This function allows a widget to hook its own event handler into the normal widget event handling mechanism. For example, a widget that derives from one of the standard widget interfaces could insert its own event handler that will be called by the base widget object each time an event is processed.
The event handler is stored in a HandlerDesc data structure as a PFNHANDLER, along with a pointer to a block of memory allocate and owned by the widget. This pointer will be passed into the widget's event handler with each event to be processed. The HandlerDesc data structure also contains a PFNFREEHANDLER that will be called when the widget is released so that the widget may free the private memory allocated and passed to the event handler.
IWidget_SetHandler() associates a given HandlerDesc data structure with a particular widget.
void IWidget_SetHandler(IWidget *pif, HandlerDesc *pDesc);