations will be directed to this target until another GRAPHIC ATTACH or GRAPHIC DETACH statement is executed, or the graphic target is deleted. All PowerBASIC graphical displays are persistent -- they will be automatically redrawn even if minimized or temporarily covered by another window.

Yet if I GRAPHIC ATTACH a GW then subclass it and attempt to, say, GRAPHIC BOX on it from the subclass proc or a function called from it, the GW is blank - unless I have included a GRAPHIC ATTACH statement in the subclass proc etc. Changing the scope of the GW handle to GLOBAL does not seem to make much difference - I mean, I don't think that the scope of the ATTACHment is the same as the scope of the window handle.

OK - this could be cruelty to GRAPHIC WINDOWS - or flagrant mixing of DDT and SDK styles - but it would be nice to know what the specific rules are for GRAPHIC ATTACH, or indeed where to find them, if not in the online help.

Also, is there a downside to re-attaching a GW? Any special handling required when closing the GW?

Just in case I have completely misread the situation - it has happened - here is some some code to show what I mean. If you comment out the GRAPHIC ATTACH in the "drawit" SUB, nothing is shown.

Code:

' to investigate scope of GRAPHIC ATTACH
' Chris Holbrook 13 Dec 2008

#compile exe
#dim all
#include once "win32api.inc"

global GrDialogProc as long               ' used in subclassing the grapic control to get mouse msgs
global GrOldProc as long                  ' used in subclassing the grapic control to get mouse msgs
global currentrect as rect
global hWin, hGW as dword
'--------------------------------------------------------------------------------
function GrProc(byval hWnd as dword, byval wMsg as dword, byval wParam as dword, byval lParam as long) as long
    static startpt, endpt as pointAPI
    static htimer as long

  select case wMsg
     '
     case %wm_mousemove
         if wparam and %MK_LBUTTON then ' we're dragging
            endpt.X = lo(word,lParam)
            endpt.Y = hi(word,lParam)
            drawit(startpt, endpt)
            startpt = endPT
         end if
    '
    case %wm_lbuttondown
        startpt.X = lo(word,lParam)
        startpt.Y = lo(word,lParam)
    '
    case %wm_lbuttonup
        startpt.X = 0: startpt.Y = 0
    '
  end select
 function = CallWindowProc(GrOldProc, hWnd, wMsg, wParam, lParam)

end function
'----------------------------------------------------------------------
sub drawit ( startpt as pointapi, endpt as pointapi)
    local colour as long
    
    colour = %black
    offsetrect currentrect, endpt.X - startpt.X , endpt.Y - startpt.Y
    ? currentrect.nleft, max(70, currentrect.ntop),currentrect.nright, currentrect.nbottom
    graphic attach hwin, 0, redraw
    graphic box (0, 70) - (500, 500), 0, -2, -1, 0
    graphic box (currentrect.nleft, max(70, currentrect.ntop))-(currentrect.nright, currentrect.nbottom),0,colour,-2,0
    graphic redraw
end sub
'-----------------------------------------------------------------------------------
function pbmain () as long
    local skey as string

    ' Create and show a Graphic window on screen
    graphic window "", 450, 130, 500, 500 to hWin

    graphic attach hWin, 0, redraw
    hGW = GetWindow(hWin, %GW_CHILD) ' Retrieve hWnd of graphic window

    ' Subclass the Graphic control
    GrOldProc = SetWindowLong(hGW, %GWL_WNDPROC, codeptr(GrProc))

    setrect currentrect, 100, 70, 250, 250
    'lastpt = gMousePt
    graphic print "to investigate scope of GRAPHIC ATTACH by Chris Holbrook 13-DEC-2008"
    graphic print "drag a box on a GRAPHIC WINDOW"
    graphic print "start with cursor in the graphic window(this one)"
    graphic print "Use the mouse with left button down to drag it"
    graphic print "<Esc> to exit"
    graphic redraw
    ' main program loop
    ' wait for the punter to press ESC then exit
    ' let the subclass proc do the work
    do
        skey = ""
        graphic inkey$ to skey
        if skey$ = $esc then exit loop
        sleep 50
    loop

    SetWindowLong(hGW, %GWL_WNDPROC, GrOldProc) ' remove subclassing
    graphic window end

end function

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