Technical Q&As

TB26 - Avoiding DragDrawingProc Pixel Trails (25-October-96)

Q I've just implemented a DragDrawingProc. To start, I've tried simply to duplicate the default behavior of the Drag Manager (so it will look as if I had not in fact attached a DragDrawingProc). Unfortunately, when the user drags into a valid drop area and the potential drop receiver calls ShowDragHilite, my DragDrawingProc seems to be responsible for leaving a trail of pixels on the screen. What am I doing wrong?

A This happens because Drag Manager does not always pass the entire "old" or "new" region to the DragDrawingProc. Here's a function which mimics what Drag Manager does when you don't attach a DragDrawingProc to a DragReference before calling TrackDrag:

static pascal OSErr LikeDefaultDragDrawingProc
        (   DragRegionMessage message,
            RgnHandle showRegion, Point showOrigin,
            RgnHandle hideRegion, Point hideOrigin,
            void *dragDrawingRefCon, DragReference theDragRef   )
    {
        OSErr err = noErr;
    
        RgnHandle   xorMe;
        long        oldA5;
        Pattern     gray;
    
        switch (message)
        {
            case dragRegionBegin:
    
                oldA5 = SetA5 ((long) dragDrawingRefCon);
                gray = qd.gray;
                SetA5 (oldA5);
    
                PenPat (&gray);
                PenMode (notPatXor);
    
                break;
    
            case dragRegionDraw:
    
                xorMe = NewRgn ( );
                if (!(err = MemError ( )))
                {
                    XorRgn (showRegion, hideRegion, xorMe);
                    PaintRgn (xorMe);
                    DisposeRgn (xorMe);
                }
                break;
    
            case dragRegionHide:
    
                PaintRgn (hideRegion);
                break;
        }
    
        return err;
    }

The call to XorRgn is the key. It's also very important to pass the correct value for the 'dragDrawingRefCon' to SetDragDrawingProc:

    SetDragDrawingProc (dragRef,LikeDefaultDragDrawingProc,
        (void*)SetCurrentA5 ( ));

(The above works for 68K code; UniversalProcPtr creation omitted for simplicity.)

By the way, be careful not to mix the use of SetDragDrawingProc and SetDragImage. See Technote 1043:On Drag Manager Additions for details.

-- Pete Gontier
Worldwide Developer Technical Support