The custom draw handler is stored in a DrawHandlerDesc data structure as a PFNDRAWHANDLER, along with a pointer to a block of memory allocate and owned by the application. This pointer will be passed into the draw handler each time the draw decorator is asked to draw itself, providing access to private application storage. The DrawHandlerDesc data structure also contains a PFNFREEHANDLER that will be called when the draw decorator is released so that the decorator may free the private memory allocated and passed to the event handler.
- p:A pointer to an IDrawDecorator object.
- pd:Pointer to the data structure that contains the custom draw handler, private memory pointer, and the callback used to free the private memory.
void IDrawDecorator_SetDraw(IDrawDecorator *p, DrawHandlerDesc *pd);
The draw handler installed by a draw decorator to perform custom drawing should itself remember to call DrawHandlerDesc_Call() so that the drawing chain will continue -- most notably, so that the object wrapped by the draw decorator will be drawn. The draw handler may call DRAWHANDLERDESC_Call() at any point -- either before it performs its own drawing, after, or someplace in the middle.
An application that hooks into both a draw decorator's draw handler and event handler (by calling IDrawDecorator_SetHandler()), should exercise care in managing the private data released by the PFNFREEHANDLER stored in the DrawHandlerDesc data structure. Draw handlers and event handlers both manage data structures that contain pointers to private storage, as well as a pointer to a function that will be used to release this storage -- which could be different... or could be the same! Each of these PFNFREEHANDLER routines will be called when the draw decorator is released. Therefore, an application that uses the same block of private storage for both its custom event and draw handling procedures should be careful to check the validity of the storage to be freed before actually attempting to release the memory.