TransRepliShow: Dragging Replicants Transparently

This article follows up on Dr. Reh's previous newsletter article, RepliShow: A Replicable Image Viewer. The code discussed is available here for your convenience.

Just remember Seth Flaxman's RepliShow: While dragging the replicant you only see the border lines of an empty rectangle - not very Be-like. However, only a few lines of code are necessary to obtain a rectangle containing the dragged image and looking transparently. Because the dragging action is managed by the BDragger class we need to do some subclassing. We create a new RepliDragger class inherited from BDragger. When this is done we override the hook function virtual void MouseDown( BPoint where ):

void RepliDragger ::MouseDown( BPoint where )                            // mouse down
{ 
    BPoint cursor;
    uint32 buttons;
	
    SetMouseEventMask( B_POINTER_EVENTS, 0 );                            // view can receive mouse-events
    GetMouse(&cursor,&buttons);

    if ( (fBitmap != NULL) && (fArchive != NULL) && (buttons & B_PRIMARY_MOUSE_BUTTON) )
    {
        BPoint origBitmap;
        BRect bitmapRect  = fBitmap -> Bounds();                         // get boundaries of image
        BBitmap *bitmap  = new BBitmap(bitmapRect, B_RGB32,true);        // create bitmap for drag-and-drop
        memcpy(bitmap->Bits(), fBitmap->Bits(), fBitmap->BitsLength());  // copy bitmap
        origBitmap = BPoint(bitmapRect.Width(),bitmapRect.Height());     // position relative to bitmap
        origBitmap = origBitmap - BPoint(7-where.x, 7-where.y);
        DragMessage( fArchive, bitmap, B_OP_BLEND, origBitmap, NULL); 	
    }
	
    BDragger :: MouseDown(where);                                        // BDragger mouse down events
}

void DragMessage(BMessage *message, BBitmap *image, drawing_mode dragMode, BPoint offset, replyTarget = NULL) is responsible for the drag-and-drop handling. message is a BMessage object containing all information that will be dragged and dropped to the BShelf container view. In our special case the message is fArchive, our archived view. Using the dragMode B_OP_BLEND lets you drag the BBitmap image transparently a